Speedometer off by 20mph.

1FatPony · Oct 1, 2006

I bought my 87 gt about 3 weeks ago and Im in the process of dropping a new engine in but on my way home from the previous owners house I noticed that the speedometer read 20mph faster that I was actually going. I am not sure if this car has different gears or not but I do know the tranny was replaced about 300 miles ago. Could a new tranny cause a 20mph difference. There are two speedo gears right? one in the tranny and one outside. Could I just change the outside gear to compensate for the inside one? How would I know which gear to get?
Thanks

marine_2003 · Oct 1, 2006

your speedo gear needs to match up with the gears in the rear end. the gears that go into the tranny, are colored for each rearend gear.

speedytang · Oct 1, 2006

Yes if they replaced the 5.0 with a 2.3. You will need to find out the gears in the rear first then find out if they replaced the transmission with a 4cyl.

1FatPony · Oct 1, 2006

how can I find out what gears are in the rear end and what speedo gear is in the tranny?

Darkwriter77 · Oct 1, 2006

The easy way is if the rear-end still has the factory tag on there, it will say 3.08 or 2.73 or maybe 3.27somewhere on there, assuming the gears haven't been swapped out. Then, get on 50resto.com and look up the corresponding speedo gear you'll need for that rear-end ratio.

The hard way is to jack up the rear of the car and measure out how many times your rear wheels spin for one full revolution of your driveshaft. Mark your starting positions on the driveshaft and one wheel (use a grease pencil or chalk or something on the tire and fender), put the car in neutral (make sure your front wheels are chocked up securely), and start turning the driveshaft by hand. You'll have to either have someone watch the wheel's rotation for you as you turn it, or mark the tire on the inside where you can see it while you're laying there. It's a ghetto method and darn near impossible to guess EXACTLY which gears you've got by doing this method, but it'll at least get you in the ballpark. Example: If the wheel turns just a tad bit more than four full rotations per one full rotation of the driveshaft, you've got 4.10's.

There's a huge difference between tall stock gears like 2.73's and 3.08's versus the more common aftermarket gears like 3.73's and 4.10's, but if there's 3.55's in there, it might be tough to really distinguish between 3.27's and 3.73's unless you're reeeeeeally exact with measuring it out.

Hope this helps!

JChalfan · Oct 1, 2006

If I remember correctly, even with 5.0 T5's, there was a difference in the speedo drive gear on the output shaft over the years. Take a look at this chart, it shows which trannys had which gear:

http://www.geocities.com/mgnedell/notchback/tech/speedo.html

Of course if you don't know what your trans came from, that makes it more difficult. You can probably pull out the speedo cable, look up in the hole in the trans and count the teeth. That's probably the easiest way.

To figure out roughly what rear end gears you have, jack the back of the car up and put it on jackstands. Put the trans in neutral. Put a mark on the driveshaft and one of the rear tires. Start turning the driveshaft by hand, and count how many turns the driveshaft makes per one rotation of the tire. If you have 3.08's, the driveshaft should turn 3 times for the wheel to make one complete turn. 2.75 times = 2.73's, 3.5 times = 3.55's, etc...

Or you can pull the rear cover off the diff and count the teeth on the two gears. Divide the big number by the little number, and that's your gear ratio. It's a bit messier than counting rotations though.

EDIT: Damn it darkwriter, I guess I took too long to type my response.

Jeff

trbkrb · Oct 1, 2006

Darkwriter has it backwards (easy to do), JChalfan is correct. The driveshaft turns more than the tire. 2.73 gears = 2.73 turns of driveshaft per one tire rotation.

Then you have the issue of what tranny you have. If the replacement tranny has an 8 tooth drive gear (the one in the transmission) and the driven gear (the one on the speedo cable) is the same one from the 87 tranny, then your speedo reading would be off also. The 8 tooth gear is in 90 up T5s, while the 7 tooth gear is in the earlier models. I'm willing to bet that is the problem. The 8 tooth gear is spinning the speedo cable faster, resulting in a higher reading. Here is a website with all the ratios and gear info: http://www.steeda.com/store/-catalog/speedometer.htm

GTOreturns · Oct 1, 2006

Just for a quick guesstimate. What rpm's are you at when do 60mph in 5th gear.

1FatPony · Oct 1, 2006

Thanks for the info! Is there also a way to find out what gear is in the tranny without taking the whole thing apart? Like could I just look in there and see what color the gear is?

I am not sure what rpms im at at 60mph in 5th gear...i dont have an engine in yet . By the end of this week I should be running with the new engine and this speedo problem will be my first project.

1FatPony · Oct 1, 2006

Is there someway to check the speedo gear in the tranny while the engine is out? I have the engine out right now and I plan on putting it back in tomorrow but if its easier to check the gear with it out I want to do that.

Flavadave4 · Oct 1, 2006

JChalfan said:
If I remember correctly, even with 5.0 T5's, there was a difference in the speedo drive gear on the output shaft over the years. Take a look at this chart, it shows which trannys had which gear:

http://www.geocities.com/mgnedell/notchback/tech/speedo.html

Of course if you don't know what your trans came from, that makes it more difficult. You can probably pull out the speedo cable, look up in the hole in the trans and count the teeth. That's probably the easiest way.

To figure out roughly what rear end gears you have, jack the back of the car up and put it on jackstands. Put the trans in neutral. Put a mark on the driveshaft and one of the rear tires. Start turning the driveshaft by hand, and count how many turns the driveshaft makes per one rotation of the tire. If you have 3.08's, the driveshaft should turn 3 times for the wheel to make one complete turn. 2.75 times = 2.73's, 3.5 times = 3.55's, etc...

Or you can pull the rear cover off the diff and count the teeth on the two gears. Divide the big number by the little number, and that's your gear ratio. It's a bit messier than counting rotations though.

EDIT: Damn it darkwriter, I guess I took too long to type my response.

Jeff

Count the rotations

trbkrb · Oct 1, 2006

If you have a small flashlight, you can try to see in the hole that the speedo cable goes into to check the color. I know the 7 tooth is a yellow gear, and I think the 8 tooth is green?

speedytang · Oct 2, 2006

And since you have the engine out put the transmission in 1st and count how many rotations the rear shaft makes when turning the front shaft 1 rotation. The same idea as the read end.

trbkrb · Oct 2, 2006

speedytang said:
And since you have the engine out put the transmission in 1st and count how many rotations the rear shaft makes when turning the front shaft 1 rotation. The same idea as the read end.

That won't tell you anything about the speedometer gear in the tranny, just the actual gear ratio of the tranny.

TRWXXA · Oct 2, 2006

How fast do you think you were going? If the error is caused by an incorrect speedo gear, your speedo is going to be off by a percentage, not just "20mph". If it were off by 20mph, it would say 20mph while you're stopped (if that's the case then you have a broken speedometer).

If you think you were doing 70mph, and your speedo was reading 90mph (37% error), that would be consistent with a 2.73 to 3.73 gear swap. If you think you were doing 40mph, and the speedo said 60mph (a 50% error), that would be consistent with a 2.73 to 4.10 gear swap.

1FatPony · Oct 2, 2006

could this effect why my speedometer is off by 20mph?
it is hard to see but that wire is burnt and melted from the exhaust pipe. Is that even the speedo wire?

1FatPony · Oct 2, 2006

TRWXXA said:
How fast do you think you were going? If the error is caused by an incorrect speedo gear, your speedo is going to be off by a percentage, not just "20mph". If it were off by 20mph, it would say 20mph while you're stopped (if that's the case then you have a broken speedometer).

If you think you were doing 70mph, and your speedo was reading 90mph (37% error), that would be consistent with a 2.73 to 3.73 gear swap. If you think you were doing 40mph, and the speedo said 60mph (a 50% error), that would be consistent with a 2.73 to 4.10 gear swap.

yea it was a percentage off. and I think i was actually going 70 when it said 90. Does this mean it is the rear end gears and not the speed gears??

TRWXXA · Oct 2, 2006

1FatPony said:
yea it was a percentage off. and I think i was actually going 70 when it said 90. Does this mean it is the rear end gears and not the speed gears??

A common cause for a big speedometer error is non-stock rear gears. A lot of people will change the rear end ratio, and forget to change the speedometer gearing to compensate. The speedo is just counting the number of revolutions the transmssion's output shaft makes for each mile driven. With a higher rear end ratio, the transmission has to turn the driveshaft more revolutions per mile. The speedo senses this as a faster speed. Non-stock tire sizes will have a similar, but much less drastic, effect.

The tranny has two speedometer gears in it -- a "drive" gear on the output shaft (hard to change), and a "driven" gear on the speedometer cable (easy to change). You need to reduce the number of times the speedo cable turns per mile. You do this by increasing the number of teeth on the "driven" gear on the end of the cable.

The number teeth on the "driven" gear you need to get depends on the year of your car, the rear end gear ratio, and what tranny you have.

1FatPony · Oct 2, 2006

since the tranny was replaced, do you think it is jsut a matter of what tranny I have? and not the drive gear? How can i tell what tranny it is?
Thanks

JChalfan · Oct 3, 2006

Follow the suggestions above to see if you can determine which drive gear is inside your speedo.

Like someone said above, check your rear end to see if the factory tag is still on there. It would be held on by one of the cover bolts. If the tag is still there it usually means the diff hasn't been opened up, and it should say the gear ratio on the tag.

There's a pretty good chance that your speedo is off because your transmission was replaced by a different year with the different drive gear.

Once you figure out which drive gear your tranny has, and what ratio your rear end gears are, then you can figure out which speedo gear you need to get your speedo accurate again.

Jeff

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