I recently bought new tires for my 07‘ Chevy Silverado and I was asking about larger rims and tires possibly 20 inch or more and the gentleman said that larger rims and tires on newer vehicles will affect my on board computer and he does not recomend it. Is this true? I see vehicles all the time with larger rims and tires.
Yes, it will definitely affect the calculations. The onboard computer does not know what size tires you put on - it has been setup by the factory to assume the diameter of the factory recommended tires that are on the tire sticker (usually located on the driver side door frame). If you are lucky to have a computer that you can reprogram then good for you - most do not allow changing the tire size AFAIK. Those other people who have changed their tires are also having issues, but for the most part they might not realize the calculations have been off, since the change might be small for them.
It can affect the computer. It is a possibility, but probably it's not enough of a difference to matter. Your truck's on board computer controls many things that rely on calculations based on the revolution of the wheel. These calculations are based on how often the hub turns, but if you place a larger diameter wheel on the hub, the truck actually goes farther on one revolution of the hub than the computer thinks it does. Systems that may be affected by this are: anti-lock braking system, tire pressure monitor, speedometer and odometer. I am not a fan of changing wheel diameters on vehicles, however it actually is not that big of a deal in most instances.
by going for a larger overall tyre diameter, you are gearing your car faster for the same rpm. this will make your speedo read LOW. it will make your engine work harder on hills etc, though it may wind out to an higher top speed. it may or may not improve mileage, depending on whether the car was originally undergeared or not. rim diameter affects sidewall height choices and handling, tyre diameter affects gearing, braking, economy and speed. tyre width affects speed, handling and wear.
