Do bigger rims after speedometer

[A3mikeA3]

Do bigger rims but tires that are the same diameter affect the speedometer? If I go from 17x8 rims and 225 45 17 tires to 18x8 225 40 18 tires, will it screw up my speedometer

[moyenecorniche]

No it's called a +1...I suggest going to Tirerack webs and looking up their definition and specs on wheel and tire upgrades to larger wider...
Search A-zine wiki and the tech sections, should find just about any answer to most questions...

Welcome Aboard....

[Dad A3]

Sound advice, bought my tires 9mounted and balanced) from tire rack.
Welcome aboard.

[Lunchsacks]

Wheels*

[rotts4u]

Do bigger rims but tires that are the same diameter affect the speedometer? If I go from 17x8 rims and 225 45 17 tires to 18x8 225 40 18 tires, will it screw up my speedometer

The change in speedo vs actual will be determined by one and only one thing. The circumference of the new tires (not wheels) vs the original tires. So check the circumference in the spec sheet of your old and new tires and that is what matters. It may only give the diameter so you will have to use the conversion between diameter and circumference which is pi x diameter 3.141 is pi

[JRutter]

http://www.1010tires.com/TireSizeCalculator.asp

The A3 speedo reads a little high anyway...

[LWNY]

The change in speedo vs actual will be determined by one and only one thing. The circumference of the new tires (not wheels) vs the original tires. So check the circumference in the spec sheet of your old and new tires and that is what matters. It may only give the diameter so you will have to use the conversion between diameter and circumference which is pi x diameter 3.141 is pi

The speedo will also be affected by tire pressure. Extremely low pressure will decrease its circumference, thus making you think you are going faster.