Most Brownie boxes have an OD ratio of around 0.8:1, or about a 20% overdrive. If you're running 45 MPH in top gear now, a 20% OD would bring the top speed up to 54 MPH or so. You might also be able to nurse a little more speed out of it by installing taller tires- - - -what size is it running now?
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!