speedo is off, trust me
The RPM/speedo comparison won't tell you anything, because the speedo works off the rear of the tranny, so the computer has no idea what tires you're running. If you slapped a set of 12" atv tires on, it would still read "correct", but you'd be going a damn sight slower! A GPS will prove it if you don't wanna take my word for it, but I promise it's true -- you're going faster than it reads.
Some geek math:
For every revolution a 29" tire makes, it moves forward 91.06".
For every revolution a 31" tire makes, it moves forward 97.34".
So for any given RPM, a 31" tire on a vehicle calibrated for a 29" tire cause an increase in speed of 106.9%, which means when it reads 55 MPH, you're actually going 58.8 MPH. For 65 MPH, that's 69.5. So if you're doing 70 in a 65 because you figure that won't get you for 5 over, you're really doing 74.83, which I'm sure they'll call 75. Now you're 10 over. Oops.
Not much of a difference, but enough to make for a nice ticket if a picky cop runs across you. You can spring for the speedo correction if you don't want to have to think about it, or just keep the speedo at 5 under the speed limit in a 65 zone, and 4 under the rest of the time.
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