I'm guessing you just want the overall speed of the vehicle, and I'm guessing its outside (since you're worried about slippage) so,
If $99 is an ok price for you, then just get a RS-232 GPS receiver, they spit out speed as part of their very-easy-to-decode databursts. You get 1 second updates, an as an added bonus, you know where your robot is!
Here's Microsoft's available at some stores for $99 (on their site $130). Just cut the wire running from the GPS antenna and the USB<->RS232 converter and just solder your serial lines and power supply (+5V) line onto the wires. OR solder on to the board directly, make your own connector, etc. I believe it runs at 4800bps, but you'll have to check that on HyperTerm.
PM me with any questions.
-Q