Tire Size question.
 

right now im looking for tires... The RSA's have to go


Could I get away with running on the stock wheel a 245/45/18 on the fron and a slightly larger 245/40/18 on the rear ?

I'm looking at http://www.marangonicanada.com/products_main.htm

40's are SMALLER, not larger.

Moving to General Chat.

you should look for 255-40-18 in the back to get about the same height or 265 if they fit

245-45-18 are 26.7in
245-40-18are25.7in
255-40-18are 26.in
265-40-18are26.3

P245/40R18

P = Passenger tire
245 = is the width of the tire/tread
40 = is called aspect ratio meaning the hieght of the tire is 40 % of the width
R = Radial
18 = rim size

first off. Why? and second you may get a sensor tire presure reading in your driver info center.

... It's a FWD car?

As long as the pressure is the same between the 2 tires on either side it shouldnt throw it.

Stock size is 235/50/18 a 245/45/18 would be a larger tire no ? but not wider ok i was a little duh there.

they have 255/50/18 but im not to sure about them
http://www.marangonicanada.com/products_maxo.htm

then again for 180.00 per the BFGoodrich g-Force T/A KDW 2 look nice.

http://www.miata.net/garage/tirecalc.html

I run 245/45/18s on my GXPs


255/45/18 would be closer but still smaller outer diameter than the stock size.

I'm not sure if you understand tire sizing.

The first number (say the "245" in 245/45/18) is the width of the tire in millimeters. The second number (the "45") is the aspect ratio, or sidewall height. It is expressed as a percentage. For this tire, the sidewall height is 45% of 245mm, or 110.25mm. The third number, obviously, is the rim size in inches.

When changing sizes, you need to keep the overall rolling diameter the same. So if you've got a 235/50/18 now, you've got a rolling diameter of 27.25":

(235 x 50% = 117.5mm) x 2 sidewalls = 235mm + (18" x 25.4 mm/inch = 457.2mm) = 692.2mm or 69.22cm or 27.25"

You want your original new tire diameter to be within one cm of this to ensure your speedometer is accurate (unless you reprogram) and your front + rear diameters to be within 1 cm of eachother so you don't throw an ABS/low tire light. Your car likely doesn't have pressure sensor valves, it just works off differences in wheel rolling speed and the assumption that a low tire has a smaller rolling diameter and therefore rolls faster.

The 245/45/18 tires have a rolling diameter of 26.68":

(245 x 45% = 110.25mm) x 2 = 220.50 mm + 457.2mm = 677.7mm or 67.7mm or 26.68"

As you can see, it's almost an inch smaller in diameter. Your speedo will be out slightly. A 255/45/18 would be a better choice at 686.7mm or 68.7cm if you're set on wider tires. They are almost exactly the same in diameter to your stockers and should be cheaper too buy and easier to find, too, as they are the stock front size on the Grand Prix GXP. The Potenza RE050As that come on that car (buy the GM spec tire, not the generic RE050A, they are specially wrapped to reduce torque steer and handle better) are amazing in my experience.