I would like to put new wheels on my GTP but keep the factory ratio or rev/mile my question is what have you ( anybody who can help ) been putting on your Pontiacs. When I went to http://www.miata.net/garage/tirecalc.html
to compare the ratios of the overall sizes the two tires that are almost perfect are 245/50-17 and 245/45-18 but from grandprixstore.com I heard that those are not that common so 255/50-17 and 255/45-18 should be what I should look at.
Has anyone had those or larger sizes rub agains the wheel wells. What are someof the sizes YOU have.
Thanks

Hello,
I was looking into getting the "exact match" for the circumference of our tires in a bigger rim size.
When I went to a Firestone dealer, he said, "those sizes don't exist". So, I'm assuming they only make aftermarket tires in sizes that are the same as factory sizes. And, with that being said, there must not be any cars that come w/ the numbers that you are looking for (245/50-17 and 245/45-18). I don't know about the 18" tire, but I'm sure I was looking for the same size 17". Therefore.....U must change the size a fraction, and I'm sure someone will chime in, and tell you what the affects will be.
Later now,

I bought rims and tires from the tire rack. They have 17 tires in the 245/45 18 size. Look here http://www.tirerack.com/tires/Compare1.jsp?startIndex=0&vehicleSearch=true&width=245%2F&ratio=45&diameter=18&sortCode=54050&minSpeedRating=H&ratio=45&width=245%2F&diameter=18&searc h=true&pagelen=20&pagenum=1&pagemark=1&do=Search&speed_rating=HR&speed_rating=VR&speed_rating=WR&speed_rating=YR&speed_rating=ZR

The 17 inch tires are the hard ones to find. Good luck

Buddy of mine found 245/45-17 but I was wondering if your 245/45-18 ( which differentiation from the factory settings is only -0.2% ) gave you any problems with rubbing when you corner tight. Also what wheel did you put them on 18X8 ?

Sorry to take so long to respond. I bought a wheel\tire package from tirerack. The wheels are ASA JS5's and they are 18/7.5. http://www.tirerack.com/wheels/DisplayWheel.jsp?wheelMake=ASA&wheelModel=JS5&wheelFinish=Silver+w%2FMachined+Lip I have had these wheels and tires for about 6 months now and have had no problems what so ever. 18's are a heavy set for the car and will degrade acceleration and braking some. Even though these are 18 inch wheels with 245/45 the tire does not look like an aggresive low profile tire. Some people like the looks of big wheels and lower profile tires but my combo does not look very low profile. Some people can't believe I have 18 inch wheels on the car until they read the tires. I do not know what the maximum tire you can put on the car with out rubbing. I wanted a tire that would be as close to factory settings as possible and the 245/45 18 are an exact match.

Thanks rrfleury , thats exactly what I needed. How badly is the weight differentiation between factory and your new 18" ers? What is the braking and acceleration difference like, is it fairly noticeable?

I do not know the exact weight difference. When I changed my wheels and tires I picked up a stock set and then picked up the new set. There is quite a difference in weight. As far as braking, the car braked well to begin with so there is not much difference in braking but you are asking you brakes to slow down a larger wheel mass so they are working harder. Again the larger wheel mass does slow down acceleration because now the engine must spin the larger wheel mass. Torque steer is still evident but with larger tires and heavier wheels it is easier to handle on heavy acceleration. I also do not know what the difference in 0-60 times are with the bigger tires but I do know the larger tires are slower and I can feel the difference in take off. For normal everyday driving I love the bigger wheels and tires. If I thought about racing or dragging I would definetly find a different combo than I have now. If weight is a concern you can find 18" wheels that weight less than the stock 16" wheels but that is going to cost extra money. This is all food for thought. If there is anything else I can help you with I will gladly help. -Ron

Uh. Max size rim.. i know a guy wiht a GTP that has 20's....to big for my taste

20's !!! that would be too much but thanks for the incite Ron, definitly a lot of help for me when I buy my combo. On a unrelated topic, have you ever wrapped or otherwise insulated your hoses, especially the upper rad hose, I have my headers and X-over pipe wrapped and a heat shield for overkill and have reflective tape as well as well as wrap left over and think of wrapping the hose but heard that it might be damaging as the hose wil retain lots of heat but then again I saw the new G-Force with a steel braided hoses that were excellently insulated. Any thoughts.
adam