2003 GT stock rearend????

rickster99 · Dec 11, 2004

Guys help me out here. I have been thinking on changing the rearend on my car however I thought I would do some research first. You guys are my first source of information so here it goes.

1. What is the rearend size on a stock 2003 GT?
2. Did they provide different sizes without special ordering or is it all standard on 2003 GT.
3. If I do change it what size should I go with? I noticed that 411 is a bit much for a daily driver?
4. What gains am I looking at performace wise?
5. Do you recommend a specific brand?

Any info you can give me on this would be great. I am all ears.

bdcardinal · Dec 11, 2004

GTs have a Ford 8.8" diff. they mostly came with 3.27 gears. i do not believe there was a factory option for different gearing. most people change to 3.73 or 4.10 gears. for gears, Ford Racing are fairly inexpensive, around $140, and are very high quality.

rickster99 · Dec 11, 2004

bdcardinal said:
GTs have a Ford 8.8" diff. they mostly came with 3.27 gears. i do not believe there was a factory option for different gearing. most people change to 3.73 or 4.10 gears. for gears, Ford Racing are fairly inexpensive, around $140, and are very high quality.

Ok cool. So the question remains, what kind of gain am I looking at in the 1/4 mile for example.

bdcardinal · Dec 11, 2004

a few tenths maybe, maybe up to half a second or a full second depending on planet alignment, and other mods.

jrc_03_gt · Dec 11, 2004

4.10's

tomustang · Dec 11, 2004

if you throw 4:10s you'll regret this post. but if you want to change the entire rear, go for the 9 inch converson.

Greg Barnett · Oct 30, 2016

OK guys. I have no idea how to determine which gear I have. 2003 gt. I have 19" rear wheels. My speedometer is off about 7 mph compared to gps. I have a 5 speed and and at 2500 rpm at 70mph on the gps. I just bought the car and don't know any history with it. Help!!!

Az Pete · Oct 30, 2016

Find what you have first..
https://www.google.com/search?q=how...droid-samsung&sourceid=chrome-mobile&ie=UTF-8

trinity_gt · Oct 30, 2016

Super-rough calculation for you:

Assuming your 19s have a similar-to-stock outside diameter, they'll turn roughly 810 revolutions per mile. At 70MPH you'd see roughly 945 wheel RPM. If 5th gear in the transmission is 0.62:1 then engine RPM is given by:

Engine RPM = Wheel RPM x Rear Gear x Transmission Gear

Solving for Rear Gear you get

Rear Gear = Engine RPM / (Wheel RPM x Transmission Gear) == 2500/(945 x 0.62) == 4.27

4.27 is pretty close to the "standard" value of 4.10:1 for Ford 8.8s so I'd wager that you've got 4:10s in the back assuming all the info you've given is accurate and the assumption of revs/mile on your wheel/tire combo is right.

mogs01gt · Oct 31, 2016

Greg Barnett said:
OK guys. I have no idea how to determine which gear I have. 2003 gt. I have 19" rear wheels. My speedometer is off about 7 mph compared to gps. I have a 5 speed and and at 2500 rpm at 70mph on the gps. I just bought the car and don't know any history with it. Help!!!

I used an axle calc but since I do not have the full specs of your tires, I can only guess. Supply your full tire specs and I will correct it for you.

rear gear: 4.10
tire width: 245
aspec ratio: 35
wheel dia.: 19

68.7mph @ 2500rpm

Mustang5L5 · Oct 31, 2016

need tire sizes to answer that question accurately

kalvick · Nov 3, 2016

mogs01gt said:
I used an axle calc but since I do not have the full specs of your tires, I can only guess. Supply your full tire specs and I will correct it for you.

rear gear: 4.10
tire width: 245
aspec ratio: 35
wheel dia.: 19

68.7mph @ 2500rpm

what does tire width have to do with anything?

Mustang5L5 · Nov 3, 2016

kalvick said:
what does tire width have to do with anything?

Well it's kinda important to determine overall tire diameter, which you need in determining RPM at a given speed with a certain gear. it's part of the calculation

A 245/35/19 is 25.75" in diameter. A 275/35/19 is 26.58" in diameter.

mogs01gt · Nov 3, 2016

Mustang5L5 said:
Well it's kinda important to determine overall tire diameter, which you need in determining RPM at a given speed with a certain gear. it's part of the calculation

A 245/35/19 is 25.75" in diameter. A 275/35/19 is 26.58" in diameter.

this

35thGTLimited · Nov 6, 2016

Yeah it definitely seems like it has aftermarket gears. My current car, '00 GT auto, does 68-70mph at 2,000rpm, stock gears. It seems like overdrive in my auto is exactly the same as my old 5 speed's 5th gear, is this true? It's been years since I had the 5 speed and it's hard to recall exactly, but it seems like it.

I wonder what 3:73 or 4:10 gears would feel like in my current car.. Wish I could just drive someones car so I could know before an upgrade.

Anyway, I'm also interested, I hope this guy comes back and posts his tire sizes. I'm considering the same upgrade.

Neuron · Nov 6, 2016

I just bought an '02 GT Auto(bad knees) I am on the fence for 3.90 or 4.10. My '98 Cobra had 3.27(245 45 17) and I went with 3.73 and was disappointed in the difference it made although the gas mileage wasn't much worse at all if any. Definitely installing an oil cooler for the tranny first though. If it was a stick no hesitation on the 4.10 but the torque multiplication may make the 3.90 better, food for thought I guess.

Greg Barnett · Nov 9, 2016

mogs01gt said:
I used an axle calc but since I do not have the full specs of your tires, I can only guess. Supply your full tire specs and I will correct it for you.

rear gear: 4.10
tire width: 245
aspec ratio: 35
wheel dia.: 19

68.7mph @ 2500rpm

They are 275/45/19

Greg Barnett · Nov 9, 2016

mogs01gt said:
I used an axle calc but since I do not have the full specs of your tires, I can only guess. Supply your full tire specs and I will correct it for you.

rear gear: 4.10
tire width: 245
aspec ratio: 35
wheel dia.: 19

68.7mph @ 2500rpm

Thanks for the info. My tire size is 275/45/19

Greg Barnett · Nov 9, 2016

mogs01gt said:
I used an axle calc but since I do not have the full specs of your tires, I can only guess. Supply your full tire specs and I will correct it for you.

rear gear: 4.10
tire width: 245
aspec ratio: 35
wheel dia.: 19

68.7mph @ 2500rpm

Ok. Correction. 275/40/19

Greg Barnett · Nov 9, 2016

35thGTLimited said:
Yeah it definitely seems like it has aftermarket gears. My current car, '00 GT auto, does 68-70mph at 2,000rpm, stock gears. It seems like overdrive in my auto is exactly the same as my old 5 speed's 5th gear, is this true? It's been years since I had the 5 speed and it's hard to recall exactly, but it seems like it.

I wonder what 3:73 or 4:10 gears would feel like in my current car.. Wish I could just drive someones car so I could know before an upgrade.

Anyway, I'm also interested, I hope this guy comes back and posts his tire sizes. I'm considering the same upgrade.

I posted reply. They are 275/40/19. I appreciate all the help

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