Most people use either 3rd or 4th - the dyno is calibrated accordingly so should make no difference.

Boost will come in later in a lower gear but you will get a higher wheel power reading too. Most DD rollers only measure the wheel power and then add a transmission loss for the flywheel power, this is not the same as a coastdown calculation.

This has also been proven at G-Force a few years ago. JohnS asked for a power run to be down in 4th and the operator said it made no difference, so did a run in 3rd and a run in 4th, both results in the same power (1bhp here or there)

It can depend on the type of Dyno being used as to whether 3rd or 4th is used. On the Dyno Dynamics Dynomometer which was used here then this rr is calibrated to a max of around 150mph, if the car is ran in a gear that could exceed this then the results become exagerrated (I proved this in the Cerb). For the Fiat Coupe 20VT then running in 3rd and 4th shouldn't make that much difference. The other important thing to remember is that the ideal is for peak power to occur at between 100-115mph for accuracy. I ran my 335d on a Dyno Dynamics last night in the same setting as used for the Coupe in this thread (Shoot 6F), but mine was run in 4th as it is a six speed auto / tiptronic / flappy paddle type box.

Jules.