In the past we had something like the Venom Speed meter
VENOM, Speed Meter ,(VEN-0615)

This device did hit the market under several brandnames like Robbe and even Traxxas so probably you should be able to find one.

On the other hand a GPS meter does not cost that much. In the search of a Venom look-a-like I came to this:
Review: DYNAMITE Passport GPS Speed Meter
But a bit pricy
Dynamite [DYN] Passport GPS Speed Meter DYN4401 | eBay