I have a GT with original drivetrain/weight, and I have 17" alloys with 225/45 Michelin Pilot Sport summer tires. Any ideas what tire pressures I should be using for ordinary driving? I have been getting some pretty wide-ranging opinions. I have no idea how to do the math on this one. Any suggestions? Oh yeah, the car also has KYB self-adjusting shocks, Eibach 1" lowering springs, and Addco sway bars fore and aft. Oh yeah, and poly bushings throughout. Pretty much stock car other than those items. Thanks!

Use the tire pressure recommended in your owner's manual. Whatever you do don't go by what is printed on the tire, those are maximum pressures and will cause a very harsh ride or even tire failure. If you set your tire pressure to 32 psi you should be good for daily driving.


http://ask.cars.com/2007/05/how_do_i_find_t.html

[This message has been edited by avengador1 (edited 05-10-2008).]

That's a good rule for stock size tires for sure, but this is a very different situation.

Funny, I do stick to the numbers on the door on my cars with stock size wheels and tires. But try to get a shop to do that! Most small cars with 14 inch wheels take about 29 lbs. But if you let any of those apes at a garage get their hands on it, they will put in no less than 35. Idiots who don't know how to read....

You need to inflate the tires to match the weight you plan to put on them.

If you look on the sidewall of your tires, you'll see the maximum weight the tire can support and the pressure you need to put in the tire in order to support that much weight. Your Michelins probably say something like [Max Load 1365 Lbs @ 51 PSI].

What this means is if you plan to put 1365 pounds of weight on each tire, then you need to put 51 PSI in them. But note that you have four tires, 1365 LBS X 4 tires = 5460 LBS. I dont' think your Fiero weighs that much If you inflate the four tires with enough air to support over 5000 LBS, then they will be way overinflated. This will result in reduced traction and an unnecessarily harsh ride.

What you need to do is 'derate' the tire pressure to match the actual weight of your car plus passengers plus luggage. There should be a sticker on one of the door pillars that tells you the GVRW (Gross Vehicle Weight Rating ) of your car. The GVRW is the weight of the car, plus two people and about 100 Lbs of luggage. It varies by year and model, but an average Fiero should be rated around 3650 Lbs.

Now the math: (I used a calculator so I'm not going to show my work )

3650 LBS is about 67% of 5460 LBS.. To get the proper air pressure needed to support the fully loaded weight of your car, you need to determine what 67% of the 51 PSI maximum is..

67% of 51 PSI works out to about 34 PSI. If I got the figures right about the GVRW and what it says on the side of your tire, then that's about what you need to use. Personally, I'd use around 35 - 36 LBS.. Over inflating slightly will help gas mileage a bit.

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1988 Fiero Coupe 2.5L (Written Off in an accident in 2006)
1990 Trans Am GTA 5.7L

My suspension/wheel setup is similar to yours. However, my tires are slightly thinner than yours (215/45-R17). I've found that 42 PSI front / 46 PSI rear yields even tire wear and good handling. You may need to use slightly higher or lower pressure, depending on your weight distribution and total vehicle weight.

For a Fiero with a stock drivetrain (i.e. no extra weight in the back), you could probably start with 40 PSI front / 44 PSI rear and work from there.

I keep mine at 42 pounds front and rear

Best way to determine the correct pressure is to get one of those crayons they use at tire shops and put some lines across the tread and drive a short distance. You want the crayon mark to wear evenly across the tread. Worn away in the middle and not the edges is too much pressure and visa versa.