Filed under: 1966 Ford Mustang (Coupevertible)

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Ran across an interesting post that explains this well:

Re: How does rear end gear ratio effect your Dyno numbers? (hogurt)
The dyno measures engine RPM and rear wheel RPM. From this data it computes a multiplication ratio that it uses in calculating the output at the rear wheels. The dyno can’t distinguish between a 4th gear 1:1 through a 3.42 axle and a 3rd gear 1.30 through a 2.64 axle. It has no way of knowing where the overall driveline multiplication is coming from or what gear the transmission is in. The dyno doesn’t fudge the TQ and HP or make any compensation for driveline inertia. It simply measures the acceleration of the drum over time. It can then compute the force (TQ) and plot this against time to yield power (HP) at the drum. It then uses the measured ratio between the engine RPM and the wheel RPM to compute the final numbers. The rotational mass from inside the transmission all the way to the front of the engine is constant. Any angular momentum (inertia) stored in these components upon spin up is an integral to the engines output and relative to engine RPM. A rear wheel dyno has no method see it as separate in anyway. Let me try to restate the fundamental idea. The engine produces 400lb.ft. of twist at the input shaft. If you run 1st gear 2.97 times an axle ratio of 3.42 (2.97*3.42=10.1) you have a mechanical advantage of about 10 to 1 at the wheels. If it were 100% efficient, we would have 4000lb.ft. of twist at the wheels for the dyno to measure. When a force of this magnitude is applied to the dyno rollers it will spin up to 50 mph much quicker than it would in 4th. This would mean the raw torque measurement would be much larger. But the dyno then takes the 10 to 1 mechanical advantage and divides the raw numbers by it. This division of the raw torque number by mechanical advantage factor corrects torque numbers so cars with different gearing can be compared. To put it another way, if in the above example we substitute 4.10 rear axle ratio we get the following: 4.1*2.97=12.2 mechanical advantage. A twist of 400lb.ft.becomes 4960 lb.ft. The extra 960 lb.ft. accelerates the car harder and twists the dyno drum harder. But in the case of the dyno, it sees the new mechanical advantage of 12.2 and uses the new higher number to divide the new higher raw number. In the end the bigger correction factor erases all of the gains in the measured raw torque.