Your statement remains entirely unsubstantiated save anecdotally. I'm starting to get the impression you don't really understand why this isn't very... compelling.
Depending on the way the dyno measures torque, you want to eliminate as much mechanical advantage from the tranny as possible. On dynos where torque is calculated by the rate of acceleration of a weighted drum (like Dynojets, I believe) you can get a run to read artificially high by taking advantage of gearing. This is why everyone suggests you use the gear closest to a 1:1 gear ratio. 4th is 1.171:1, and 5th is 1.085:1. Assuming you understand how ratios work, you'll notice 5th is closer to 1:1.
Now, on dynos that don't measure torque this way, you could conceivably run any gear you want to get accurate numbers, and there would be no issue trying to use a specific gear ratio.
I asked about 1:1 gear ratios because I wanted an informed answer, not your "BECAUSE" answer that tells me nothing beyond that you have strong and possibly misinformed opinions about this subject. If you can bring up anything specific, or some data or numbers or something, awesome. If you know something better about how dynos correct for gearing, or what effect the final drive ratios being different for 1st - 4th and 5th - 6th gears may have on trying to use the gear closest to 1:1, cool. I asked you to defend your statement, and you haven't.
If your only rebuttal is "Hey man I have dyno charts and my claim remains unsubstantiated, but I'll say it again anyway" you can keep it to yourself until someone who actually knows something about this can chime in.
