Nick86

iTrader: (1)

I have done quite a bit of searching around here and have come up with every different answer I could imagine - some plausable, some outright stupid.

My car ('86 GXL) came with stock 205/60/15 tires. On the Door Jamb it is written that the reccommended tire pressure for this car (with those tire sizes) is 32 psi. (IIRC).

Recently I have upgraded wheels and tires:
I am running 17x7 and 17x8 with 215/45/17 all around. So:
WHAT TIRE PRESSURE SHOULD I BE RUNNING?

I've heard everything from:
"Keep it at 32 - size has nothing to do with pressure."
To:
"Put it at about 40 psi."
To:
"Whatever the MAX pressure on the side of the tire is."

What do you guys think?

Thanks.

rynberg

You follow the 32 psi recommendation (or whatever you were running before if you like the tires slightly softer or harder).

As long as the volume of air in the tire can still support the weight loads (which those tire sizes can), you are good to go.

DamonB

It can be a difficult question.

What supports the weight of the car is literally the molecules of air in the tire. Because of this as the volume of air the tire can hold goes up, the required air pressure to support the vehicle weight goes down. Ideally you figure out the exact volume of air inside the stock tire size and that with the inflation pressure would allow you to compute how many molecules of air are inside the tire. Now no matter what size tire you use you merely fill it with the same number of molecules of air. Unfortunately we don't count the number of air molecules when we inflate tires, we measure pressure which is completely different. If you put bicycle tires on your car the pressure would need to be really high, if you put 15" wide slicks on the pressure would be really low.

There are some things we can say with complete certainty. You DO NOT inflate the tire to the pressure on the side of the tire. This pressure is merely the maximum pressure that the tire can safely run and it will be much too high in nearly every situation.

To entertain myself I estimated the volume of air inside your stock 205/60/15 (15" wheels) tires and that inside the 215/45/17 (17" wheels) and found that the 17" tires hold only about 85% of the air volume the 15" tires did. With that in mind then 32psi + 15%= 37 psi. This is merely an estimate. Size has everything to do with pressure. It's not strictly the width of the tire though, it's actually the internal volume of air the wheel and tire assembly hold.

Your new tires are wider so you would expect the air volume to go up, but since they are also on wheels which are much larger in diameter your new setup in fact has less internal volume than your original. That means the required tire pressure will be higher. Start at 37psi and see what happens.

rynberg

What's your math, Damon, I get that 215/45 17 is 93% of volume of 205/60 15. That equals increase pressure to 34 psi.

If he did a proper +2 sizing, that would be 225/45 17, which is 2% more volume than 205/60 15 or 31.X psi required.

Nick86

iTrader: (1)

Thanks for the insight!
It's nice to know that someone thinks the same way I do! lol

My First thought was to do a simple calculation of the Tire's cross section. Of course this would be a very general rough estimate - but a guide nonetheless. A 205/60 tire would have a 205 tread width, and a 123 sidewall height. While a 215/45 would have a 215 tread width and a 96.75 sidewall height. The area off this cross section for the 205 would be bigger: 25215 mm sq. VS the 20801.25 mm sq for the 215. That rough calculation showed that the 215's were 82.5% smaller.

At that point I was reluctant to just put the pressures up 17.5% as it seemed a bit too easy - but that said, it made sense lol!

I think that 36-38 PSI seems like a safe way to start.

Nick86

iTrader: (1)

Ok, I did a more precise calculation:

Here are the general measurements I used: (all in mm) Of course this all assumes that the tire tread width is the same as the wheel width - and we all know this isn't true, but these are rough calculations:

205/60/15

Tread width: 205
Sidewall height: 123
Radius of entire wheel and tire: 313.5
Radius of just wheel: 190.5

Volume of Tire in mm cubed:
39924490.4

215/45/17

Tread width: 215
Sidewall height: 96.75
Radius of entire wheel and tire: 312.75
Radius of just wheel: 216

Volume of Tire in mm cubed:
34553312.4

That works out that the 17" wheel has 86.55% of the volume the 15" tire.
Again, this is by no means an exact calculation - more of a rough guess. If you translated that into PSI increase, we are still looking at about 37 PSI. I'll give it a shot.

DamonB

I compute the diameter of the tire and divide by 2 to get the radius. I then compute the volume of that. From that volume I subtract the volume of the wheel and the result I call the volume of air the wheel and tire assembly holds.


205/60/15

area of a circle= pi(r)^2

volume= pi(r)^2(width)

Diameter of tire= (section width)(aspect ratio times 2) + wheel diameter= 62.7 cm

15x7 wheel volume= 20269 cubic cm

volume of tire/wheel assembly= pi(31.35)^2(20.5)= 63296 cubic cm - 20269= 43027 cubic cm

205/60/15 on 15x7 wheel holds 43027 cubic cm


215/45/17

Diameter of tire= 62.53 cm

17x8 wheel volume= 29756 cubic cm

volume of tire/wheel assembly= pi(31.265)^2(21.5)= 66024 cubic cm - 29756= 36268 cubic cm

215/45/17 on 17x8 wheel holds 36268 cubic cm



The 215/45/17 holds 15% less air than the 205/60/15 so it's pressure is increased by 15%. 32psi + 15%= 37psi

DamonB

Just realize our calculations are still an estimate since we are in fact estimating wheel volume by figuring it's a normal cylinder, which it is not. Undoubtedly it's safe to say that you're going to need 5 psi or so higher than what stock was however.

rynberg

Ok, I must have made a slip on the ol' calculator to get 93%....I didn't realize such a small difference in tire volume would require such seemingly large changes in psi required.

Yet another point for properly plus sizing, the air volume stays approximately the same.

DamonB

I dunno, it's Friday where I'm at too. Wouldn't hurt for someone to check our numbers

What makes it so different in this case is the fact that wheel diameter was changed at the same time. If you keep the same wheel size and go to a wider tire then profile of the wider tire goes down and the volumes of the narrow and wide tire are more equivalent. Once you drastically change wheel diameter as well you can bet pressures are going to change.

Nick86

iTrader: (1)

A proper +2 of 225/45/17 would be about 5% less volume, so an increase of only a couple psi. At the time, 215/45 became available at a great price so I couldn't turn them down. The overall height and circumfrence of the tire vs a 225/45 is comthing like 1.02%, so it's nominal. Certainly not worth worrying about!

POM HB

Great info!

Tyler, so what psi are you using on your set up now? I have the same-size tires.

POM HB

rynberg

I've been running 31-32 psi in my 235/40 and 265/35 18 tires.

Doing the calculations with MY math (directly calculating the volume of the tire itself), I get that the 235/40 is 98% of stock and should run 32.5 psi and that with the 265/35, has 9.9% more than stock and should run 29 psi.

No wonder Howard Coleman is always saying to run 30 front/27 rear. With a staggered setup like mine, there IS a 3 psi differential. Damn, he might just know something!

Great thread, as a result, I'm going to try running my tires at 32/29 and see how it feels/wears.

Herblenny

iTrader: (13)

Rynberg,

I would like to know how it turns out.