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Old 09-07-2009, 03:20 PM   #1
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Speedo is off...

in my 98 gt my speedo is about 7mph fast. now i dunno if the rear gears have been changed or not and i have stock size tires at wheels... what speedo gear should i look for...
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Old 09-07-2009, 11:53 PM   #2
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Re: Speedo is off...

if it was swapped it should have a new tag on the real diff. you can check ur year and model to see if that is the stock gears. if not you will need a speed calibrator.
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Old 09-08-2009, 12:05 AM   #3
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Re: Speedo is off...

i thought only the 99 and up year models had electronic speed sensors vs the speedo gear in the tranny?
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Old 09-08-2009, 07:16 AM   #4
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Re: Speedo is off...

that would be 94 and earlier
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Old 09-08-2009, 08:27 AM   #5
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Re: Speedo is off...

its doubtful they put a new tag on the rear end, you can by removing the rear cover or turning the wheel one full turn and counting the drive shaft revolutions, that will get you in the ball park. If you are about 7mph off it sounds like 3.73 gears to me~ there is no 100% way to get the speedo correct, you can probably use a speedcal unit as I had to in my 01 it works and is very easy to hook up.
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Old 09-08-2009, 12:58 PM   #6
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Re: Speedo is off...

7 mph off sounds like 3.73 gears to me too.

You can check what gears you have by jacking up the rear end and letting off the parking brakes so your rear wheels will spins (remember to block the front wheels!). Get under and spin the drive shaft until the rear wheel makes one complete turn and count how many times the driveshaft revolved. It will be a fraction and probably slightly off but it will give you an idea of what gears you have. Also mark a point on your driveshaft so you see when it completes a turn and to keep count.

You could also do between 70 and 75mph in 4th gear and see your RPMs which will tell you, e.g. 3730 RPM = 3.73, 3230 RPM = 3.23 gears, etc.

You can try picking up a new gear, a speed calibrator or get a new tune to fix the problem.
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Old 09-08-2009, 03:17 PM   #7
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Re: Speedo is off...

^ why did I never think about 4th gear 1:0... duh... yup do that lol
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Old 09-08-2009, 03:56 PM   #8
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Re: Speedo is off...

for every 1 rotation.....nothing happens?
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Old 09-08-2009, 05:49 PM   #9
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Re: Speedo is off...

haha thanks fellas! 3.73 gears seem pretty popular... is that better for 1/4 mile runs or what? i know zip about vehicle rear ends
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Old 09-08-2009, 06:17 PM   #10
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Re: Speedo is off...

3.73 is a good middle ground

generally the higher number gives you faster acceleration, but also means you have to turn relatively higher rpm for the same speed. So if you drop a set of 4.xx:1 in, your car will sound like its doing 100mph at 50mph woot (my dad got a ticket for this once back when he was my age "it sounded like you were speeding"). They also hurt your gas mileage.


I know dick about 1/4 mile runs, but generally I think you want to gear so that you are staying in your peak power band through the gears, but not having to shift into another gear right at the end of the run.
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Old 09-08-2009, 11:03 PM   #11
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Re: Speedo is off...

is there a programmer ya'll recommend? and ill stay with the 3.73's that car is fast enough haha... for now
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Old 09-09-2009, 08:21 AM   #12
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Re: Speedo is off...

you can not fix your speedo with a programmer, you will have to use a speedcal unit to correctly fix it unless you want to spend more on the speedo gear and remove the output shaft on the transmission to put in a new gear as well so it reads right~ just fyi a speedo gear wont be accurate if you change it.

3.73 usually the best daily driver choice for gearing
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Old 09-09-2009, 05:52 PM   #13
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Re: Speedo is off...

ight well scratch that... at least its reading faster than it really is mayb that will equal less tickets...
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Old 09-10-2009, 12:29 AM   #14
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Re: Speedo is off...

Also make u look like u have a slow mustang. I have a radar detector that has gps and it tells me how fas i'm going. Right now mine is about 2 MPH higher and i only have bolt-ons.
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Old 09-10-2009, 08:51 AM   #15
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Re: Speedo is off...

Hmm so if you are in 4th gear going XXXX rpms how do you figure out what rear gear you have as your MPH is going to be different...
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