Question for everyone with larger aftermarket rims. Do you learn to drive slower or do you just follow traffic? I checked out this calculator http://www.miata.net/garage/tirecalc.html and it says from going to stock 18s to 20s sets your speedometer off by 19 percent.

To those who have upgraded: Do you get a lot more speeding tickets now, or are you able to gauge things pretty well based on a mental adjustment you make to your speedometer make when driving?

 

Did you factor in tire size? ie
205/45/17
to a 205/35/19?
Ive only even gone 17 to 17, so I cant talk much.
Personally I dont think one should put larger rims on a car if it did come from the factory like that. Plus added rotational mass is a definite con, depending on the wheels I guess.

 

You have to adjust your tire size according to width as well. I have 20s and my speedo is the same as my stock wheels. Stock profile is 45, that calculator helps you adjust to find you need 35 profile now... unless you go super wide then more changes.

245/45-18 stock

245/35-20 upgrade
I have 20x8.5 so wheels are 1" wider. more your tire size needs adjusted.

this is only a 0.3% difference hardly noticeable. Use a GPS to compare the speeds.

 

I have 20s on mine and compared to my Garmin GPS, I'm +3mph over.

 

according to Garmin I am slightly over with both stock and my 20s.

 

I drive faster with my larger wheels LOL.

I have 245/45/20 aftermarket tires on my car vs my stock 245/45/18.
According to that site... when my speedo says 60mph - im really doing 60.2mph.

Nothing to worry about.

 

If you get the correct tire size, your speedometer should not be 19% off. In fact, the usual "acceptable" range is only about 2 percent or so. You should use the tire calculator to find the right tire size for a 20" rim that will give you closest to stock diameter, and as a result, the lowest speedometer error.

This is a very good one in addition to the Miata.Net one:

http://www.rimsntires.com/specs.jsp

Basically, what NismoMax80 already posted...