I was talking with my father today about putting rims on my car (he raced SCCA pro series for a long time so i figured he is the man to talk to) i asked him if i were to put 19" rims on the car if it would slow the car down dramatically?

He said just from his experience that with a larger rim it sometimes will throw off the calibration of the speedo if anything. He did say it would slow it down..i guess that’s kind of obvious, but has anyone heard of 19 inch rims throwing off the calibration of the speedo?

 

As long as you stay with the same OD you'll be fine. More rim less tire.

 

Yes, there are calculators out there to help you determine this. When you go up a size in rim diameter, you must decrease the sidewall height to compensate. Keep the same OD and you'll be fine.

http://www.miata.net/garage/tirecalcold.html

Here's a good calc. Just input stock wheel tire size and use it to figure out the size you would need on 19's.

As for bigger wheels slowing you down a tad, that is true. Usually, larger diamter wheels are heavier, but even if you had a 19" wheel and tire setup that weighed the same as the 17" wheel and tire combo the car would still be a tad slower with the 19's. The reason is because the majority of the mass (wheel rim) is further away from the center of the wheel on the 19's. The farther away from center, the more energy it takes to spin.