Over-inflating... how much is to much? - Fuelly Forums

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Old 05-02-2008, 03:49 AM   #1
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Over-inflating... how much is to much?

I have a set of Firestone Destination A/T 33X12.5RR15's on my Jeep.

I filled them to 37PSI last night. I have seen discussion on the forums of people who say you can overinflate... what would be a safe number for these tires?

P.S. I know they ruin my fuel economy but replacing them would cost at least $400. The cost would not equate into more savings at the pump.
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Old 05-02-2008, 04:25 AM   #2
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Well, it looks like your tires are the lowest of the bunch. Legally speaking, no one that sells or supports them will tell you anything higher than 35. I personally have run tires at 50+ w/o issue on tires that were marked for 35 though...

Everyone will tell you something different though. What it comes down to is what you feel safe running, and what kinds of compromises you are willing to make.

I enjoy overinflated tires, and plan to run 50+ into the foreseeable future.

Why not try bumping your pressure up by 2-3psi every day and see where that takes you?
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Old 05-02-2008, 05:34 AM   #3
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Agreed. I just drove 15 miles to work and the difference is remarkable. Driving I-210 bridge in 6th gear required little throttle and coasting @ 50 MPH was a pedal tapping affair! The tire pressure last night ranged from 25 PSI to 30 PSI.

This morning, after the 37PSI fill up last night, only one tire was 35 PSI. Any clues why?

These tires are to big... can anyone recommend a better tire that will fit a 15x10 rim and have a ride height of approx. 30"?

Side note: I had to refill my 5 gal air tank to 100PSI twice to fill these huge tires.
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Old 05-02-2008, 06:31 AM   #4
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Originally Posted by rgathright View Post
{snip} The tire pressure last night ranged from 25 PSI to 30 PSI.

This morning, after the 37PSI fill up last night, only one tire was 35 PSI. Any clues why?
Well ruling out any small leaks that may have shown up at the higher pressures (old repairs, unseen nails, leaky bead), my first guess would be that the tire is 'settling/stretching' slightly as it adjusts to the new pressure. You've made an approximate 35% pressure increase above where they were before. It should settle down in a couple days.
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Old 05-02-2008, 08:23 AM   #5
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That's what I was thinking too... honestly!

So you're saying my Jeep is just getting fatter huh?

What kind of aerodynamic drag will this produce?
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Old 05-02-2008, 08:57 AM   #6
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33x12.5RR15 .... can anyone recommend a better tire that will fit a 15x10 rim and have a ride height of approx. 30"?
30x12.5RR15?

Luckily this tire size designation format has the diameter and width indicated directly. The "P-metric" format requires some math skills to determine the overall diameter and width. A 315-70R15 is a very close approximation to what you presently have in your 33x12.5R15.
A 315-60R15 will maintain the width (about 315 mm) and the rim diameter (15 in), but will make the side wall only 60% of that 315, rather than 70%, getting you close to 29.9 inch height.
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Old 05-02-2008, 08:59 AM   #7
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I never inflate beyond the tire's rated maximum pressure as stamped on the tire. Stuff like that can potentially bite you in the buttocks (silly forum censorship) if there was an accident and the insurance company wanted to find a way to blame you for being negligent.

I do inflate beyond the vehicle's recommendation, though. My pickup has load range E tires that are rated for up to 80psi. I have been running that pressure for at least 4 years and 120,000 miles.

If you're already shopping for tires, be sure to get tires that can accept higher pressure.
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Old 05-02-2008, 09:03 AM   #8
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What kind of aerodynamic drag will this produce?
More air drag.
The over-inflated and thus taller, wider tire will have more frontal area and will raise the vehicle allowing more air underneath. These will increase the air resistance.
Now ask if the rolling resistance is reduced more than enough to offset the almost imperceptible increase in air drag. Yes, most definitely.
Will you be able to tolerate the more staccatto ride characteristics? That's your call.
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Old 05-02-2008, 09:27 AM   #9
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The microscopic frontal area difference caused by the miniscule height difference after overinflating is probably compensated for by the change in the tire's shape, which will bulge at the center of the tread when overinflated vs. being flat and square at lower pressure.
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Old 05-02-2008, 09:36 AM   #10
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I would be watching the side walls of that tire that lost pressure over night. Spin it in the air and make sure that you do NOT have a buldge developing in the sidewall somewhere and that it is still round.
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