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You have to be careful how high you go on the gearing or you'll be causing more problems with your fuel mileage especially with a Turbo. Go too high in the gearing and you'll be in the Turbo more than you want. The slightest incline and the motor will be boosting.

Most car manufactures bump the gearing up by one set to improve the drive ability for a slight decrease in fuel economy. My experience with Volvo's is if the car came with 3.73's in an automatic the best economy would be with 3.54's. If it came with 3.54's with a manual transmission then the 3.31's would be the best for economy.

There are gear calculators out there when you can play with gearing for the transmission as well as the differential and tire sizes. You can tune the rpms to vehicle speeds with these online calculators.

With an Eco Boost four cylinder, manual transmission Mustang, I'd be looking at rpms running around 1750-1800 at 65 mph for the best fuel mileage. With a V8 Mustang you can run 1200-1300 rpms at 65 mph because the engine produces so much torque at low RPM's it isn't an issue.