Upsizing a tire has several ramifications you should consider. You can widen your tires for handling, or narrow them for ride and longevity, but they must remain the same height. Height effects your vehicle's drivability. The computer comes programmed for a certain number of engine revolutions per minute (RPM) at a set speed. If you change the height of your tires, the RPM and speed change because you have, in effect, changed the gear ratio. The transmission shift points will change as well as the speedometer.
Look in the owner's manual if you have access to one, and open it to the section on tires. If will tell you the optional size tires the vehicle will accept with no adverse effects. If the owner's manual is not available, all tire shops have a fitment book that gives all optional tire sizes. These sizes give the appropriate clearance so they do not rub.
Examine your car and needs to determine the results you seek in changing your tire size. The wider the tire, the stronger the sidewall, and the better the handling. Conversely, the narrower the tire, the poorer that the handling, but the better the longevity and ride. To illustrate this concept, push a vehicle with a common whitewall tire from side to side. Watch the tire as it flexes and bends. Now try to do the same with a car with much wider, low profile tires---the car barely moves from side to side.
Look at the size of your tire. It will have three sets of numbers such as 195/75/15. In this case, the tire is 195 mm wide and 75 percent as tall as it is wide, and it fits a 15-inch rim. To upsize this tire and keep the same height but go wider, increase the first number, 195, by 10 to 205, decrease the second number, 75, by 5 to 70 and always keep the same wheel size. You now have a tire with the same height as the first but much wider. The next widest tire would then have the number 215/65/15, then 225/60/15 and so on.
