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  #1  
Old 08-14-2009, 12:49 AM
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Decreased MPG after inflating tires to 35psi???

Hey guys, I run into a situation that's a bit hard to explain.

So I heard that in order to improve your gas mileage, one of the easiest things you can do is to over-inflate your tires. I know the factory recommendation for my 09 fit sport is 33psi all 4 tires, so I inflated all 4 to 35psi in the hope to perhaps getting a few extra MPG.

But since I've done that, my actual MPG as calculated per fill-up actually went down! There's been no change in driving habits. If anything, the 2 tanks I've driven since the tire pressure change have been highway driving primarily and that should have helped the MPG instead of decreasing it.

I'm a bit bummed, I can't really think of a reason why the MPG went down.

You can see my MPG log here, the tire pressure was changed on sometime b/w 7/31 and 8/4
2009 Honda Fit Sport (5sp MT) (Honda Fit) | Fuelly
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  #2  
Old 08-14-2009, 01:32 AM
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Dude, read about fuel efficiency. You sound like you are majorly off in udnerstanding of the topic. You may have eaten an extra pound, or the wind blew the other direction, or didnt fill up the tank the same way... What are you talking about...
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Old 08-14-2009, 01:47 AM
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I honestly think that there are too many variables like john said to blame the 2 psi on your decreased MPG.

I think the biggest variable you can probably account for is the temperature outside. From what I've heard, different temperatures can change the psi +-2 at any given moment, so you might just be the victim of that.
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Old 08-14-2009, 02:34 AM
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The other guys are right. There are so many variables in measuring fuel economy / mpg, that so few events will most likely not give meaningful data. IMO, an increase of 2 psig will probably not give much of a difference either. Pump 'em up about 5 psig over the recommendation and give that some time.

Good luck, enjoy the car and don't worry to much about small changes in mpg. Get a good baseline established.
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Old 08-14-2009, 02:41 AM
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Thanks for the replies. I love this car.
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Old 08-14-2009, 02:47 AM
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I know the 09 Fits manual says 33 psi recommended but the tirewall of the factory Dunlaps on mine have a max psi rating of 51 psi, so I keep mine around 45-50 psi. I have averaged between 38 and 45 mpg with this. Do this at your own risk and I will not be held responsible for your tire exploding.
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Old 08-14-2009, 11:03 AM
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How much stiffer does it ride with that kind of tire pressure?
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Old 08-14-2009, 11:12 AM
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Quote:
I know the 09 Fits manual says 33 psi recommended but the tirewall of the factory Dunlaps on mine have a max psi rating of 51 psi, so I keep mine around 45-50 psi.
WHAT!?!?!
Congrats, now you can wear out the centers of your tire and spend a ton of cash replacing them for the small amount of mpg you get.

Not to mention, you are putting yourself and others around you in a dangerous position. Since you are basically just riding on a very narrow path, not the entire tread as it was designed, your car will be much more unstable in turns and especially in the rain.

I worked on a lady's car that did that and it actually slid over an entire lane in the rain due to over inflation of her tires.
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Old 08-14-2009, 11:16 AM
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Quote:
Originally Posted by JPGC View Post
I know the 09 Fits manual says 33 psi recommended but the tirewall of the factory Dunlaps on mine have a max psi rating of 51 psi, so I keep mine around 45-50 psi. I have averaged between 38 and 45 mpg with this. Do this at your own risk and I will not be held responsible for your tire exploding.
I always go by the side wall figure. 33psi is way too low for a max psi of 50 psi. If its max psi of 50 I keep them at 45psi. Tires under inflated just leads to safety issues, excessive tire wear and loss of money in my wallet.

Reference....
http://www.drivetosurvive.org/tire_safety_article.htm
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Old 08-14-2009, 11:26 AM
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Quote:
Originally Posted by lostpacket View Post
WHAT!?!?!
Congrats, now you can wear out the centers of your tire and spend a ton of cash replacing them for the small amount of mpg you get.

Not to mention, you are putting yourself and others around you in a dangerous position. Since you are basically just riding on a very narrow path, not the entire tread as it was designed, your car will be much more unstable in turns and especially in the rain.

I worked on a lady's car that did that and it actually slid over an entire lane in the rain due to over inflation of her tires.
They are not overinflated if he's below the parameters of the max psi. There are a lot more risk to under-inflating than keeping right below the max psi level. Tire manufactures put the max psi there for a reason. See my link that I posted above.

"not the entire tread as it was designed, your car will be much more unstable in turns and especially in the rain."

Thats a pretty ignorant statement given you are under the Manufactures max psi.
Now if your at 60 psi at a max rating of 50 psi... they yea your a d*mbass and should not be operating a vehicle. In that case yes your vehicle could possible become unstable.

I'm going to believe what the tire manufacturer states on their actual product over what Honda has in there manual. In the case of a tire failure Honda is not liable the tire manufacturer is. Thus I go off what the tire manufacturer states as acceptable psi.
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Last edited by Committobefit08; 08-14-2009 at 11:32 AM..
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  #11  
Old 08-14-2009, 12:35 PM
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Quote:
Originally Posted by Committobefit08 View Post
I always go by the side wall figure. 33psi is way too low for a max psi of 50 psi. If its max psi of 50 I keep them at 45psi. Tires under inflated just leads to safety issues, excessive tire wear and loss of money in my wallet.

Reference....
Tire Safety Article
So an expert reference is a writeup from some guy that owns a safety store catering to mostly heavy-duty and commercial vehicles? I'm not saying he's wrong, but it's two entirely different animals when talking about tire pressures of commercial vehicles which are carrying loads which may REQUIRE the max pressure to achieve the load rating and a subcompact passenger vehicle on which the tires will almost never see their rated load weight.

from Michelin...

Recommended Pressure

If you don’t know the proper inflation air pressure for your tires, what do you do?
Easy. Check at one of the following places on your vehicle:
  • In the vehicle owners manual.
  • On the vehicle’s door jamb. (Often, a vehicle information placard is located on the door jamb along with the recommended tire inflation pressure.)
  • Inside the fuel hatch filler flap. (In some vehicles.)
  • The glove compartment door. (In some vehicles.)
    • But NOT on the tire. The inflation pressure shown on the tire sidewall is only the maximum tire inflation pressure. In most situations, the vehicle manufacturers recommended tire inflation pressure is shown on the vehicle placard. However, on some older vehicles, the recommended pressure listed by your vehicle’s manufacturer may be shown as two numbers: One for city speed driving and normal load and one for highway speeds and/or maximum capacity vehicle load. Be sure to inflate your tires to the recommended inflation pressure for each type of driving situation.
from Pirelli...

Please note that 'suggested inflation pressure levels' do not exist. The correct inflation pressure value is given by the vehicle manufacturer and can be found in the vehicle log book. Inflation pressure checks should be made only when tires are cold. Tires are considered to be cold when they have not been run for at least one hour or have only been run at low speed for not more than two or three kilometers. Check cold tire pressure frequently (at least once a montgh) and always before long journeys, not forgetting the spare tire!

from tiresafety.com (Bridgestone)...

Inflation Pressure
Proper inflation pressure is essential for achieving maximum performance and mileage. Improper tire inflation pressure can cause severe internal tire damage, which can lead to sudden tire failure and resulting in serious personal injury or death.Improper inflation pressure may result in rapid or irregular wear. Pressures should always be checked when the tires are cold and at least monthly. Under normal tire operation, approximately 1psi of tire pressure will escape every month. Also, for every 10 degrees F change in ambient temperature, tire pressure will change by approximately 1psi.
Vehicle manufacturers list recommended tire pressures for original vehicle tires in the owner's manual or on a placard on the end of the driver's side door or in the glove box.

For continuous high speed driving, tire pressures should be increased by 3 to 5psi above the normal cold inflation recommended.
However, for passenger tires, never exceed the maximum inflation pressure molded on the sidewall. The inflation pressure for light truck tires may exceed that molded on the tire by 10psi. Any recommended front to rear pressure differential should be maintained.

Recommended Inflation Pressure
Many people ask "what air pressures do you (meaning tire companies) recommend for cars under normal conditions?" The answer is "we recommend what the vehicle manufacturer recommends."

What criteria do the car manufacturers use to determine inflation recommendations
* Ride * Load capacity * Traction, wear * Fuel economy/Rolling resistance .....are all correct. But, consider another question: "Why did Ford recommend 30 in the front and 34 in the rear on a 1997 Crown Victoria?"
The additional air stiffens the sidewall and makes it more stable. In order to achieve the handling, tracking, and other driving characteristics, one of the things the manufacturer can do is balance front to rear handling by adjusting the tire pressures. In adding the same air pressure in the front as the rear you will change the handling characteristics from what the manufacturer intended. Quite a few station wagons and light trucks have different pressures for the front and rear. But, it's not just station wagons and light trucks; it's also Porsches, Nissans, Corvettes, Hondas, and BMW's to name just a few.
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Old 08-14-2009, 12:44 PM
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The Max tire on the sidewall is just that, a maximum that the tire can take not what it should run at.

The recommended pressure from Honda takes more factors into consideration like the weight of the vehicle for example.

What I do is measure the tire wear with a tread depth gauge and adjust the pressure so that the tire wears evenly across the whole tread.
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Old 08-14-2009, 12:44 PM
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I've been getting 32 MPG w/ 75-80% highway and I'm trying to figure out why I'm not getting better mileage. Checked my tires this morning and my tires were too low. My rears were at 30 PSI and my fronts were at 32 PSI. I decided to up my PSI to 35 all around. I'll let you know if I get better or worse mileage.
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Old 08-14-2009, 12:56 PM
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Quote:
Originally Posted by bojackson63044 View Post
Hey guys, I run into a situation that's a bit hard to explain.

So I heard that in order to improve your gas mileage, one of the easiest things you can do is to over-inflate your tires. I know the factory recommendation for my 09 fit sport is 33psi all 4 tires, so I inflated all 4 to 35psi in the hope to perhaps getting a few extra MPG.

But since I've done that, my actual MPG as calculated per fill-up actually went down! There's been no change in driving habits. If anything, the 2 tanks I've driven since the tire pressure change have been highway driving primarily and that should have helped the MPG instead of decreasing it.

I'm a bit bummed, I can't really think of a reason why the MPG went down.

You can see my MPG log here, the tire pressure was changed on sometime b/w 7/31 and 8/4
2009 Honda Fit Sport (5sp MT) (Honda Fit) | Fuelly

Just as in showroom stock racing, there is no real change in tire performance with a 2 psi change. Inflate to 42 psi and see what happens.
There aree too many uncontrolled variables involved with the air pressure for example. We have found very few gages to be accurate within 2 psi. We calibrated ours with equipment at the lab to insure we knew what we were doing and I suspect this may be your major problem.
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Old 08-14-2009, 01:21 PM
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Quote:
Originally Posted by txmatt View Post
So an expert reference is a writeup from some guy that owns a safety store catering to mostly heavy-duty and commercial vehicles? I'm not saying he's wrong, but it's two entirely different animals when talking about tire pressures of commercial vehicles which are carrying loads which may REQUIRE the max pressure to achieve the load rating and a subcompact passenger vehicle on which the tires will almost never see their rated load weight.

from Michelin...

Recommended Pressure

If you don’t know the proper inflation air pressure for your tires, what do you do?
Easy. Check at one of the following places on your vehicle:
  • In the vehicle owners manual.
  • On the vehicle’s door jamb. (Often, a vehicle information placard is located on the door jamb along with the recommended tire inflation pressure.)
  • Inside the fuel hatch filler flap. (In some vehicles.)
  • The glove compartment door. (In some vehicles.)
    • But NOT on the tire. The inflation pressure shown on the tire sidewall is only the maximum tire inflation pressure. In most situations, the vehicle manufacturers recommended tire inflation pressure is shown on the vehicle placard. However, on some older vehicles, the recommended pressure listed by your vehicle’s manufacturer may be shown as two numbers: One for city speed driving and normal load and one for highway speeds and/or maximum capacity vehicle load. Be sure to inflate your tires to the recommended inflation pressure for each type of driving situation.
from Pirelli...

Please note that 'suggested inflation pressure levels' do not exist. The correct inflation pressure value is given by the vehicle manufacturer and can be found in the vehicle log book. Inflation pressure checks should be made only when tires are cold. Tires are considered to be cold when they have not been run for at least one hour or have only been run at low speed for not more than two or three kilometers. Check cold tire pressure frequently (at least once a montgh) and always before long journeys, not forgetting the spare tire!

from tiresafety.com (Bridgestone)...

Inflation Pressure
Proper inflation pressure is essential for achieving maximum performance and mileage. Improper tire inflation pressure can cause severe internal tire damage, which can lead to sudden tire failure and resulting in serious personal injury or death.Improper inflation pressure may result in rapid or irregular wear. Pressures should always be checked when the tires are cold and at least monthly. Under normal tire operation, approximately 1psi of tire pressure will escape every month. Also, for every 10 degrees F change in ambient temperature, tire pressure will change by approximately 1psi.
Vehicle manufacturers list recommended tire pressures for original vehicle tires in the owner's manual or on a placard on the end of the driver's side door or in the glove box.

For continuous high speed driving, tire pressures should be increased by 3 to 5psi above the normal cold inflation recommended.
However, for passenger tires, never exceed the maximum inflation pressure molded on the sidewall. The inflation pressure for light truck tires may exceed that molded on the tire by 10psi. Any recommended front to rear pressure differential should be maintained.

Recommended Inflation Pressure
Many people ask "what air pressures do you (meaning tire companies) recommend for cars under normal conditions?" The answer is "we recommend what the vehicle manufacturer recommends."

What criteria do the car manufacturers use to determine inflation recommendations
* Ride * Load capacity * Traction, wear * Fuel economy/Rolling resistance .....are all correct. But, consider another question: "Why did Ford recommend 30 in the front and 34 in the rear on a 1997 Crown Victoria?"
The additional air stiffens the sidewall and makes it more stable. In order to achieve the handling, tracking, and other driving characteristics, one of the things the manufacturer can do is balance front to rear handling by adjusting the tire pressures. In adding the same air pressure in the front as the rear you will change the handling characteristics from what the manufacturer intended. Quite a few station wagons and light trucks have different pressures for the front and rear. But, it's not just station wagons and light trucks; it's also Porsches, Nissans, Corvettes, Hondas, and BMW's to name just a few.
Yea like any dealer or joe shmo tire shop put 40 psi in the back and 30 in the front to make a car more stable...please. they put them all at 30psi so the ride is comfortable to the customer...in no way to save tread life. In that case all dealers should put full synthetic oil in the car for you...but they don't they put crappy cheap oil. Dealers put the tire psi at the minimum to wear the tire quicker so they can get more money out of you. Its business.... thats why I do all my own car work.
Ok I agree if the car manufacturer states a certain psi its legit only for a minimum psi. Its for comfort to get optimal customer satisfaction on ride quality...not tread life. When the tire manufacturer say the auto manufacture place card psi is the minimum pressure ever to be put in the tire then I think its safe to say I think I rather stay between 33 (placecard) and 51 psi (sidewall max)...maybe like 45? at least in the middle.

Straight for tire manufacturer...

Cooper Tires
"Tire placard on the vehicle. These pressures must be maintained as a minimum. Do not exceed the maximum pressure indicated on the tire sidewall of passenger car tires."

My point stay above the tire pacecard psi (the min.) but below the Max stated on the side wall and you'll be safe. NO loss of vehicle control and serious danger like Lostpacket was inferring.
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Last edited by Committobefit08; 08-14-2009 at 01:34 PM..
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Old 08-14-2009, 02:00 PM
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Too high of a pressure would "balloon" your tires, thus causing wear in the middle treads. Too low would cause wear on the outside.

Personally, I go with high 36-38 on my street driven cars. This is mainly where I find even wear across the tires.

When I used to do low 30's (recommended by manufacturer), I tend to get wear on the outsides.

If I was racing, I usually go low to mid 40's using street tires. Again this is to get as much tire on the ground for cornering. I get pretty good tire wear across the width, but I think the reason it wears ok under racing conditions is because of higher loads on the tires. Just driving straight up and down the streets and freeways would not put as much load, thus wearing the center tires a little quicker.

I think tires can handle pressures beyond the stamped max pressures pretty well. I've seen people do some high pressures to tune some rotation in for racing, espcially for FWD cars. But I don't think 50+ psi will give you much more on the streets. Probably better off driving 1-2mph slower...

Anyway, the stock tires on the fits were made to last forever (at least compared to the summer only tires i usually use). So it would probably be hard to see any wear on the tires. I probably have to change the tires due to deterioration of the rubber before the tread wears out. lol...

Tirerack has a handful of writeups on tire pressures.
Tire Tech Information - Air Pressure - Correct, Underinflated and Overinflated
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Old 08-14-2009, 02:27 PM
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I average roughly 20- 30k miles a year on my vehicles and I have never had a problem running my tire pressure right below the maximum psi listed on the tirewall. I have also never had a problem with uneven wear or ballooning. There is a reason that tire manufacturers list the max psi on the tire because that is the maximum safe psi for the tire. Yes, there are variables but tire size takes care of a lot of that, for example...You shouldn't be running a 185/ 50 R16 on a car or truck that weights 5000 pounds. Too big of a vehicle for those tires. Every tire has specs on it to show how much weight they can handle. I have never followed the car's manual on tire pressure, always the tire manufacturers specs. Every tire from different manufacturers is different.
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Old 08-14-2009, 02:29 PM
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Quote:
Originally Posted by Committobefit08 View Post
They are not overinflated if he's below the parameters of the max psi. There are a lot more risk to under-inflating than keeping right below the max psi level. Tire manufactures put the max psi there for a reason. See my link that I posted above.

"not the entire tread as it was designed, your car will be much more unstable in turns and especially in the rain."

Thats a pretty ignorant statement given you are under the Manufactures max psi.
Now if your at 60 psi at a max rating of 50 psi... they yea your a d*mbass and should not be operating a vehicle. In that case yes your vehicle could possible become unstable.

I'm going to believe what the tire manufacturer states on their actual product over what Honda has in there manual. In the case of a tire failure Honda is not liable the tire manufacturer is. Thus I go off what the tire manufacturer states as acceptable psi.

Thanks, I was bout to comment the same way, lol
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Old 08-15-2009, 04:30 PM
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I have always inflated at least 5psi over maximum

On all my vehicles over the years, and never had un-even tire wear or any other tire related incidents.

IMHO 5 psi is not that much, but will give you a stiffer ride - you loose some cushion, but gain less rolling resistance - sort of like riding your bicycle with hard tires vs. low inflated tires.

There is a big difference in the fit when coasting in neutral with tires inflated high.

Your car, do what you want. I could think of a lot more dangerous things that you could do to it....

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