Drivetrain Lowering Gear Ratios For Improved 6th Gear Mpg.

[Slappy72]

Please move this question to the appropriate sub-forum, if I've misplaced this post.

My circumstances:
I'm considering buying a 2015 Mustang, with a 6-speed manual transmission. My only disappointment in my current 6-speed, is that 6th gear runs at 3000 rpm at 75mph, instead of being closer to 2500 rpm or even 2000rpm at 75mph. I'd love a little more fuel efficiency when I'm just cruising in high way traffic, with enough drafting to feel as though I'm just wasting power. I'm loading the engine less in highway traffic at 75mph than when I'm driving 50mph with no one in front of me. I'd love to be able to up-shift to 6th in highway traffic for better mpg on commutes and longer trips, and still enjoy sporty driving in 1st through 5th gears.

My Questions:
1. Roughly how expensive would it be to have someone modify my final drive ratio, or just 6th gear, to bring me closer to 2000rpm at 75mph in 6th gear?

2. Which of the various engine options offered for the 2015 Mustang would suffer the least if I'm not vigilant about downshifting, so I wind up lugging the engine sometimes, with too much load at too low an rpm?

3. Are there other drawbacks to this modification, beyond lugging the engine or causing accelerated engine wear, that I haven't considered?

Thank you,
-Slappy72

 

[Mattstang04]

I have no idea what it will cost to change gears in the new IRS diffs. The old solid axles were pretty affordable but we're in new territory. Ask the SN95 Cobra guys what they paid. It will probably be a little more than that (just because its new and mechanic shops are like that).

V8 will suffer the least from lugging. It's the engine with the most low end torque.

Drawbacks to this mod are slower acceleration. Would be a little less fun stoplight to stoplight. Plus the engine is going to be working a little harder in city type driving. May hurt in-town mpg's.

The lowest rear end gear that i have seen Ford put in these stock cars is 3.31. I probably wouldn't go any lower than that. My 04 GT started life with 3.27s and it really took away from the excitement of driving the car. I just didn't know it until i put 3.73s in it.

 

[welchy]

My 2016 turns ~2300 rpms at 80 in 6th with 3.73 gear ratio.

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[Jim Harper]

I have no idea what it will cost to change gears in the new IRS diffs. The old solid axles were pretty affordable but we're in new territory. Ask the SN95 Cobra guys what they paid. It will probably be a little more than that (just because its new and mechanic shops are like that).

V8 will suffer the least from lugging. It's the engine with the most low end torque.

Drawbacks to this mod are slower acceleration. Would be a little less fun stoplight to stoplight. Plus the engine is going to be working a little harder in city type driving. May hurt in-town mpg's.

The lowest rear end gear that i have seen Ford put in these stock cars is 3.31. I probably wouldn't go any lower than that. My 04 GT started life with 3.27s and it really took away from the excitement of driving the car. I just didn't know it until i put 3.73s in it.

I have a stock 3.15 in my 2017. 2000 at 75mph

 

[I Bleed Ford Blue]

The IRS used in our S550's has 34 spline axles, because of that, standard 8.8" gearsets will not fit. Ford calls this new setup the super 8.8" Also super 8.8 gears will not fit a standard 8.8. So, in the end changing gears will cost you more than a regular gears swap. And AFAIK ford is the only one who makes gearsets for the super 8.8 and the S550 mustang is the only vehicle that uses it, and that all adds up to expensive.