i currently have a 98 1.8TQ. i bought the car with 17 rims on them and stock came with 15. do i still use the tire pressure on the gas door or do i add more air. i currently have 38psi on all 4 and do long cummutes. i know that larger rims can kill my mpg does it also make a diffrence on acceleration, speed reading, and stoping distince.

 

38 seems a little high to me - but it depends on the tire as well. I have 33psi in my 18"s.

Going from 15" to 17" tires does not change your speed reading unless the circumference of the tire/rim has changed - which is unlikely (although possible if they did not do their homework when the previous owner bought them).

Having too much air in your tires will cause them to wear more in the center of the tire.

 

the ratings on the gas lid are good for stock tires yes, but if you run different tires, the rims nothing to do with tire pressure. the pressure ratings should be labeled on the tire itself, and that shoukd be the maximum. unless your running some really weird tires, max pressure shouldnt be over 40 psi. a general rule of thumb with car tires is 35 psi. as for running different tires versus stock, check this out. it will give you just how far off you are. http://ctny.audiworld.com/mark/a4/calculator/

 

thinner profiled tires need more tire pressure. i had 235/35 19's and ran about 40psi. depending on what your tire size it 38 should be fine.

 

I run 51 in my 18's lol. Gotta read the side wall and it will tell you the max and I try to run about 5-10psi under the max on customer car's. On mine I run the max though.

 

well my tire size is p245/45ZR17 95W is this larger then the factory tire size. there is only a max pressure on the side wall which is 50psi and im pretty sure that it is way to much

 

So when the tire heats up, and you are over the max pressure, then what...?

Running the maximum pressure stamped on the side-wall yields a terrible ride. There is no reason to run the tires that high on your A4, even with 18"s. Experiment until it feels right. 38psiF/36psiR usually seems to work well with 17"s or 18"s.

 

Those tires are way too huge for your B5 and should not be used. You'll likely rub, even at stock height. Keep a close eye on those inner side-walls where they will likely be touching the struts.

 

would running this larger tire kill my gas millage.

 

That's the least of your worries, but it will certainly be heavier, so it's a possibility.