Swapping rear end gears on a PP?

[Jimdohc]

So 3:31 is my best bet?

I'm just here for the discussion. Trying to elevate dialogue not give conclusions.

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[sqidd]

So 3:31 is my best bet?


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Don't forget that the GT500 has a lot taller gearing in the trans. For a blown S550 I wouldn't want to go any higher than a 3.08 and would prefer a 2.73.

 

[sqidd]

I drive all sorts of blown cars with power anywhere from 450 to 1000.

A common combo I drive is 500-550hp with a GT500 trans (2010-2012 gearing) and a 3.55 rear end. With 315 rubber on the back the car will roll the tires hard in 1st and buzz them all the way through 2nd.

To duplicate that gearing with a MT-82 you would need to run a 2.73 rear gear. That will give you the same overall gearing. Yes, the MT-82 is geared that short.

Here are the gearing numbers. Plug them into a gearing calculator and check it out.

TR6060 (stock GT500 2010-2012)

1st 2.97
2nd 1.78
3rd 1.30
4th 1.00
5th .74
6th .50

MT-82

1st 3.66
2nd 2.43
3rd 1.69
4th 1.32
5th 1.00
6th .65

 

[Old 5 Oh]

And..... money spent vs overall difference between 3:73/3:55 may not be worth it.

The taller tires on the 15 make the 3.73 effectively the same gear as a 3.50 on the earlier cars. You'll be thrilled. MT82 has a little deeper first gear and a little higher sixth than the five-speeds did.

 

[Jmeo]

Don't forget that the GT500 has a lot taller gearing in the trans. For a blown S550 I wouldn't want to go any higher than a 3.08 and would prefer a 2.73.

But would I be hindering my everyday driving experience (normal driving) switching to 3.31 let alone 3.08 or 2.73?

I don't want to "have" to be in boost to enjoy the car.

Jaime


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[sqidd]

But would I be hindering my everyday driving experience (normal driving) switching to 3.31 let alone 3.08 or 2.73?

I don't want to "have" to be in boost to enjoy the car.

Jaime


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Have you ever had a blown car? What kind of blower are you considering?

 

[Jimdohc]

For example, lets compare 2 horsepower and torque numbers:
435hp and 400lb-ft- NA S550 with some bolt-ons
675hp and 585lb-ft- S197 Roush Phase 3 supercharger kit (for S197, using for good realistic comparison.)

At what level does S550 chassis & tires lose traction on street tires?

I built this is very simplified calculator one night a couple years ago. Plugged in the Mustang's numbers.

*note/disclaimer: I guesstimated most of these numbers. If you want to contract me for $200 per hr or 10% of the projects budget. I'll find the real numbers not guesstimates
**Didn't raise the car for calculation, supercharger gives the car a higher center of gravity.

You can easily build a car with more drivetrain force than tire traction. But it's a waste of money if you can't put the power down. Only good for smoking tires and Texas Mile top speed runs.

 

[Jimdohc]

example of poor tuning.
1st gear is useless.

 

[Jimdohc]

One more... to bring it back to original topic.
3.73 vs 3.15 highway MPG.

Looks like, at 70mph 6th gear RPM will drop from ~2,000rpm to ~1,650
350rpm move will change the MPG. How much? Please test before and after and share results.

 

[Five Oh Brian]

Can I assume by that statement you will be leaving your car 100% stone stock?

My '14 GT is not a track pack or some factory specialty model, just a regular GT that came with EPA/CAFE friendly 3.15 gears for the masses. It wasn't geared from the factory for 1/4 mile drag racing, which is my passion. Therefor, I calculated how much gear I would need to run the 1/4 mile crossing through the traps in 4th on a 28" drag radial. That math led me to go with 4.10 gears. My car has just a few simple - yet effective - mods and runs 11.70's @ 117+ in the 1/4 mile, and the gearing works great.

On the other hand, Ford carefully chose a combination of parts for the '15 GT PP, including the 3.73 gears, with road racing in mind. Hence their reference to how it matches or betters the '12/'13 Boss 302. The GT PP is purpose built. Ford obviously thought this out and didn't just pick 3.73's at random. I'd bet they even tried different ratios to see which ones yielded the best lap times around whichever road course they were running.

 

[908ssp]

My '14 GT is not a track pack or some factory specialty model, just a regular GT that came with EPA/CAFE friendly 3.15 gears for the masses. It wasn't geared from the factory for 1/4 mile drag racing, which is my passion. Therefor, I calculated how much gear I would need to run the 1/4 mile crossing through the traps in 4th on a 28" drag radial. That math led me to go with 4.10 gears. My car has just a few simple - yet effective - mods and runs 11.70's @ 117+ in the 1/4 mile, and the gearing works great.

On the other hand, Ford carefully chose a combination of parts for the '15 GT PP, including the 3.73 gears, with road racing in mind. Hence their reference to how it matches or betters the '12/'13 Boss 302. The GT PP is purpose built. Ford obviously thought this out and didn't just pick 3.73's at random. I'd bet they even tried different ratios to see which ones yielded the best lap times around whichever road course they were running.

I kind of agree but I have to caution you Ford won't do anything that might hurt sales even if it is not the correct or "best" thing to do. The common thinking that lower gears are better determine sales whether that is true or not. Do the low gears "feel fast" sure but "feel fast"and "are fast" is not the same thing. Only empirical testing on a road course can determine that with a stop watch. Did Ford test the car at the tacks you are most likely to run? Road racers almost never use first gear except to leave the pits. And you can easily get the PP gearing to get rolling and well I might add in second gear so the whole first gear is dead weight on a road track car. For a street car where you almost never push the car in first a taller 6th would be nice when you have to make long distance run and want to save gas.

 

[Jimdohc]

...I calculated how much gear I would need to run the 1/4 mile crossing through the traps in 4th on a 28" drag radial. That math led me to go with 4.10 gears...

Good call! '15 w/3.73 does 0-60mph at the top of 2nd gear. Also, finishes 1/4mile end of 4th. Looks like Ford was using same technique to maximize some key performance metrics.

Only empirical testing on a road course can determine that with a stop watch...

I think you've found yourself a project, proving Ford was wrong. Just make FD your first modification and compare lap times.

 

[Swoope]

You guys need to spend more time going around corners. The gearing is WAY too short.

I could care less about MPG. But having to grab 5th for anything but a long straightaway drives me nuts.

but the no power in the first three gears doesnt matter?

you need to buy a vowel vanna!

beers

 

[D3adch1ld]

This is a good write up on rear diff gearing and tire diameter influence:

http://www.hotrod.com/how-to/transmission-drivetrain/how-gear-works/

Enjoy

 

[DivineStrike]

After logging a tank of gas on the car, I'm going to have to agree with those saying 3.73's are too short for track work. But not by much IMO, of course I will have to eventually take it to the track to be able to really comment further. I feel that the 3.55's would be about perfect. All the gears just feel a little bit too short, except for 6th gear of course lol. That gear is ridiculous, my gti has much better pick-up in 6th than this thing does. Obviously that's due to a much shorter 6th gear and more than 600lbs less weight.

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