every nuvi I've owned records max speed in the odometer section.

I built these, based on a light GPS module that's accurate to better than 0.5 km/h. Standard software holds top speed achieved and button resets it. They also have a continuous speed readout mode that some people like to use on their 1:1 car :). Some guy at work wanted a 4-digit clock while the car's stationary, so there's a minor rev with a 4-digit display.

Overall weight is under 15g; works off any voltage from 1s-3s and reverse-voltage protection.

Phil.