Swapping rear end gears on a PP?

[Grintch]

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.

Yes, 3.73 is the prefect answer for the Boss (with a higher redline), the old track pack, and the new performance pack? Seems kind of unlikely. I am betting it is because Ford wants a standard "performance" configuration to improve their economy of scale (maybe they have a big stock of Torsen 3.73 diffs they want to use up).

You want a taller gear for autocross to avoid time wasting 2-3 shifts. You want a taller gear for Road Racing on longer tracks so you reach top speed in 5th and avoid the big rpm drop of shifting into the fuel economy 6th gear. And on the street a taller gear gives you better fuel economy, a potentially higher top speed, and avoids some useless wheel spin.

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[D3adch1ld]

It does seem like the 3.73 makes the gears go quickly in sacrifice of better low end response. You can full around with the wheel size to compensate a bit, I'll wait to have the car and modify as I upgrade. It all comes down to the ultimate goals, i want the responsiveness and retain a decent top speed, I intend to road race with it, but the car will be mostly street driven.

 

[Mountain376]

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.

100% Agreed.

Most people who buy sports cars just race them around street lights and maybe the drag strip (because it's the cheapest and more-safe way to let loose in your car - it also doesn't require as much skill to do correctly).

Any "Performance" package is going to be a compromise for all motor-sporting a person would do with the car, plus some. Also, as mentioned, the sportiest package is the one that will be testing through the magazines for 0-60MPH stats (dang, look at the 0-60! It's 0.05-seconds quicker than Chevy).

From my experience with the MT-82 on 27" OD tires and down to 26" OD tires (and everything between) anything less than a 3.31 rear gear is garbage due to the transmission ratios (daily driving, road course, autocross and drag strip). Transmissions shifts are too busy. PERIOD. First gear is useless. PERIOD. Second gear is near useless. PERIOD. Sure, 3.73's felt fast and fun (gee, I wonder if Ford made that as a compromise). I can maybe see a 3.55 if all you care about is straight line (again 27" OD tires or less).

Yes, the Boss came with 3.73's and it was a "track" car. That car had at least 500 more RPM too. And, again, compromises because it was a street "track" car (LS or not).

On a 2015, a 3.55 might work fine with almost a 28" OD tire. A 3.73 might be O.K.. I wouldn't touch 3.73's though.

 

[Mountain376]

One more thing people seem to forget about 3.73 gears: the can be noisey on new cars and God help you if you ever have rear end issues... I've dealt with this many times, I know people who have dealt with it many times and I've read about people who have had to deal with it many times. The chances of getting a 3.73 gear replaced and it being quiet is slim, even when set up by a highly competent person. Why? All the tolerances in the differential assembly (housing, differential, ring and pinion) and the harmonics associated with the RPM (not engine RPM!) the gear will regularly see during use. If you differential assembly happens to be one with components that match pretty well and the gears are installed correctly, you're golden.

 

[analogman]

So today a bit of monkey wrench was thrown into my plans to swap out the 3.73 rear end gears for 3.31 on my soon to be delivered GT PP.

Both the Ford dealer, and a local Ford performance shop, told me that the ECU will need to be reflashed to work with the different gear ratio - and the software to do that won't be available for 4-6 months. Does that sound right to the wise and learned here, the need to reflash the ECU software with a rear end gear change?

It's also going to be more expensive than I thought. The Ford dealer quoted $980 in labor, plus parts (about $200 on line), the local speed shop less for labor but more for parts so the total is about the same. As much as I want to lower the rpm's at cruise speed, at $1200 I don't know if I would save enough gas over the life of the car to make it worthwhile.

The quest for a cost-effective numerically lower gear continues...

 

[908ssp]

The software issue is probably just for the speedometer and a dealer wouldn't let you drive out with a speedo reading that far off. It is possible there are other issues I can't think of them. Taller tires do the same thing. You can probably put a 28.5 tire on the back that would give you a 3.55 ration or very close to it.

 

[Mountain376]

A rear gear change will change your speedometer reading (holding the tire size the same). If you make a small change, like 3.55 to 3.73 or vice-versa, you could probably get away with it and not really notice much difference with the speedometer. Why? because due to the various tire size options, variations in exact tire size between manufacturers, rear axle options and other minor factors like air pressure, the factory speedometer reading is all that accurate (I've checked this in datalogging against a good GPS speed and some of those Police digital speed limit displays they put out sometimes).

Swapping rear gears on a manual car will only affect the speedometer reading. On an automatic car, it can mess with the shifting/shift strategy some.

A hand-held tunner will allow you to change rear end gear ratio and tire size (revolutions/mile). The newer cars use a sensor on the output shaft of the transmission for speedometer reading. In the old days there was a gear you could swap out to correct the speedometer. You could also use part M-4209ADPT-AC if you really wanted to. You could also do like 908ssp said and change your tire size, but the 2015 tire sizes (OD) are already pretty tall and getting near-truck tall.

I suggest that you don't mess with the car until you drive it some and really figure out what you want to do.

 

[Mountain376]

Hmm,, looking back, that Ford Racing part might not work. The input signal for the speedometer on the 2011-15 cars might be different (sign wave vs square wave). I forget what kind of sensor the new and old cars have.

 

[aeropaul]

Yeah, my only gripe about the car is that 70% of the time I drive it as a 4 speed, sometimes a 3 speed, because there isn't enough difference between 3, 4, & 5, so I pick one, then go to 6th. I feel like 1st is a tall enough gear that going to 3.55 rear from 3.73 wouldn't be sacrificing much.

 

[Old 5 Oh]

Yeah, my only gripe about the car is that 70% of the time I drive it as a 4 speed, sometimes a 3 speed, because there isn't enough difference between 3, 4, & 5, so I pick one, then go to 6th. I feel like 1st is a tall enough gear that going to 3.55 rear from 3.73 wouldn't be sacrificing much.

Seems like big, torquey V8s don't really need so many gears. My GTI benefitted from its close-ratio 6-speed when I was in a real hurry, but most of the time I went 1-3-5-6 or 1-2-4-6. Just too many gears to stir, and that was with a 2.0 turbo 4!

 

[DivineStrike]

Seems like big, torquey V8s don't really need so many gears. My GTI benefitted from its close-ratio 6-speed when I was in a real hurry, but most of the time I went 1-3-5-6 or 1-2-4-6. Just too many gears to stir, and that was with a 2.0 turbo 4!

With my gti generally my shifts were...1,2,6 unless getting in to the freeway haha

You also have to remember our little 2.0s have a retarded amount of low-end torque and below 2800rpms no less ;)

I can't do it as often in the mustang due to the insanely tall 6th gear.

However in my opinion, 3.73s are way too short.

3.55s or taller

 

[Old 5 Oh]

With my gti generally my shifts were...1,2,6 unless getting in to the freeway haha

Yeah, the GTI needed a 7-speed, with a real freeway gear. 6th was too short, good down to about 30 mph and really spinning on the freeway.

 

[908ssp]

Seems like big, torquey V8s don't really need so many gears. My GTI benefitted from its close-ratio 6-speed when I was in a real hurry, but most of the time I went 1-3-5-6 or 1-2-4-6. Just too many gears to stir, and that was with a 2.0 turbo 4!

I had an E30 M3 1990 2.5 liter NA with a bunch mods reved to 8000 all day. The gear gaps in the transmission were horrible who in their right mind would put a gap like that between 2nd and 3rd. I also had a gray market 635 it had a close ratio gear box, it died so I swapped the box into the M3 what an amazing difference. Every gear was perfectly spaced with about a 700 rpm drop going up gears. Just transformed the car you could keep those Shrick cams on the boil what fun. But a Mustang is not a 2.5 liter engine it has torque. One more thing the close ratio box had first gear left and down....yup down so that the reaming four gears [it was a 5sp] were in the H pattern that's how race boxes were built back then.

 

[Blacksheep]

Not a gearing "expert" but this is what I have been quietly thinking in my head.

I am coming from an 05' with 3:55 gears (Whipple Charged) and have been wondering if next month when I Whipple Charge my 15' am I going to want to change my gears?

I have been driving almost every incarnation of Mustang for 29 years and have had many size gears but have never had 3:73. I am very much looking forward to the snap of 3:73 and I live in a very hilly area. I think the 3:73 will help me stay in performance range better in my situation NA but am concerned if I may need to change to 3:55 when supercharged.

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

Jaime


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I'm thinking about the same thing myself. From what I gathered 3.73 gives you that snap. I'm thinking of adding a whipple sc someday. You said you're not tracking your car. I think it should be ok. It's not like you're gonna
Launch your car from every stop light. For me I would supercharge it for "more" spirited driving too. And for a snappy highway passing power. And so I can say I have a supercharged stang. I think most people who are advising against 3.73 are looking at it at the drag racing point of view. I never owned a mustang or drove with 3.73 gear or supercharged car. It's called a left and right foot. Just modulate them I guess so you won't be eaten up by a billow of smoke from the tires.

 

[Jmeo]

I'm thinking about the same thing myself. From what I gathered 3.73 gives you that snap. I'm thinking of adding a whipple sc someday. You said you're not tracking your car. I think it should be ok. It's not like you're gonna

Launch your car from every stop light. For me I would supercharge it for "more" spirited driving too. And for a snappy highway passing power. And so I can say I have a supercharged stang. I think most people who are advising against 3.73 are looking at it at the drag racing point of view. I never owned a mustang or drove with 3.73 gear or supercharged car. It's called a left and right foot. Just modulate them I guess so you won't be eaten up by a billow of smoke from the tires.

We are on the same page. Driving the 2015 with 3.73 is fun and I would probably just leave them if I wasn't going to SC it but since I am I feel the gear change is a necessary evil to make the care feel "spirited". The gears go by quick and once boost is involved they will feel even shorter. I just want to use the boost efficiently and get a nice steady incline of boost/power and not need to roll through gears one after the other. I'm afraid the 3.73 gears will just spin the tires off the car with all that power to tires.

My '05 with SC and 3.55 would let me build a nice long boost/power build up because it's legs were long enough to do so. The new 6 speed is a whole new animal to me and I'm thinking 3.31 are in order with a SC.

Jaime


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