S650 R&P - 3.15 v 3.55 v 3.73 v 4.09 (updated to include 3.15 R&P)

[Garcia172]

S650 R&P - 3.55 v 3.73 v 4.09

A few people here have done it. Please chime in.

What's are shops getting to change the R&P?

*This is based on the stock manual transmission, base 300A, S650 Mustang GT 18" OEM wheel & tire package > 255/45r/18 tire

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[MNPony]

I went from 3:15 to 3:55 but I did complete differential. Local dealer did the work. About $2300 all in.

 

[Will2]

I went from 3:15 to 3:55 but I did complete differential. Local dealer did the work. About $2300 all in.

Do you feel a significant difference in acceleration or fuel efficiency?

 

[MNPony]

Do you feel a significant difference in acceleration or fuel efficiency?

I honestly pay little attention to fuel economy with this car but the change absolutely woke the car up. Coyote's have notoriously poor low end torque and the lower gears really help keep the engine in the power band. It's definitely more fun to drive now.

 

[DHGOOD]

I have a Dark Horse 6-speed and a 2024 GT 6-speed. If I could snap my fingers and change I’d switch the two. The GT is fine with the 3.55s but would benefit from the 3.73s in the DH because its transmission is wider ratio and has two overdrives. The DH is kinda geared too low with 3.73a and the 19” wheels with the 30 tires and 1:1 5th. Pretty big 5-6 drop but it’s ok on the interstate in 6th just doesn’t have a good gear on a 2-lane road. Definitely a GT manual with 20” wheels would be good on 3.73s but a 4.10 would be too much for me. And I’ve had a lot of vehicles cars and trucks with 4.10s.

 

[schuvwj]

I went from 3:15 to 3:55 but I did complete differential. Local dealer did the work. About $2300 all in.

Was there a big difference? I also have a 3:15.

 

[MNPony]

Was there a big difference? I also have a 3:15.

Yes. Really woke the car up. So much more fun to drive now.

 

[Garcia172]

I went from 3:15 to 3:55 but I did complete differential. Local dealer did the work. About $2300 all in.

I saw that Ford sells the complete 3.55 differential assembly. I wish did the same for the 3.73. So much easier than replacing just the R&P.

 

[schuvwj]

manual ratios, adding 3.15 is just proportional scaling from its 3.55 column. At 7,000 rpm:

Gear	3.15	3.55	3.73	4.09
1st	55.2 mph	49.0	46.6	42.5
2nd	85.0 mph	75.4	71.8	65.5
3rd	125.8 mph	111.6	106.2	96.8
4th	178.9 mph	158.7	151.0	137.7
5th	219.6 mph	194.9	185.5	169.2
6th	287.5 mph*	255.1	242.8	221.4

Sixth-gear cruise RPM

Speed	3.15	3.55	3.73	4.09
65 mph	1,583 rpm	1,784	1,874	2,055
80 mph	1,948 rpm	2,195	2,307	2,529
85 mph	2,070 rpm	2,333	2,451	2,687

 

[schuvwj]

288 mph is moving in a Mustang!!!

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