[MPerry]

I would be curious to know the cruising RPMs for a 555 and a 6v92, that might help you determine if you need to change the current ratio. I believe top gear on a 747 is 1 to 1, and suspect the top gear on the 555 was also 1 to 1. So if the desire cruising RPM on the 6V92 is close to the 555 you may be ok as is.

My 903 runs about 1900 rpm at 60mph. (though not exact, using this and tire size i estimate my ratio at 3.82)