fxdsconv2000

When a Car is Dyno'ed the runs are normally done in 3rd gear right? I assume this is done because doing a run in first gear would not show the correct horsepower right? So 3rd gear is close to a one to one ratio. Is this one to one ratio, one revolution of the motor to one revolution of the tires? What if 3rd gear is not a perfect 1:1 ratio is that compensated for in the dyno?

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RX-8 friend

It's not a 1:1 ratio. The closest gear to that is 5th. Dynos need to be spun at a certain RPM range for the data to store properly. This varies between dynos. First gear isn't used because it's too hard to control the tires on the dyno drums (too much torque so slight mistakes on the accellerator could cause the wheels to jump out). Too high a gear will spin the dyno drum very fast and that's dangerous. Third gear is a good compromise.

Other than safety and convenience of the electronics, any gear could be used. Some cars do special things in first and second. The 3rd gen RX-7 for example has a switch on the transmission for 1st and 2nd gear - it's not clear exactly what it does but we like to avoid problems so we like to avoid those gears.

DemonRX-8

RX-8 friend said it all, but to expound on that, the relationship between engine rpm and tire rpm is dictated by the transmission gear ratio and the differential final gear ratio. So generalizing that third gear gives you a 1:1 ratio will never be accurate for any given transmission/final drive ratio. As a matter of fact, it will be no where near for most cars. The 8 has a third gear ratio of 1.645 (US) and I believe the final is 4.44 - this gives a ratio of 7.3038 engine to wheel rpms in third gear if my math is correct.

IKnowNot'ing

I've personnally experienced rolling dyno runs in 4th gear also. But 3rd seems to be the norm, which might actually lead to some issues : some cars have torque truncation in 1st, 2nd and even 3rd gear due to gearbox protection. I don't know of any US-sold car like this, but the EU Ford Puma 1,7 had torque truncation up to 3rd gear because of its weak Fiesta based gearbox. So if you dyno it in 3rd, you won't measure full torque.

And it's never done in first gear because you would be in fluctuating transient conditions all along the torque curve. 3rd or 4th give you conditions a bit closer to steady state.