Often times customers will have worn two of their tires out, but the other two are in fine shape. The customer will purchase two tires. The question now, where should those two best tires go? Contrary to what seems to be common sense, the two best tires should go to the rear of the vehicle. This logic holds true whether the vehicle is front, rear, or all wheel drive.
The reason being, the best tires will grip more, and if the best tires are in the rear, over-steer will result in wet, snowy or icy conditions. This same logic applies to rotating tires. Experts claim, if there is a 25% difference in tread depth, the best tires must go to the rear. It seems like a scam to make customers buy new tires for the front wheels as well, but I assure you the proof is in the pudding.
The lesson to be learned? Rotate your tires regularly to keep them even.