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

I have a set of Firestone Destination A/T 33X12.5RR15's on my Jeep.
https://www.tirerack.com/images/tires..._owl_ci2_l.jpg
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.

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?

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.

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.

That's what I was thinking too... honestly!

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

What kind of aerodynamic drag will this produce? :rolleyes:

30x12.5RR15?:rolleyes:

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.

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.

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.

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.

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.

Well, this subject comes up from time to time and it is really something only you can decide. Personally I run just over 50 psi in tires whose side walls say 35 psi and haven't seen any strange side effects, unless you call saving gas a strange side effect :)

I guess I'll just put some cash aside and fill my tires.

You guys are pretty daring, call me what you will but I think if you're doing highway speeds anything over 25% above rated is just crazy. You have the added pressure of the air plus centrifugal forces working to rip that tire to shreds, I wouldn't be worried about the tread area since that's steel belted, it's the nylon reinforced side walls that would worry me.

Don't be doing that on the cheap tires or tires that show any signs of aging. That's just me tho I don't run anything over the side wall rating. Safety first, you lose a tire and cause an accident the cars sitting on the freeway backed up will pollute way more than any reduction you get from increased pressure, plus your cost of replacing a car or your insurance rates getting raised because you were at fault for 3 other cars crashing. (worst case of course)

I am @ 40 psi on all my 35psi tires, but I had them at 32psi for the snow. Did try 50 psi once, lost 60% of the traction, thing skittered all over the road, 40psi seems just right, seem to keep the traction, get very crisp, predictable and responsive handling.

A year or two ago, someone posted a link to an article about a police driving school where police officers learn the extreme driving techniques they sometimes need to use in their jobs. Things like high speed manuevers, riding on 2 wheels, etc. The cars in that school keep their tires inflated at 100psi.

That's not the sort of think that's likely to happen. Tires (well, common modern automotive pneumatic tubeless radials) are very overbuilt against that sort of thing, because there's other things that are much harder on them. Mainly, when you hit a bump hard with the tire inflated far past its maximum rating, you have an increased risk of the bead breaking loose. Since highways tend to be pretty smooth, this is a city driving issue.

These tires don't pop like a bicycle tire, and they don't fly apart unless they're underinflated and overheated. It's funny, this is the ONLY forum where I've ever had to take this side of the tire pressure argument. There was a "Poor man's mods" thread on golfmkv.com where I tried to explain the advantages of inflating past the car's recommendation but not beyond the tire's maximum, and the FUD was just awful, so I finally gave up.

That's fine for controlled conditions and limited time/mileage. If they were operating IRL at 100psi on tires rated for a maximum 50, they'd blow them on potholes and bumps. Riding on 2 wheels would necessitate high pressure, considering the extra weight and the sidewall forces. J-turns would need high pressure to reduce the contact patch.

Yeah I'd think the 100psi in the training cars would be so that you can practice breaking traction at much lower speeds than needed in real life.

I seem to remember reading a Tom McCahill article in Mechanix Illustrated back before most people on here were even alive (and it was a old magazine even then) suggesting around 45-50 psi in the old nylon cord (no steel belt) bias ply wide-whitewall tires for best mileage and longest tire wear.

Back 35 or so years ago, I drove a Corvair. The sticker specified about 15 psi for the front and 26 psi for the rear. I ran 20 psi in the front and 40 in the rear. Usually had junky used tires...never a problem. BTW, the reason for the pressure differential on that car was to introduce a bit of understeer to keep the car from getting away from you in precarious situations. The problems people had with Corvairs often came from not maintaining that pressure differential...thus the first chapter in Nader's "Unsafe at Any Speed"

Thank you! That is a more reputable source saying the things I've been saying but people think I'm crazy.

That guy must have been following me around on the internet, reading all my posts that each say part of that (but in few of which I remembered ALL of those things). :D

https://www.tirerack.com/tires/tirete...e.jsp?techid=1 also says some of that, but also cautions against overinflation (as if there's been many cases of intentional overinflation that caused some consequence!). Even so, here's how the part cautioning against overinflation ends:

Well having done the experiment on my tires, I'd say there is a point where handling gets worse, and I'd think that 100psi in stunt cars is aimed at keeping the tire on the rim when strangely loading it, whether through rolling on 2 wheels or skidding.

I do believe that over inflation has saved my arse on several occasions, the stock inflation would take half a second to respond to steering input, but I've been able to "teleport" the vehicle I've been driving to the next lane to miss dopey drivers trying to merge into me by inches... with the delay in response with the stock inflation, I'd have had contact a couple of times by now I'm sure.

Yup, I agree. On my pickup, I run 80psi all around. The rear, when empty, can get a little skittish, especially in low traction conditions or on rough pavement. I could probably run the rear at 65 without losing any FE or tire life, but the occasional oversteer is fun and exciting for me. :D

WOW! you drove a corvair and survived? ;)
personally I think corvairs rock! :cool:
But not from a FE point of view.

I dunno, if you took a Corvair, and crossed it with a Pinto, downsized the brakes, motor from a trabant, and put a row of spikes on the dashboard... you'd have the perfect economy and safety car... or at least one that would make people really concentrate on driving more slowly and safely.

Or a car for the next Mad Max movie. :)

Just wanted to followup and say that my tires are now at 40PSI and behaving fine. No bulges on the sidewalls. The Jeep really rolls alot easier now.

I think it's very cool that you have a Corvair.

I like a lot of things about Ralph Nader, but I've always resented the fact that he launched his career by slamming the Corvair. After all, it was an American attempt to build a small car, and one that wasn't boring (it wasn't a Nash Rambler). And yes, it was economical.

I wonder if Detroit would have taken more chances in that direction, in the Sixties and Seventies, if not for Nader.

I think that's the best advice for starters. Your headroom to overinflate is greater with a tire rated for more. Likewise, you won't need to exceed the rating as much for a similar result.

I have no concerns about my 51psi rated Sumitomos inflated to 55 psi on the Escort. The cheapo M&S tires rated for 35 psi on the Tercel however give me a little more concern jacked up to only 45 psi.