Swapping rear end gears on a PP?

[Old 5 Oh]

If I were sticking with the MT-82 I would run 3.08's.

I would rather just switch over to a T-56 Magnum with a 2.66 1st/.50 OD or a TR-6060 with a 2.66 1st/.50 OD though and keep the 3.73's.

And this wouldn't be "track" gearing. Just daily driver/back road gearing.

I beg to differ. With the tall tires and a 2.66 first, you are talking about 13% LESS low gear than the old 2.73-rear Foxes had. Even with the low-end torque of the 5.0 Windsor, those were clutch-slip specials around town, and benefitted mightily from steeper gears for daily driving.

I think a Coyote with those gears would be really flat off the line, though once you had it going it would be fine. You'd just never get out of third gear until you hit the freeway, LOL. (Just like those old Foxes.)

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[sqidd]

I beg to differ. With the tall tires and a 2.66 first, you are talking about 13% LESS low gear than the old 2.73-rear Foxes had. Even with the low-end torque of the 5.0 Windsor, those were clutch-slip specials around town, and benefitted mightily from steeper gears for daily driving.

I think a Coyote with those gears would be really flat off the line, though once you had it going it would be fine. You'd just never get out of third gear until you hit the freeway, LOL. (Just like those old Foxes.)

The Coyote makes more low end than the 5.0 H.O motor did.

 

[Jimdohc]

When accelerating through a corner you can run out of gear and have to shift. This upsets the chassis, it doesn't matter how good the driver is.

It's not that 5th gear is bad for cruising. It's that I don't want to have to throw the lever all the way up over there to go 50mph. Especially when it's geared so low that it buzzes the tires through the first two gears with only 400ish hp.

And having to row it all the way up to 5th to go 50mph is just stupid.

What style of corner requires enough of a speed change to require a gear shift? If your corner-entry speed is 70mph and corner exit is 100mph. If started in 3rd, you run-out of gear at ~85-90mph and are required to shift. Isn't it possible to start the corner in 4th gear and avoid the shift?

Wasn't 5th gears was specifically installed to cruise at 50mph? I don't understand how this is a problem. Is a 4 speed transmission better?

 

[Old 5 Oh]

The Coyote makes more low end than the 5.0 H.O motor did.

Seems to me the old 5.0 was rated at 300 ft/lb (at the crank) at 2800 RPM or so. And then it was fairly flat to about 4,000 before it fell like a rock. Allowing 13% for "at the wheels," that would be about 260 or so.

The Gibson Exhaust dyno shows the new Coyote to be at 260 ft/lbs "at the wheels" at 2,900 rpm. No info on what happens below that point. So I think from off-idle to maybe 3,000 rpm, the two engines are at least comparable. Obviously what happens from 3,000 to 6,500 is as different as it can be.

So, for driving in a civilized manner away from a stoplight (DD use) I still think the 3.08s are going to be really tall with a Tremec first gear. On a road course? I'll take your setup all day long.

 

[Five Oh Brian]

I got to thinking today after posting and a few thoughts came to mind...

• Ford has to meet CAFE ratings and tries to get away with as tall a gear as they can, while balancing some sort of minimum performance.
• Ford is claiming that the PP GT's match (or better) the '12/'13 Boss 302's lap times around whatever road course they chose.
• Ford likely spent some R&D time/money on finding the ideal gearing to satisfy CAFE requirements while matching the Boss 302's lap times.

Therefor, I think it's safe to say that Ford's engineers believe that 3.73's are the right gears for the PP GT's. Who are we to question their research.

 

[Todd15Fastback]

I got to thinking today after posting and a few thoughts came to mind...

• Ford has to meet CAFE ratings and tries to get away with as tall a gear as they can, while balancing some sort of minimum performance.
• Ford is claiming that the PP GT's match (or better) the '12/'13 Boss 302's lap times around whatever road course they chose.
• Ford likely spent some R&D time/money on finding the ideal gearing to satisfy CAFE requirements while matching the Boss 302's lap times.

Therefor, I think it's safe to say that Ford's engineers believe that 3.73's are the right gears for the PP GT's. Who are we to question their research.

Valid point!

What was the gearing in the Boss? Guess I will google and find out.

 

[sqidd]

Who are we to question their research

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

 

[Jmeo]

I got to thinking today after posting and a few thoughts came to mind...

• Ford has to meet CAFE ratings and tries to get away with as tall a gear as they can, while balancing some sort of minimum performance.
• Ford is claiming that the PP GT's match (or better) the '12/'13 Boss 302's lap times around whatever road course they chose.
• Ford likely spent some R&D time/money on finding the ideal gearing to satisfy CAFE requirements while matching the Boss 302's lap times.

Therefor, I think it's safe to say that Ford's engineers believe that 3.73's are the right gears for the PP GT's. Who are we to question their research.

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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[sqidd]

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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Have you had a car with a MT-82? If not throw out all of your gearing assumptions.

MT-82

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

 

[Jimdohc]

...Whipple Charge my 15' am I going to want to change my gears?
...overall difference between 3:73/3:55 may not be worth it.

Found a gear ratio calculator online. Was bored so I filled it out and took a screen shot. Hope this helps.
Maybe you could just launch using 2nd gear

 

[Jmeo]

Have you had a car with a MT-82? If not throw out all of your gearing assumptions.



MT-82



1st 3.66

2nd 2.43

3rd 1.69

4th 1.32

5th 1.00

6th .65

Nope newest has been 05' 5 speed.

So... What is the best gear for me once I supercharge ? Never track, just spirited driving. This will be my daily driver in spring/summer/fall. I am looking for just the best over all setup not at all concerned about mpg just seat of the pants feel.

Thanks,
Jaime


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[Jmeo]

Found a gear ratio calculator online. Was bored so I filled it out and took a screen shot. Hope this helps.
Maybe you could just launch using 2nd gear

Yah, I'm not that smart. Not exactly sure what I'm looking at lol.




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[Jimdohc]

If you're going supercharger. GT w/3.73 vs GT500 might be useful to compare.

 

[Jmeo]

If you're going supercharger. GT w/3.73 vs GT500 might be useful to compare.

So 3:31 is my best bet?


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[Jimdohc]

Yah, I'm not that smart. Not exactly sure what I'm looking at lol.

I'm sure you know from experience. With more torque you'll lose traction in low gears. If first is geared too low, it will be useless.

Without doing any math to calculate torque to the wheels. You can compare transmission & FD by looking at max MPH in that gear.

GT w/3.73 vs 3.55... max MPH is almost the same for the first three gears. So I don't think it will help get more traction on your supercharged car. 6th gear cruising at 70mph looks like RPM will drop about 100, ~3,000 to ~2,900. Doubt that will significantly change MPG.

GT500's first is good for bit over 60mph (depending on how far you're willing to rev it). GT w/3.73 can only do 40-42mph. Big change in transmission & FD ratio to accommodate supercharger torque. I would think it's mostly for traction reasons.

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