Your on the right track, but you don't need to complicate this with weight and wheel diameter.

The goal is to select the gear that achieves max rear axle torque at any given speed. This is simply engine torque X gear ratio at that speed and gear. You can neglect the final drive ratio because it is fixed.

So tabulate points for axle torque vs vehicle speed through the range of interest for each gear, then see where changing gears up increases the axle torque.

You will find this occurs when max area under HP curve is acheived.

I say we all switch to CVT's and be rid of the whole issue :)

Brad