Well we know that the highest velocity traveled (backwards) on the game was 12 Undecillion MPH.
But can we go higher? Whats the highest number velocity a player can reach if they always held the backwards key? 1.8e+308 MPH? 32-bit integer?

It will increase forever

The speed of the Rigs is unbound by the integer bits of mortals.

the rigs are able to exceed the limits of the physical universe because they are gods

Well we know that the highest velocity traveled (backwards) on the game was 12 Undecillion MPH.
But can we go higher? Whats the highest number velocity a player can reach if they always held the backwards key? 1.8e+308 MPH? 32-bit integer?

You say it's 12 Undecillions but the ULTRANAV's speedometer actually reaches infinite, the BIG RIG is literally everywhere at the same time, but obviously nobody can see it, it's easier to see subatomic particles than to see that moment. In fact not even your computer can render it in a single frame, that's why we only see "YOU'RE WINNER !", but believe me, BIG RIGS have no speed limit.

thats basically the same

the universe is composed entirely of a single big rig at all points in space and time that started going in reverse a few moments before the big bang