3.73s worth it?

[06 T-RED S/C GT]

not with a Centri

With a centri, I agree. Twin screw/PD, however, I would not recommend 4:10's. From personal experience, in 5th gear at 61mph, my car is approx around 2200 RPM, then at 66MPH the RPM is between 2400-2500 RPM in 5th gear. With a 3:55 gear ratio, in 5th gear at 2200 RPM, your approx 10MPH faster at 71MPH and at 2500RPM in 5th gear you're again 10MPH faster over the 4:10's at approx 76MPH. At any rate, with the 10MPH gain difference that 3:55's provide over the 4:10's in 5th gear, is more than enough reason for making the switch back to the stock 3:55's IMHO

 

[Norm Peterson]

wouldn't that really depend on the track one races? FWIW - a track with short straights would that issue be a non-event, like Lime Rock witch is fairly local to me, the entire track is only 1.5 miles.

Depends on many things, the whole point with a TR3650 being to avoid having (or "wanting") to use 5th because it's so far away from 4th (and obviously, not getting momentary help from a torque converter). Depending on other things such as tire size, rev limit, cornering capability, and driver skill, 4th may or may not work at Lime Rock with 4.10's. I know I would not be able to use 4.10's at either of the NJMP courses without having to choose between grabbing 5th at least once a lap or just pacing along in 4th for a few hundred feet.

Pentalab nailed it by identifying the TR3650's specific gear ratios as causing the problem. 2nd is too far below 3rd and 5th is too far above 4th so you're stuck with a top speed defined by 4th and your axle ratio on the high end while hoping that 3rd is low enough for the slowest corner (because you'd be in and right back out of 2nd and not WOT in it much at all because that's when you'd be near the apex).

Just off the top of my head . . . ratio-wise, using 3rd, 4th, and 5th of an MT82 with 3.31's and 25.9" tall tires would be a better fit for a 4.6L Mustang on many road courses. I'm ignoring the apparent reliability issues with that transmission and looking at ratios only.


With 3.55's and 285/35-18 tires (short enough to duplicate OE-height tires and 3.73's), 3rd is a decent gear between about 55 and 96 mph, and 4th reaches out to about 126. If you were to go up to 19's, your tire choice should probably be 305/30-19 on 19x11's, which isn't enough taller to change those numbers by more than about 1 mph (26.2" vs 25.9", 127 vs 126 calculated @ 6000 rpm with a 3% correction for tire tread circumferential compression in the contact patch assumed - it's pretty close for most tires).


Norm

 

[Pentalab]

Big difference between a 25.9" tire...and a 28" tire. A 28" tire would imply the use of something like
a drag radial, in which case, a 4.10 would make more sense for street / strip use. 28/25.9 = 1.081 or 8.1% higher geared..which is a bunch. I can't see 28" strip tires being used for a road course anytime soon, like in Norm's case. Can you even install a 6R80 onto a 3V ? 5th gear on a MT-82 is a 1:1 ratio..which is the same as 4th gear on the TR3650 / 5r55s. The MT-82 would have some more wiggle room on a road course. 3-4-5 could be more effectively used on a MT-82.
Too bad these cars didn't come with either a quicker change rear end... or more optimized tranny gears. That isn't about to happen any time soon, so the best you can do is optimize for a specific range application.

 

[Norm Peterson]

Definitely the tranny. I can't see the 2.00 to 1.32 jump from 2nd to 3rd in the 3650 doing the drag racers much good either if you're going to be grabbing 4th before the traps anyway.

I think if I was ever going to get into swapping axle gears back and forth that I'd have a locker with the deeper gears for the strip and a T2R with the taller gearset for everything else. And shims/spacers for the pinion instead of crush sleeves.


Norm

 

[06 T-RED S/C GT]

Big difference between a 25.9" tire...and a 28" tire. A 28" tire would imply the use of something like
a drag radial, in which case, a 4.10 would make more sense for street / strip use. 28/25.9 = 1.081 or 8.1% higher geared..which is a bunch. I can't see 28" strip tires being used for a road course anytime soon, like in Norm's case. Can you even install a 6R80 onto a 3V ? 5th gear on a MT-82 is a 1:1 ratio..which is the same as 4th gear on the TR3650 / 5r55s. The MT-82 would have some more wiggle room on a road course. 3-4-5 could be more effectively used on a MT-82.
Too bad these cars didn't come with either a quicker change rear end... or more optimized tranny gears. That isn't about to happen any time soon, so the best you can do is optimize for a specific range application.

Sure, a 28" tire is fine if you're running a staggered setup with 28" just in the rear. However, for those running a square setup in all 4 corners, a 28" inch tire more than likely won't clear the inner and outer fender up in the front and even then your tire selection is pretty much limited in 28" diameter 18" sizes to begin with anyhow

 

[NUTCASE]

My car came with 3.55s and I went 3.73. I felt a little extra push, but really for doing road trips I could have saved my money and left the 3.55s in. I do think I made out on road trips and daily driving with the 3.73 compared to 4.10s, but on the track your gonna wish you had 4.10s.

If I could go back in time I would have done 4.30s and just not DD it. I think I would have been way happier.