even though your tires are of different sizes like 315/30 and 275/35 or whatever its ultimately comes down to the overall tire diameter that matters. with a 315 tire its wider but the sidewall height is shorter due to a lower profile(30) than the 35 series of the 275. as long as the total height is the same or very close to eachother between front and back your computer could care less and honestly doesnt know you changed anything. Stick with the recommended sizes and you will have smooth sailing. for the fun of it you could even run a stock 17 inch front tire and a 22 rear with a 305/30 or something similar and your baby wouldnt even notice