Auxiliary ("Brownie") transmissions have been around about 3 days past forever. Most of them have an overdrive gear that can be used to increase top speed, as long as the engine has enough torque to pull it. Gas mileage will suffer horribly once you get out of the torque band and start lugging the engine down, however.

How about brakes and steering? Getting the rig moving is only one third of the equation- - - - -stopping and steering one at speed can be the world's most effective laxative! My 120 MPH ride down Grapevine hill hanging onto 39 tons of runaway Freightliner one night was a real eye-opener!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!