Gear Dyno Difference

BGTV8

The dyno is measuring torque (actually tractive effort which is not quite the same thing) and the gear box and final drive are "torque multiplication" devices and there making the gearing shorter will be default provide for less "torque multiplication" which affects the Nm measure and by direct implication the Kw number (or lbs/ft and hp for non-metric folks).

Biggest risk is that with lower gearing, you are seeing a bit more slip on the rollers which some might interpret as "transmission losses".

This is why I prefer to set my engine up on an engine dyno and get numbers from the crank, but that is a PITA for anything other than a race engine build.

I need some quiet time outside the orifice to figure out the equivalence factors so you can compare readings with different FD ratios.