2015 Mustang Ecoboost K/SA?

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Originally Posted by Bobby Lundholm

With the stiffer gear and it shifting into over drive I would think the 4.09 gear should work fine.

If you were going to switch to a C4, then the 4.09 might be needed, but if you're using the stock 6R80 transmission, I'd gear it to go through in 4th and leave it.

The 6R80 transmission has a 4.17 first gear. If you combine that with a 4.09 ring & pinion, that would give you a launch ratio equivalent to running a C4 with a 6.50 rear gear. I just don't think that's necessary for an engine that makes almost all of it's torque by about 2,500 rpm.

The calculations I ran, had the car going through in 4th gear, which is 1.14. 5th gear is .87, and 6th gear is .69. This engine makes so much torque and has such a wide powerband, with a horsepower peak at a relatively low 5,500 rpm, I don't think it would need to use that many gears to get down the quarter mile efficiently.

With a 30" tire, a 3.73 ring & pinion, and a 1.14 4th gear, the car would do just about 115 mph at 5,500 rpm through the lights.

With a 28" tire and the stock Performance Package 3.55 rear end gears, it would do 113 mph at 5,500 rpm in 4th.

With a 28" tire and the stock non Performance Package 3.31 rear end gears, it would do about 115 mph at about 5,300 rpm in 4th.

I'd be tempted to try it with a 28" tire and the stock gearing.