blong10

So I have a 1999 mustang gt with the 4R70w transmission it is a automatic. I drive probably around 20 miles on the highway a day and 10 in town for my job. And I have a camp about 80miles away I probably go there once a month with my car so about 160 miles total. I'm pretty sure my car has 3.27 gears. I run about 2,000rpm at 65mph on the highway. I was leaning towards 3.73 how much would the rpm goes up if I went with 3.73. Would I even notice a difference coming from 3.27 to 3.73. Thanks

Got Torque

I cant speak for the difference between stock and 3.73's as the only Mustang I've driven with a stock rear end was bone stock versus a slightly modified with 3.73's. However, I have 3.73's on my 2001 GT and at around 70 or 75mph (I cant remember exactly) I run at about 2500 RPM even so if you only run 65 on the highway you should be fine, probably at around 2300 RPM's. If you do a lot of highway driving I would not go any higher than 3.73's.

Drastang

for your auto I would do the 3.73. I have 4.10 in a 5 spd and it is perfect match. been told that 3.73 is comparable on auto's. you will lose a good chunk of your top end on the automatics fyi.

WhiteFoxGT

auto needs 4.10s, as well as 5spd (as long as forced induction isnt in the near future)

Dont forget when you change gears in an auto mustang you have to use a handheld tuner (SCT x3 or equivalent) to change gear ratio settings for computer... it wont shift right

Gadgetech

I replaced the 3.27 gears with 3.73 in my 4R70W and it was a great, but required because my rear end was going out, upgrade. I used the SCT to calibrate the speedo and that's about it. What's really nice is about 70mph or so, a slight push of the pedal gets a much better result than with the 3.27 gears. I don't mind the higher RPMs on the interstate but I don't drive it that often either. I've had some emission control problems with the car that I'm now being forced to correct since I'm trying to register the car in Georgia, afterwards I may look at some other bolt on mods.

Z28KLR

Swapping gears and realizing all of the wonderful things a shorter ratio has to offer isn't as simple with an auto trans equipped Mustang as it is with a manual. With a manual, you install gears and recalibrate speedo and go. Emphasis on "go" lol.

I don't have an auto so I can only speak to what I know of others' experiences, but from what I understand just doing gears by themself is a bit underwhelming. Also highly recommended is a higher stall speed torque converter, and definitely need to reprogram the shift points and line pressures. If it were me, I would do the above plus J-Mod valve body. After that it will f***ing scream.

To address your original question though, if it were my car I would do 4.10's or .30's. Maybe 4.56 if you're feeling a little extra spicy, but those could begin to cut in on the daily driver abilities of the car.