Dude.. a difference of 10rpm is nothing... that's pretty accurate I'd say.
See, on your car, the Rear tire will wear out more than the front and at some point, the RPMs on the front and rear won't even match on the same set of tires. Because the overall diameter woulb be different due to uneven wear in the front and rear..

and it's prefectly normal! That's why we rotate tires too.~

 

no.. not the rim size.
For each car, there's a stock tire size, which is manufacturer's spec. Now the speedometer is calibrated according to the overall diameter of the stock tire.

For example, say a car came out with stock 17" and the sidewalls are 2".. so overall diameter is 21" (17 + 2 + 2). To keep the speedometer accurate when you upgrade to 18", all you need to do is keep the overall diameter same. For this example, the sidewall of the tire would have to decrease to 1.5" as the wheel size increases by 1"

Because overall diameter changes as tires wear out, it is almost guranteed that your speedo is not accurate, but within an acceptable and usuable range.
Usually performance tires come with 10/32" of treads. A tire is considered worn out if it reaches 2/32" or lower. So, if the speedometer was accurate with brand new tires on the wheel (10/32"), by the time the tires are worn(<2/32"), the overall diameter will decrease at least by 1/2" or .5" (10/32 - 2/32 = 8/32*2 = 16/32 = 1/2), and that's lot more that 10 rpm difference per mile with the same tire.

Here's a link with a Java applet where you can enter your stock tire size and then try out different other sizes to see how it changes everything. It's good.

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

If you have any questions about Wheel/Tire sizing like offsets and backspacing and all that.. let me know..