OK, let's see if I can get the math right!

If you figure a 40 inch wheel (a factor), that's .333 inches diameter so 1.05 inches circumference.

Top speed is say 70 miles per hour, figure 44 feet per TT scale mile so that's 3080 feet per hour or 51.3 feet per minute or 616 inches per minute.

So if you divide 616 by 1.05 that's 586 RPM to do 70 MPH in TT scale

So if the typical max speed of the 1024 from the chart is 12400 rpm at 12 volts, 12400/586 is roughly 21:1!

Now that assuming you'd want to run full throttle for top speed, I'd rather keep everything a little less stressed. And that's not including any of the engineering variables like friction losses, etc.- no idea how much it would affect it?

If that's right, higher than 14 or 15 to one would be better? And the only other thing might be when you start to get into gear or pulley diameters, I seem to remember Geoff from Hollywood saying if there's a bigger gear ratio that the gear sizes increase? I think that was an issue in the GP38, the gear version would work but the 15:1 belt drive might have had clearance issues when swiveling on tight radius curves.