So I thought by doing my research I could get my speedo right but apparently not, lol. I have an 85 GT with stock speedo, TKO trans, Black speedo gear, 3.55 gears, and 235/45/17s, but the speedo is off by 10-15 mphs. I even posted on here and everybody said that with a tremec and 3.55s to use the black gear. I know the tires arent stock but they arent that off to cause 15 mph difference.
Cant get speedo to read right!!
- Thread starter SmockDoiley
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You just change the speedo gear, but since I'm already using the one I'm suppose to have and its wrong; I guess Im screwed. Just get under your car and follow where the cable goes into the tranny. There should be one small bolt holding it in. Then just pull it out and remove the little c clip holding the gear on. Its really easy.
Using the formula -
Driven Gear Teeth = Drive gear teeth (7) x axle ratio (3.55) x tire revs/mile (826) divided by 1000 you are at 20.52. You could move up to a 21 tooth gear and see what that does or you might have a sender problem. My 87 GT speedo was broken about half of the time I owned it.
Driven Gear Teeth = Drive gear teeth (7) x axle ratio (3.55) x tire revs/mile (826) divided by 1000 you are at 20.52. You could move up to a 21 tooth gear and see what that does or you might have a sender problem. My 87 GT speedo was broken about half of the time I owned it.
Michael Yount
Mustang Master
It takes time to explain - and it doesn't lend itself to being written about. Talking live speeds up the process on this one.
You need to know 1) how many teeth on your drive gear (the one inside the tranny on the output shaft)? 2) how many teeth on your driven gear (the black one you installed)? 3) Exactly how far off is your speedo with your current gears and tires? (10-15 isn't close enough - use the mile markers along the interstate to check the odometer, preferably over a 10-20 mile run).
I'll make up some numbers to show you how to do the calculation. Let's say you've got an 18 tooth driven gear, and a 7 tooth drive gear. And let's say your speedo is reading 20% high - that is it's reading 72 mph when you're going 60 mph. So, you need to adjust the ratio of your drive to driven gears in the same proportion that your speedo needs adjustment. A higher number of teeth on the driven gear, or a lower number of teeth on the drive gear will speed the speedo up.
7/18=.39 You need a 20% adjustment in that ratio, in the direction so the speedo shows a faster speed. .39X.80=.31 So you need a set of gears that results in a .31 ratio - or something close. Keeping your 7 tooth drive gear, a 23 tooth driven gear results in a .304 ratio - pretty close. However, it's been my understanding that the 23 gear teeth don't always last a long time. So, an alternative is to switch to the 6 tooth drive gear. If you did that, 6 teeth on the drive gear and 19 teeth on the driven gear gives you a .315 ratio - pretty close.
Remember, all this was based on made up data. You need to know exactly how far off your speedo is, and what gears are causing that. Then you can use the methodology above to solve the problem. Capice? By the way, you can't count the number of teeth on the drive gear by shining a flashlight through the tranny hole. Use the color of it to match up the number of teeth. To count them, you have to be able to view the drive gear from the side - which requires pulling the tail shaft housing off.
You need to know 1) how many teeth on your drive gear (the one inside the tranny on the output shaft)? 2) how many teeth on your driven gear (the black one you installed)? 3) Exactly how far off is your speedo with your current gears and tires? (10-15 isn't close enough - use the mile markers along the interstate to check the odometer, preferably over a 10-20 mile run).
I'll make up some numbers to show you how to do the calculation. Let's say you've got an 18 tooth driven gear, and a 7 tooth drive gear. And let's say your speedo is reading 20% high - that is it's reading 72 mph when you're going 60 mph. So, you need to adjust the ratio of your drive to driven gears in the same proportion that your speedo needs adjustment. A higher number of teeth on the driven gear, or a lower number of teeth on the drive gear will speed the speedo up.
7/18=.39 You need a 20% adjustment in that ratio, in the direction so the speedo shows a faster speed. .39X.80=.31 So you need a set of gears that results in a .31 ratio - or something close. Keeping your 7 tooth drive gear, a 23 tooth driven gear results in a .304 ratio - pretty close. However, it's been my understanding that the 23 gear teeth don't always last a long time. So, an alternative is to switch to the 6 tooth drive gear. If you did that, 6 teeth on the drive gear and 19 teeth on the driven gear gives you a .315 ratio - pretty close.
Remember, all this was based on made up data. You need to know exactly how far off your speedo is, and what gears are causing that. Then you can use the methodology above to solve the problem. Capice? By the way, you can't count the number of teeth on the drive gear by shining a flashlight through the tranny hole. Use the color of it to match up the number of teeth. To count them, you have to be able to view the drive gear from the side - which requires pulling the tail shaft housing off.
Check to see if there is a speedometer shop in your area. If there is, ask about the Stewart-Warner speedometer gearbox. I believe it is either a 777 Series Drive Joint Kits and Parts or 666 Series Drive Joint Kits and Parts. It is a small gear box that fits between the speedo pickup gear on the transmission and the speedometer. It has quick change gears that allow you to choose almost any tire and rear end gear ratio you want. This will allow you to get the accuracy with within 2%-3%. The drawback is that it isn't cheap.
You could always take it to san diego speedo/tach off of rosecrans and they can verify and or recalibrate your speedo to read correctly. It may be in the speedo head, if not they usually can tell you what gear to use if you know the amount of teeth on the drive gear. Ive never seen a car exact mph with just change of the driven gear but 10-15 mph is alot more than the 2-5 i usually see.
