say i was going to use a 245/40/18 up front and a 275/35/18 in the rear ...this would raise the front up but nothing noticable?



would this mess with the speedo also?


would i be better off sticking with all 35 series tire heights?

 

Those are so close in sidewall height, you'd never notice a thing.

Speedo? Well, what tires are on there now? It'll likely 'mess it up', the question is how much and is it correctable. Rear gear ratio please too. Speedo reads ok with the current setup I assume.

 

atm the car has 215/65/15s which come out to 26 inches tall....the 40 series 18 come out to over 26, almost 27 inches tall


the speedo reads fine atm and i have 3.73s

 

275*.35/25.4=3.79" of sidewall. Those tires with 18's are gonna be shorter than what you have now, 25.6 inches. The 40 series are 26.7 inches.
Unfortunately there isnt much in the way of speedo gears with a 3.73, so the speedo is going to be a little off if its not now. With the 35's its going to read a little faster than what you are actually going, about 1.5%. With the others it will read 2.7% slower.

I cant think of the tire size GM used with the 3.73, but if its anything like the common 245/50r16, the speedo should be off now and right on with the 275/35. In any case, 1.5% isnt much to worry about.

 

You gotta factor in the tire widths as well. The sidewall heights are different, 40 versus 35, but if you consider the widths of the tires too, the overall heights are very close, even though the sidewalls are different. So I wouldnt worry about it, just try different combos in a wheel calculator and ensure overall tire heights are similar.