45PSI Tires MPG vs. 34PSI Tires MPG
 

Since the conditions were so similar, I thought it would be releveant to present the information I found this weekend.

My car is a 2007 Fit Sport AT with 10,000 miles now. Only mods to the car are the following:

• Royal Purple Fully Synthetic Race Oil 10W30 (1/2 quart low on purpose)
• Sid Short Ram intake system
• 15"x7" Konig Next wheels with Hancook Ventus HRII 205/50/ZR15 tires filled with nitrogen
• About 500lbs total cargo (people and luggage)
• No spare tire/jack
• A/C was run sporatically both trips
• Cruise control set at 75mph constant

Trip #1
Miles = 6,500 odometer
Exterior Ambient Temp = 70
Tire Inflation = 45psi Cold
Distance = 150miles
Mileage = 43.8mpg

Trip #2
Miles = 10,000 odometer
Exterior Ambient Temp = 90
Tire Inflation = 34psi Cold
Distance = 146miles
Mileage = 40.3mpg

I knew that running less tire pressure would result in lower mileage, but with my back, I needed the extra amount of cushion afforded by the softer tires.

I must say, it was a lot more comfortable on this trip compared to the last. My wife did not complain as she did last time. :)

The steering was a little numb, and a little slow on initial turn-in response. Handling also suffered slightly with less tire pressure.

I think the loss of a little over 3mpg was worth the softer ride.

Have you tried...


34 psi cold in the front...then 45 psi cold in the rear tires? or the other way around?

I usually add +2 psi in the back since the back wheels aren't required for turning and they don't add heat as fast as the front tires.

Around Here we seem to like the A - B - A testing.

So, Try the lower tire pressure again to verify that you didn't improve your driving habits.

If memory serves correctly, you have come along way from 26mpg to 43mpg, congrats on that.;)


I am not saying you are wrong, I believe and know increasing the tire pressure increases the mpg which is why I roll at 50psi.:vtec:

Dont discount that outside air temp also, that will figure in as well. 20 degrees is pretty significant. I'd also tend to think the AC was run more often in 90 temps than in 70.

[quote=Sid 6.7;354017]Since the conditions were so similar, I thought it would be releveant to present the information I found this weekend.

My car is a 2007 Fit Sport AT with 10,000 miles now. Only mods to the car are the following:

• Royal Purple Fully Synthetic Race Oil 10W30 (1/2 quart low on purpose)
• Sid Short Ram intake system
• 15"x7" Konig Next wheels with Hancook Ventus HRII 205/50/ZR15 tires filled with nitrogen
• About 500lbs total cargo (people and luggage)
• No spare tire/jack
• A/C was run sporatically both trips
• Cruise control set at 75mph constant

Trip #1
Miles = 6,500 odometer
Exterior Ambient Temp = 70
Tire Inflation = 45psi Cold
Distance = 150miles
Mileage = 43.8mpg

Trip #2
Miles = 10,000 odometer
Exterior Ambient Temp = 90
Tire Inflation = 34psi Cold
Distance = 146miles
Mileage = 40.3mpg


Good experiment but some more data is wanted.
The 20F difference in ambient temperature is significant. Did you measure the tire pressures during the end of the drive(hot) or cold?

100% nitrogen? Air is already 78% so how does it factor in?

The coefficient of expansion of nitrogen is less than that of oxygen so the pressure increase is less.
Its only significance is in racing where heat buildup is extreme (on the order of a hundred degrees or more) and critical to the tire not overpressurizing, losing valuable contact surface.
I've forgotten what the optimum tire temps for GY race tires but it seems that it was around 220 degrees F.
Using nitrogen on the street isn't much of a help in mpg where higher pressureis the item that increases mpg.
The single useaful thing about nitrogen is that bleedout of the pressure through the tirewall (what you thought they are impervious?) is slower with nitrogen because the nitrogen molecule is larger and harder to permeate through the rubber.

I'm curious about tires wearing in the center as a result of the higher pressures. The hypermilers claim tires still wear evenly since steel belts keep the tire surface flat. One poster said he's seen tires inflated at higher pressures for use in races wear out faster in the middle.

Anyone have any data for tires used only in streets?

Interesting results, I got similar results from my data this weekend.

To get better MPG you have to reduce the surface area, basic physics tells us that running higher pressures will make the tire deflect outwards in the center thus giving less resistance and wearing that part of the tire more.

Regardless of construction everything deflects when you apply load.

This is true but remember that inflating tires to MAX side wall pressure is NOT, I repeat NOT over inflating.

Lets look at the sport tires, the max side wall pressure is 51 psig. So, if one inflates the tires to 51psig then the tire is NOT over inflated. However, if you inflate to 52, 55, or 60psig(anything above 51) then it would be consider over inflated.

The manufacture of the tire is has established operating ranges for tires.
The manufacture of the "car" has established operating ranges for the "car's" tires.

NOTE: the manufacture is subjectively concerned with comfort not necessarily performance. Notice that the recommend tire pressure by the manufacture does not take into account the type of tire that you have on your car, therefore, one must rely on the information from the tire manufacture.

True, when using max tire sidewall pressure blow outs are not a concern. Also there is some safety and comfort margin in the reccomended PSI of probably a couple of pounds.

However the tire is designed to be running with more weight at that pressure as it is a intended to fit a wide variety of vehicals. A giant heavy domestic car running the same tire will probably have a higher reccomended pressure. With that amount of pressure in the tire for a vehical which only reccomends 32psi you are without question not providing enough force downward to counteract the outward deflection that would naturally occur.

When it comes right down to it there is really no other way to explain why you get better MPG using higher pressures other than you are reducing surface area. Using that logic, if you reduce the surface area the points of contact will have more weight on them and will wear faster.

In this case we are seeing roughly a 9% increase in overall efficency. If we assume this directly relates to the tires then that is a 9% reduction of contact on the flat tread (highway's don't exactly roll the tire on corners so we can dismiss any rolling resitance along the side tread). With 9% less contact your wheels we can only assume the deflecting tire is also arching out to create that affect and has an substatial increase in weight right along the center that would not normally be there. So really the wear in the center of the tire would be somwhere between 10%-20% worse than a properly infalted tire.

good thread. anyone try 87 vs 91 octane? i've always run super in my cars but it's now at 4.85$ vs. 4.65$ for reg...ouch...

Wow, I just found this on cleanmpg.com:Before you over inflate watch this. - Page 3 - CleanMPG Forums

But this is what I wanted to share:

After this tank I may increase from 50 to 55 and then to 60 if things turn out good. Yeah, I am willing to take the risk.:p:popc:

Yes we have generally run our tires at the upper end of the specs on the sidewall for decades. And yes the tires wear out in the center first. Steel belted tires are flexible too so the center wears out first & mpg is better because the rolling resistance is less.

Yikes, that is scary they can sell tires that old :(

With gas prices they way they are now I think replacing tires early is cheaper to be honest. I still don't really endorse it just because I like to stay nimble with my car. I had a deer jump out in front of me on the highway this weekend, luckily I was able to stop barely in time. No amount of saved gas could have offset that!

This is exactly why I'm not interested in theory. I spent lots of time learning theory in physics and engineering classes, then found out what from theory actually made it into the real world. Get me some real data.

Here's more theory blah blah you haven't accommodated in your calcs - an additional reason for lower rolling resistance is less bending in the edge of the tire surface as it enters and releases from the road. Also, lower cross section means more force per area which will increase friction so in one respect, cross section could be a zero sum game (decrease in surface area is offset by increase in friction per area). In yet another, one could surmise the resistance comes from compression of the tire surface as the rubber moves through the contact patch, much like compressing and releasing a damped spring. In this case it's not the patch that matters, it's the amount of compression along the direction of travel times the width of the tire that would indicate the amount of force required to keep the tires moving (this would close to but not identical to your patch calc). My guess is the damped spring effect is likely the greatest but not sole reason for rolling resistance.

I agree the tire will only wear where it makes contact with the road (duh!) but is wear linearly proportional to force? Unless you're a tire scientist and have worked with both tests and an understanding of the materials, any commentary is all speculation. There's lots of other speculation about how much a steel belt would deflect in the middle and whether or not that effect would increase or decrease the overall useful life of the tire.

GET ME DATA!!! The hypermilers claim no decrease in useful life of the tire through overinflation, there's data that counters several forums worth of speculation, if it's true. The racers note an increase in wear in the center of tires used for racing but we don't know if this is exacerbated by race conditions or actually resulted in a decrease in useful life of the tire. If the center of the tire wears faster but I get the same amount of total miles out of the tire then I'll take the better mpg with a worn center tread.

Another factor is the newness of the tire. I've noted new tires have lower rolling resistance than old tires. Is this because of a manufacturer coating, the rubber compound being new, or the upper portion of the tread being a different forumla than rubber in the lower parts of the tread, or something else?

Do you still get good life out of the tires or does total miles on the tire seem to go down too?

I have the sport model and I am not sure how many miles these things are rated for. I have been driving with 40psi at about 11K and I have increased it to 45psi then 50psi up until my mileage now which is at 31k. Tonight I just pumped the tires up to 55psi and the ride is not much different than the 50psi and the tire does not look any different, though I have driven only ~10miles on the tires at 55psi. I must say I have had my tires rotated and balanced at least every 5k.

Yes, the tire is 'worn out' prematurely. There has been a couple of times we dropped the pressure to 'even' out the tire wear cause we wanted them to last longer but I don't remember that being successful. Normally for me, tires are replaced at 20,000 miles front on a Fit kind of car and 45000 on the rear.
Yes, I replace only one axle worth of tires at a time. The cost of rotating tires has never proven worthwhile and it keeps my out-of-pocket cost down for any one change. (I know the overall cost doesn't change but I prefer spending $150 at one time rather than $300 (for Fit tires, my other cars tires cost a lot more, especially with the crude price runup Some of thode get replaced at 5k or less miles; they seem to need newer hipo tires to go faster)

I have always rotated and balanced my tires and used the 32psi rule for tires and achieved the manufactures mileage rating. With that said this is the first time I have inflated tires to the Max psig rating and beyond (as of yesterday). The more I read at cleanmpg.com the more I am convinced I need to inflate to 60psi to achieve longer tire life and better mpg. As far as safety is concerned I haven't found anything YET that has proven over inflating compromises safe driving or handling.

Hobbit just posted something rather interesting in this link:
Before you over inflate watch this. - Page 3 - CleanMPG Forums