I remember seeing a post (but can't find it now) about how to determine what gear to use to get the speedometer somewhat close to the correct speed. Iam running 17" wheels with 3.73's in the back and a Tremec t-5 with standard gear ratio's. Anybody want to help me out and give me the formula, or better yet what gear do I need?
Thanks for the help
Yes, tires have everything to do with it. Without knowing the tire's actual diameter, you can't know the RPM. An RPM is required to figure out what gear is required.
Speedometer Drive Gear x Rear Gear x Tire RPM / 1000 = Speedometer Driven Gear
I have an 89 LX with 3.73's and 17 in. wheels all around and I got the red speedo gear from 50resto. If you go to that site and search for speedo gears you will find a link that tells you what gear you need. I know my speedo is still kind of off so tires have everything to do with it. I got new tires so I'm not changing wheels anytime soon.
tire height has alot to do with the speedo being off even if you change the gear for it. on my 89 i had 3.73's with 245-45-17's and the red 21 tooth speedo gear and it was only like a mile or 2 off from factory.
determine your tire height, if its close to stock, then compensate for the right speedo gear, if not you'll have to deal with it till your new/used tires run out of tread
__________________
84 Predator Mustang GT302R [Gone]
89GT: built fairly good (sold)
92LX: 4cyl to 5.0 conv. completed on the cheap (sold)
88GT: 302/AOD,stock 2.73's and traclok, LT's, custom 2 1/2" exhaust made by me with thrush chambered mufflers dumped at rear axle, hayden electric fan..more to come, daily driver.
what year is your car, because i believe it was 90 or 91 and up use the 8 tooth drive gear on the output shaft of the transmission, if this is your case you need the 7 tooth yellow gear, and you will be cherry. hopefully you have 245-45-17 tires, because since that is a stock size, it wont be that far off. if you are unsure of which gear you have inside your transmission, the 8 tooth will be a green, almost a glow in the dark looking color, while the 7 will be a distinct yellow. (peek behind where the speedo cable goes)
__________________
Sr. Master Ford nerd.
saludos caballeros del ovalo azul
Well, he said he had a Tremec T-5. Tremec didn't start making T-5s until '98. Mustangs w/ a V8 haven't had a T-5 since '95, only V6s did then. So either it isn't a Tremec T-5, or it is and it's from a V6 Mustang or it was bought new from a vendor.
Well, he said he had a Tremec T-5. Tremec didn't start making T-5s until '98. Mustangs w/ a V8 haven't had a T-5 since '95, only V6s did then. So either it isn't a Tremec T-5, or it is and it's from a V6 Mustang or it was bought new from a vendor.
why couldn't it be a tremec T-5 from a V8?
__________________
Currently sans Mustang
Gone but not forgotten: '83 Charcoal GT 5 spd,
'89 Black/Titanium GT 5 spd, '90 Black LX Hatch 5 spd, ...
I just went to the local Ford drealership with SVO dept...I gave them the type transmission, rear end ratio and wheel size and they found the correct speedo gear for me.
392-STE-17271-J
Automatic 8-Tooth 3.55 Ratio / Black 23 Speedo Gear $15.95
392-STE-17271-J
Automatic 8-Tooth 3.73 Ratio / Black 23 Speedo Gear $15.95
392-STE-17271-J
Automatic 8-Tooth 4.10 Ratio / Black 23 Speedo Gear $15.95
Product Details
Once you change the gears in your differential, you need a new speedometer gear to recalibrate your speedometer. They're inexpensive, and only take a few minutes to change.
These Recalibration Gears fit 1979-1998 Mustangs. Please check our Hot New Product Section for information on 1999 and 2002 Mustangs.
Instructions: Note transmission type, model and resulting drive gear. Cross reference with your rear-end gear ratio to find the correct "recalibration" gear for your application.
Manual Transmissions with 7 Tooth Drive Gear
1983-1989 T-5 OEM
Pre 1993 T-5 Aftermarket
1996-1998 Cobra T-45 OEM
All Tremec Transmissions
Manual Transmissions with 8 Tooth Drive Gear
Correcting Speedometer Gears on a Fox Body 
the first year for a mustang 5 speed 5.0 is a 1983. at least from my memory, that when i started working at a ford dealer, big G
