optimum rear gear for GT auto - calculations

I've been looking more into the idea of swapping in some 3.73s in place of the 3.27s.

According to my calculations on paper, if the engine is to be shifted at

5,500 rpm, with a 3.27 rear, the 1-2 shift occurs at about 44 mph, 2-3 shift at 81 mph, and 3-4 shift at 125. However, with a 3.73 rear, the 1-2 shift would occur at 39 mph, 2-3 shift at 71 mph, and 3-4 shift at 110 mph. From this you can see that the 3.73 rear allows peak hp to occur earlier (of course), and specifically help 0-60 mph times, and 1/4 times given the tranny ratios. It puts more average hp under the 0-60 mph run and 1/4 mile run (while not adding an extra shift point to either run.). Also, in terms of highway cruising, with the 3.27 rear, the engine should spin at around 2154 at 70 mph, while the 3.73 rear would make for 2457 rpm, just 300 more rpm. Not too bad. A change any smaller than this would not be worth the effort and expense of the gear swap... so anyone looking to try a 3.55, it's not worth it unless someone gives you a free Mach 1 rear. Going up past a 3.73 to a 4.11 would make for more useable street power, might even be perfect for best possible 0-60 and 1/4 mile times, but then the cruising revs start to climb up a bit higher than "optimum". 3.73's seem to be the perfect compromise between max street performance and good economy / high-speed cruising behavior.

Once again... does anyone know of a Mustang specialty shop anywhere in the north east that can be trusted to do a rear gear swap? I'd never let any clowns touch a rear axle unless they had a stellar reputation. So far I've read about a dozen stories on the web from guys who had their gears swapped only to result in failures and problems... have not yet heard one gear swap success story, at least not for the late model Stang. I had a highly experienced ace race mechanic rebuild my old Trans Am posi rear, and he did a perfect job, but that was 12 years ago... not sure if I can find that guy anymore, or if he is as swift with Fords as he is with GMs. I'll gladly pay top dollar to any super ace Ford mechanic or rear-end mechanic that has a flawless rep for doing such work on these rears. If anyone knows of such a person or shop, please post. Thanks.

Reply to
GT-Vert-03
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Changing the rear gear in an auto trans GT requires a chip. Both the speedo recal and changing the shift points are done using a chip, and can't be done any other way with the auto trans cpu.

Reply to
Mike King
Reply to
GT-Vert-03

I have not had any experience with the 4.6 so keep that in mind, but I have went from 3.27 to 3.73s in 302's,347's, and 351s over the years and that would be the minimum jump to do IMO. To be honest in my fuel injected Mustangs with automatic transmissions the change wasn't that noticeable. About the only thing was noticeable was the increased cruise rpm. For example back in the 5.0 liter days when most cars had 2.73s going to 3.55s was a 30% increase in gear ratio, or 36% if you went with 3.73s. Now that are starting with 3.27s going to 3.73 is only a 14% increase, and 4.10s would be 25%. I have seen a lot of people in different fourms say "I wish I had went with 4.10s" after putting in 3.73s. I know I did with my 95 GT. While the price of the gears isn't terrible the installation costs do add up so make sure you really get what you want the first time. Form your posts here about your diving style I'd highly recommend 4.10 with an automatic, unless you do some serious highway mileage all the time. Another thing to think about is that a 4.10 gear with the .7 overdrive ratio works out to

2.87 final drive. I don't know if your old enough to remember cars with out over drive, but 2.73s gears were pretty common then.

MadDAWG

Reply to
MadDAWG

Do a search on

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. You will find more info on gear swaps there than can be typed here. Also ask your question for shop recommendations in your area. I'm sure you'll get what you're looking for.

Reply to
Bard

Not entirely true, mon frere...

The speedo itself can be corrected with a calibration device like the Abbott ERA or the Dallas Mustang Speedcal for about $200 less than the cost of a custom chip.

The chip will definitely *help* with the shift schedule, but it has been my experience that you can live without it on an automatic equipped car and 3.73 gears. It is a different story with anything higher than 3.73 gears, however, as you start tagging the rev limiter on every shift.

Just food for thought. ;-)

Reply to
Tungsten

What part of the North East are you in? I have heard very good things about Jim D'Amore's shop over in Jersey. JDM Performance. A buddy of mine here in Nashville took his Lightning over to them and was very happy with the results.

Reply to
Tungsten

Oh, I thought the auto trans couldn't use a speedcal for some reason. I also thought the auto's shift points would be all f'ed up unless the programming was messed with. I guess that's why I drive a 5-speed, less to worry about. BTW, long time no see...

Reply to
Michael Bolden-King

Oops, didn't change the sig on the comp at home....

Reply to
Michael Bolden-King

thought the auto's shift points

I drive a 5-speed, less to worry

Yeah, the Speedcal works on the 1996-98 automatic as well as on the new cars with both auto and 5-speed. Or at least I think it supports both the new auto and 5-speed. 5-speed for sure. :-)

Thanks for the re-welcome... I always enjoy lurking here!

Reply to
Tungsten

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