How much will my speedometer be off installing my 3.90 gears?

Seraphitia102 · Jun 9, 2006

I let someone borrow my superchips tuner and I'll be getting it back next week, but I want to do the install on my new gears this weekend. Does anyone know how much my speedometer be off with 3.90 gears?

four.six · Jun 9, 2006

alot... my speedo was hella off just from 3.73s. just fix it when u get your tuner back.

Cobra Jay · Jun 9, 2006

It's going to be off quite a bit. Not only will your speedo be off, your odometer will be off as well. When I went to 4.10s, I didn't install my Speed Cal right away and my odometer was off about 3-4 10ths a mile.

Seraphitia102 · Jun 9, 2006

So will it be off like to the point where for example, if i'm driving at 30mph, will my speedo read 50mph?

langod · Jun 9, 2006

It should read roughly 20% high.
40mph actual = 48mph displayed
60mph = 72
75 mph = 90

And your odometer wil be clicking over 20% faster -- if you drive 50 miles, your odometer will say you went 60.

hognutz · Jun 9, 2006

it will be like 10mph or so. it kind of varies based on speed and gear since it is rpm related.

short story long you will not want to do it as it will add extra milage to you car.

you can go to a gear calculator and figure you speed based on rpm.

for example 70mph with with 3.90's is 2400rpm where 70 is 2000 rpm with 3.23. that is in 5th. so you can figure you speed out based on the tach. the problem is you odometer is going to be off and off in a direction that depriciates you car faster. it is worth the money to fix it. I bought a used diablo for $250 on ebay. you can have you computer reflashed just for gears for like $100 though.

jeffnoel · Jun 9, 2006

http://www.wcfords.com/stuff/calcs/EGTcalc/egtcalc.php

spederman · Jun 9, 2006

Good luck with those 3.90's especially if you got Motive's. I am having a hell of a time with mine, I wish i would have just got FRPP and picked either 3.73 or 4.10's. My first set whined like mad and i got them replaced through motive. We chalked up the first set to a bad install by some performance shop, but not positive and it was never proved. I got the second set installed by a different person and they are even louder. Now i know there are people on here that have them and it worked out perfect, but if your luck sucks like mine, prepare to have a nightmare of a time. I know people will say gears are gears and its all in the install, but i am now a believer that go with nothing but FRPP. The guy that installed the second set is a Ford Master tech with 17 years of experience. He has done over 1000 gear installs, so he knows what he is doing. Now you might get lucky and yours come out perfect, but if you want to avoid headaches and stress, send the gears back for some FRPP. I am stuck with mine until i go procharged and then i will just get them swapped out for some FRPP 3.73's.

drag0nfan's04GT · Jun 9, 2006

Here's another link that I used and it seems pretty accurate. This way you can know the difference at different speeds.

cooter_11 · Jun 9, 2006

Do you have to get a new speedo gear if you have a pre-99 GT when you put in taller gears?

squares98 · Jun 9, 2006

cooter_11 said:
Do you have to get a new speedo gear if you have a pre-99 GT when you put in taller gears?

gear or speed-cal

Seraphitia102 · Jun 9, 2006

spederman said:
Good luck with those 3.90's especially if you got Motive's. I am having a hell of a time with mine, I wish i would have just got FRPP and picked either 3.73 or 4.10's. My first set whined like mad and i got them replaced through motive. We chalked up the first set to a bad install by some performance shop, but not positive and it was never proved. I got the second set installed by a different person and they are even louder. Now i know there are people on here that have them and it worked out perfect, but if your luck sucks like mine, prepare to have a nightmare of a time. I know people will say gears are gears and its all in the install, but i am now a believer that go with nothing but FRPP. The guy that installed the second set is a Ford Master tech with 17 years of experience. He has done over 1000 gear installs, so he knows what he is doing. Now you might get lucky and yours come out perfect, but if you want to avoid headaches and stress, send the gears back for some FRPP. I am stuck with mine until i go procharged and then i will just get them swapped out for some FRPP 3.73's.

i got motive gears because I heard they were really good. thanks for sharing your experience, i hope my gears don't whine. i've heard of most motive gear installs went pretty well.

how did your car feel after having those gears installed?

How much will my speedometer be off installing my 3.90 gears?

Seraphitia102

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