Can anyone help me to understand and quantify the differences in the speedometer and odometer for going to a smaller diameter tire? For example, say I go from a stock size (~820 revolutions per mile) to something slightly smaller (say ~840 revolutions per mile).

Now, since this is a smaller tire, would the odometer read less miles or more miles than actually travelled?

How about speed?

Please enlighten me....for some reason, I'm having trouble trying to figure this out.

 

Here's a couple of good tire calculators

http://www.dakota-truck.net/TIRECALC/tirecalc.html

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

 

What Slimey said.

Smaller diameter tire means your speedo reads too high and your odometer will read that you've gone more miles than actual.

 

Here's another calculator that I have bookmarked:

http://gs.tolan-hoechst.com/tirecalc.htm

This one incorporates tire and wheel fitment, as well as changes from gearing. Cool.