Tyre pressures

Sat 04 Oct 08 13:58

Tyre pressures

Anyone know if there is a formula or some kind of calculation for working out what the air pressures should be on non-standard size wheels/tyres? Or would pressures remain the same as standard due to the cars body weight obviously not changing?

I have 275/40-18 rear and 245/40-18 front and don’t much fancy the idea of waiting to see if it’s the middle or the outside of the treads that wear out first just to establish over/under inflation!

Any ideas?

Thanks.

Sat 04 Oct 08 14:28

Hi Josh, they are similar size tyres to my c5 i run 30 on front & 28 rear because i was finding the rears were wearing in the middle to much hope that gives you some help MICK .

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Hi,im MICK when i was 21 i owned a 1976 t top red white leather owned it 4 years at about 12,000 miles a year had some great fun.Move on 28 years and wife, house,kids,now i have a toy again a red c5 2002 reg C5 YNK,love it RED interior 40,000 miles ,hoping to have just as much fun again on 7,500 miles policy

Sat 04 Oct 08 18:33

c5ynk wrote (View Post):Hi Josh, they are similar size tyres to my c5 i run 30 on front & 28 rear because i was finding the rears were wearing in the middle to much hope that gives you some help MICK .

Hello Mick and many thanks for that info.

I'm currently running 30f/30r (as it was when I bought it). It's difficult to judge where the wear is until they are well gone, but I think I'm getting more 'edge' wear (both edges even, so alignment is ok) than middle. Think I better find a calibrated pressure gauge or compare mine to a known good source.

Cheers.

Sat 04 Oct 08 18:53

Josh wrote (View Post):c5ynk wrote (View Post):Hi Josh, they are similar size tyres to my c5 i run 30 on front & 28 rear because i was finding the rears were wearing in the middle to much hope that gives you some help MICK .

Hello Mick and many thanks for that info.

I'm currently running 30f/30r (as it was when I bought it). It's difficult to judge where the wear is until they are well gone, but I think I'm getting more 'edge' wear (both edges even, so alignment is ok) than middle. Think I better find a calibrated pressure gauge or compare mine to a known good source.

Cheers.

When I bought my 18"s for my C3 from North Hants Tyres I asked this very question and was told 2 psi per extra 1" of diameter - in my case now 31 psi. This seems fine but as was said I'll only really know about eveness of tyre wear when they are fully worn out!

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'68 coupe - 4 speed - 3.08 - F41 - 489 cu in - 528 bhp @ 5850 rpm - 565 ft/lbs torque (766 Newton Metres) @ 3850 rpm - (225 bhp & 465 ft/lb Torque @ 2500 rpm)

Act like a juvenile before turning senile !

Sat 04 Oct 08 19:04

roscobbc wrote (View Post):

When I bought my 18"s for my C3 from North Hants Tyres I asked this very question and was told 2 psi per extra 1" of diameter - in my case now 31 psi. This seems fine but as was said I'll only really know about eveness of tyre wear when they are fully worn out!

Thanks Roscobbc,

Gotta say it confuses me. 18's have possibly less volume than 17's because of lower profile (dunno). Then there is increased width which increases the volume. I'm stumped at what's best, but the trial and error is bloomin expensive way of finding out!

Cheers.

Sun 05 Oct 08 02:26

Re: Tyre pressures

I found that my old Goodrich 235/60/15 on 8" rims would scream like a 'stuck pig' going round corners at anything approaching a reasonable speed. I tried 35 psi, then 32, 30 which stopped the squeeling but one could sense that the shockers were not dealing with it and the tyres would skip and patter on poor surfaces. 28 psi seemed a happy medium. It seems as though a number current cars with low profile tyres have higher manufacturer reccomended pressures.

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'68 coupe - 4 speed - 3.08 - F41 - 489 cu in - 528 bhp @ 5850 rpm - 565 ft/lbs torque (766 Newton Metres) @ 3850 rpm - (225 bhp & 465 ft/lb Torque @ 2500 rpm)

Act like a juvenile before turning senile !

Sun 05 Oct 08 12:25

Thanks again roscobbc.

Yes a good point that perfomance needs considering as well as wear. I'll have to experiment and hope the two are compatible!.

Many thanks

Mon 06 Oct 08 07:46

Re: Tyre pressures

I usually run my 18" at 28 all round, not sure why peeps are using a higher pressure in the front since weight distribution (with a small block) is almost 50/50 front/rear

G

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1970 Small Block Coupe, 450hp Motor, March Pulleys, twin spall fans on aluminium radiator, Steeroids, Tremec T56 6 speed c/r box, Smart Struts, Sports Rotors, Hydroboost, Semi Coil over front suspension and alloy upper control arms, 18" Alloy Replica Rallye Wheels 8" front 9 1/2" rear with Pirelli Pzero's. www.graemeinglis.com

Mon 06 Oct 08 19:22

I think it's an experimental job. You are supposed to be able to do it with a thermo gun or tyre pyrometer (thermometer basically) and aim to get an even spread across the whole tyre, however I think it's a suck it and see (try different pressures and see how you get on). It will vary by tyre construction as to what's the optimum, as well as tyre and wheel size.

Wear pattern is also a good one, but I can see that in some cases this may only achieve even tyre wear and not necessarily any other benefit.

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1995 Corvette LT1 6-Speed Coupe
1990 Japanese thingy (MX5)
2007 Honda UFO

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1986 Corvette Z51 Coupe

Mon 06 Oct 08 21:25

FranV8 wrote (View Post):I think it's an experimental job.

I fear your right, only hoping it doesn't take too many sets of tyres and the demise of my bank balance.

FranV8 wrote (View Post): You are supposed to be able to do it with a thermo gun or tyre pyrometer (thermometer basically) and aim to get an even spread across the whole tyre,

That's intereting... I've never heard of that method. I wonder if it makes any difference because my tyres have been inflated with a Nitrogen! mix... from what I'm told it's supposed to be less sensitive to pressure variations due to heat. More likely just the tyre fitters way of charging me 12 euro's to pump my tyres ... I dunno!