Im going to buy a set of nitto 555R's and was wondering how big I could go on stock rims? I dont know much about fitting on rims and how big a tire can go on, I know I can only fit 16 inch tires, but how wide?

 

Nitto only makes one size 16" tire in the 555R drag radial, so the only choice you have is 245/50-15.

http://www.nittotire.com/#index%2Etire%2Ent555r%2Esizes

 

no 16 of any kind

 

So the p245/50R/16's will fit my stock rims (rear ones only)
Also while were here, how is haveing different size tires on the front, and back going to very my speedo?

 

245/50-16 is a stock tire size (Z-rated option) on the stock 16x8" wheels. Only the rear tire diameter affects your speedo. As noted, for V8's with the Z-rated tire option, 245/50-16 is the stock size, so it won't affect the speedo. If you had the base tires (235/55-16), the 245's will cause your speedo to read about 2% higher than actual speed. PCM can be programmed for the tire size.