Hi guys,

I’m a noob to modding my Civic, but not to geometry, which is key to understanding the relationship between tire/wheel size and speedometer calibration. Ngoti8tor had it right when he said to keep your overall tire/wheel size the same to avoid inaccuracy. A few weeks ago, when trying to decide what wheel/tire combo to select as an upgrade for my ’92 CX hatchback, I put together a simple Excel spreadsheet to calculate the rolling diameter and speedo error for any tire/wheel combo I wanted to try. You input the stock wheel and tire sizes as well as those with which you are considering to replace them. The formulas calculate the change in diameter and speedo error in both absolute terms (millimeters) and as a percentage. It’s a handy tool to play around with and if anyone wants a copy I’d be more than happy to share.

Regards,

Erik