I have a chance to get a pumpkin with 4.083 gears for pretty cheap with a trade of my stock rear end (guy is going turbo), but since I drive 120 miles a day for work, I really wanted to find out the MPG loss I'd be taking with the gear swap. Has anyone ran the numbers before and after? I more interested in the difference from stock to the 4.083 gears than the actual MPG, because every setup is different. BTW, my car is an 07 Coupe 6MT with rear active steer (if that matters).

 

I'm considering the 4.083 final drive as well. The difference between 4.083 and 3.56 (stock on 05-07 6MT coupes) is approximately 15%. For the 4.083, venders represent "quicker acceleration in every gear" of 15%, so I would estimate a correlating decline in MPG by the same amount. On that basis, whatever your blended MPG is now, multiple by 85% to get a projected blended MPG following 4.083 install. Assume your long term fuel consumption will go up by at least 15% under the same driving patterns.

The 3.9 final drive works out to 10% vs. stock, but costs more since it's not Nissan OEM. Hope this is helpful.