Definitely not recommended. First you have to check the outside diameter for clearance and so the speedometer is correct. That's the easy answer. What few people are aware of is the change in "scrub radius". That is a designed-in angle that is critical for proper handling. Imagine a line drawn through the upper strut mount and the lower ball joint. That's the two steering pivots for the wheel. If you continue that line until it hits the road, it will intersect the exact center of the tire tread. Due to its rolling resistance, the outer half of the tire will try to turn out when driving and the inner half will counteract that by trying to turn in. The tire will respond by staying straight when you hit bumps in the road. That was fine for heavy rear-wheel-drive cars but most front-wheel-drive cars have a "split-diagonal" brake system with one front brake and the opposite rear brake on the same hydraulic circuit. If there is a leak, you will always still have one front brake working. To prevent the steering wheel from pulling out of your hands when that happens the engineers have modified scrub radius to put more of the tire tread inside that scrub radius line. That makes more of it want to pull towards the center of the car to fight the brake pull. In most cars that means the steering wheel will stay straight without you having to hold it when half of the brake system fails. Only GM hasn't been successful designing that into their cars. Wider tires, depending on the offset of the new wheels, changes scrub radius and will make you feel the bumps that only one tire hits and you could be surprised with a sudden unexpected brake pull if a leak develops. Insurance adjusters love to find these kinds of modifications as they are reasons to assign blame for a crash and deny claims. The only safe modification from a liability standpoint is if you can find wheels and tires that were available on the car from the manufacturer.
March, 16, 2011 AT 4:06 AM
Thank you for the reply. So it is essential to keep the outer diameter of the tyres as it is.
Just 1 more, what if I replace my standard tyres with a 17" rims with 215/45/R17 tyres? Will this keep the original diameter as recommended?
March, 16, 2011 AT 4:24 AM
You'll have to ask that at a tire and alignment shop. A friend of mine used to own a tire store and he had all of those different sizes memorized. If you keep the same outside diameter of the tire tread and the same width, changing the wheel diameter won't affect the handling related to the suspension and braking systems but there will be less flexing of the sidewalls over bumps so you'll feel it more. I should mention too that if your car has anti-lock brakes, it is critical that all four tires have the same outside diameter. If any wheels rotate at different speeds the anti-lock brake computer will pick that up and think a wheel is skidding when you apply the brakes.