kenchan: Door jam sticker says 33 f/r for 185/55-16 and 60 psi in the spare. 36-37 psi seems to be too high to begin with but also going in wrong direction as tire volume increases (205/50-16) but vehicle weight stays the same. Probably get better gas mileage.

n9cv: It might sound weird but a few psi +/- really changes the steering feel and how bouncy the rear feels when using these larger tires. Because of the short wheel base every psi seems to change the feel (good & bad) and gas mileage.

Yes, circumference-wise, the 205/50-16 and the 185/55-16 (OEM on 2011 Sport) are almost the same. I guess the lighter the car the more critical it gets (a few psi really changes how a motorcycle feels).

I was looking for the math to use as a starting point. There does seem to be a lot of advice in the pickup truck / Jeep forums but didn't know if someone had done a FIT specific calculation.

 

I think you misunderstood my point. The published maximum weight carrying figure at maximum pressure was 1201 pounds at 51 PSI and 1202 pounds at 51 psi for the other tire. Honda has determined that they want the tire inflated at 65% and 63% of maximum for the weight of a Fit. So if I look at the numbers for both tire sizes they will only vary by a few 1000ths of a percentage point.

It is not worth calculating as they are statistically the same.

I did not look at tire diameters and revs per mile on the two tires as they have nothing to do with ideal inflation pressures.

 

…n9cv: Thanks. I was trying to confirm my thinking about the subject. This is how I see it.

The OEM Dunlops (185/55-16) have a load capacity of 1074 pounds (lbs) at 51 psi, so 1074 / 51 = 21.06 lbs / 1 psi. If the door jam sticker says to inflate to 33 psi, then 33 times 21.06 lbs per 1 psi = approx 695 lbs per tire.

The slightly oversized General tires (205/50-16) have a 1201 lbs load carrying capacity at 51 psi, therefore, they carry 23.55 lbs per 1 psi. If I need 695 lbs carrying capacity in each tire, then I can achieve that with only 29.5 psi (695 / 23.55 = 29.5 psi) in each of the slightly oversized tires.

This correlates to my experience that the steering is too twitchy and rear-end is too bouncy when running the door jam 33 psi on these 205/50-16 tires. So, I’ll start out at 29.5 psi.

The issue with 29.5 psi is that the TPMS sensors don’t care and it appears that the dash alert comes on at maybe 26 psi in any tire. So, yesterday, it was in the high 40’s deg F and this morning it is almost 20 deg F, which means we almost had a 30 deg temperature swing. If tires lose 1 psi per 10 deg F of cooling, then my tires lost approx. 3 psi. So you can see that I’m close to setting off the TPMS alert depending under what weather conditions I set tire pressure and what it is today. Obviously, I don’t have TPMS alerts in the summer but only going into the winter. Usually after the first alert, I have the tires “topped up” to 29.5 psi again, I make it through the rest of the winter unless we get another drop down to zero (lose 3 psi),

Please reply if you think there is an error in math / logic. Just trying to advance the subject but this can't be the first time and I can't be the only one.

 

Whoa. I may have made a mistake. I thought I looked up both size tires and they were only 1 pound different in load capacity at 51 PSI. I'll have to go back and look them up again. I'll do it again and get back to you.

 

OK here is what I looked up again at tirerack.com

2011 FIt Sport
tire size 185/55R16
General Altimax R43
Max Load 1202 - Max inflation 51 PSI

Then I looked up the size you said you had:

tire size 205/50R16
General Altimax R43
Max Load 1201 - Max pressure 51 PSI

The numbers came out the same.

Then I looked up the Dunlop and it said 1074 at 51 PSI which agrees with what you said.

Some of the other brands listed said:

Firestone 1074 at 44 PSI
Hankook 1074 at 51 PSI
Kumho 1074 at 51 PSI
Sumitomo 1074 at 51 PSI
General 1202 at 51 PSI
Michelin 1074 at 44 PSI
Yokohama 1074 at 51 PSI
Then I looked at snow tires at the same size:
Bridgestone 1201 at 50 PSI
Bridgestone 1201 at 50 PSI
Dunlop 1074 at 44 PSI
Michelin 1201 at 50 PSI
Yokohama 1074 at 51 PSI

and also a touring tire:

Bridgestone 174 at 44 PSI

There are enough discrepancies in the list at this point that I'm not sure what to say. Usually the Max weight vs. Max pressure are the same for all manufacturers of tires. The ability to carry the weight is based on the load range, volume of air contained and Max PSI allowed. The numbers above just do not support this across all of the tires listed. So I'm confused at this point.

I still suggest that you go by the door sticker unless you can find a better source of information. I did some Internet searching for a tire inflation chart for those two size tires but did not come up with it. As for your calculations, I agree with your methods, but until we can agree on the correct input I'm not sure what to say about the results.

 

n9cv: No worry and I agree.

I went back to "ground zero" and looked at the specs for the Dunlop SP 7000 that was stock on so many FITs (mine). So, my calc of 29.5 - 30 psi seems to be very close after doing 100 miles of local driving yesterday. 29 psi might be better but then again, I'm getting closer to the psi that sets off the TPMS alert light on the dash when temperatures drop.

There are several other things we didn't discuss about ride quality and that is each tire brand / model is different, as seen in your list. That sort of means that my methodology is not foolproof although it seems to have worked for me this time.

So, if you can't rely on my method 100% of the time (individual model tire's max load ratings aren't consistent for same size tire), how do car manufactures do it to get it at least "close"? That math process would be interesting to know.

BTW, I scolded the Tire Rack customer service rep when he said to follow the pressures listed on the door jam for oversized tires. Kind of interesting that Tire Rack doesn't / unable to provide guidance when they offer oversize options (wheel / tire combos) on their website. You would think that they would have the auto industry / auto engineering connections to offer better advice.

Again, I'm going with 29 psi. The Continental DWS tires I had on before were real squirmy at 29 due to soft sidewalls. These General R43 tires seem to be a good match for me / FIT / way I drive them (~20K miles so far), in various weather / road conditions in Pennsylvania. If they are noisy, I can't hear that over my Tanabe exhaust and no inside cover (no sound insulation) on the back hatch.

 

I wish I had a set of OEM of those junk Dunlops around to read what they say on the sidewall about MAX pressure and load. I'm thinking that we are looking at a documentation (web site listing) problem. I have never seen this type of discrepancy before. I tried finding inflation charts on line from several different vendors of this size of tire. I was not successful. I can find them for larger tires like RVs and PU trucks but not for these.

Good luck with trying 29 PSI. I still think that I would go with the higher door panel figure. Please let us know how it works for you.

My daughter had those 205's on her Fit that she just totaled. She always used the door panel inflation figure. They wore well (about 60K) and we did not notice any unusual tire wear. She was talking about going back to the Yokohama Avid Asced OEM 185 size tires like she had on her 09 Fit. Then she hit a deer at 55 mph before she replaced them. She is now driving a 2016 EX.

 

Staying with 29 psi for now and will update after trying the "chalk method" in better weather or finding any new data.

I think lower psi on a larger volume tire carrying the same vehicle weight is the right direction I should be going. Just thinking about bicycle tires, you need ~90 to 100+ psi for a skinny road bicycle tire, ~30 psi for a mountain bike and ~15 psi for a "fat tire" bike to carry the same person.