Speedometer Correction

[Tiggie]

Ever since I swapped trannys, my speedometer has been slow about 6 mph. I figure the drive gear inside the tranny is different than that in my original tranny. I didn't change the driven gear (the one on the cable).

So my question is, how many teeth does my current drive gear have? Choices are 6, 7, or 8-tooth. The only clue I have to work with is that the speedometer slowed down with no change in the driven gear.

I'm thinking my car originally had a 7T drive gear and an 18T driven gear, and the new tranny has an 6T drive gear. Would that slow down my speedometer? If that's correct, my speedo is setup for 3.73 gears right now!

[gadget73]

What rear do you have, what speedo gear, and what year trans? I want to say when I went from an 8t drive gear to a 7 the speedometer slowed down before I changed the speedometer gear. I have a 3.55 rear. With the 8t, I used a purple 21 tooth, now I use an orange 20 tooth.

[Blaze86Vic]

Going fron 7 tooth to 8 tooth will increase the speed on the speedometer. Did you change wheels/tires about the same time?

Here is the speedometer gear callibration chart.

http://www.grandmarq.net/vb/showthread.php?t=2091&highlight=gear

[mrltd]

And how are you testing it?

[Tiggie]

mrltd, I am testing it using mile markers on the interstate. To go a mile, with the speedo at 60MPH, it takes me about 53.5 seconds.

gadget73, my rear axle ratio is currently 3.08, what the car came with. For my current driven speedo gear, I am guessing it's an 18 since the original tranny probably had the 7T drive gear. The speedo was right before the tranny swap. As far as the year of the tranny, I have no idea. It has the later case, but no ID tag. I am sure the tranny has been treated to a performance rebuild, possibly a Wide-Ratio kit considering the differences from before in first and second. But I am unsure because it was in a parts car I bought with no history.

Blaze, so going to a smaller drive gear slows down the reading. That tells me I have gone from either an 8T to a 7T or a 7T to a 6T drive gear.

Isn't the 6T often used in performance rebuilds?

To be sure, I will count the teeth on the current (original) driven gear. If it has 18, I'll know I had a 7T and now have a 6T, and if it has 20, I'll know I had an 8T and now have a 7T. Does this sound correct?

[mrltd]

There is no 6 tooth. 7 or 8. That's it.

[Tiggie]

Page 144 of the 2006 FRPP catalog list a 6T.

Copied from page 144:

[Tiggie]

Ahh, but now I see that's under the T-5 section.

I must have had an 8T and now have a 7T. Why would an '82 tranny with 3.08's have had an 8T? I thought they didn't come along until later?

[Lincolnmania]

the 86 aod that was in gadgets car and now in the superlinc has a 8 tooth drive gear........i'm using the purple gear with 3.55's in it

[gadget73]

Yeah, I had an AOD from an 86 LSC. It was an 8 tooth. I now have a trans from an 86 TC, and its a 7 tooth. The LSC tranny was a reman one from who knows what though, so I really dont know what year that transmission was. Its possible your 82 wasn't running on the original tranny, or it was rebuilt and someone changed the stuff in the tailshaft for some unknown reason. Hard to say really.

[Tiggie]

Mmmm... the plot thickens... both cars have a gray 18T driven gear.

With the speedometer correct before the new transmission, it would have to have been a 7T drive gear in the original '82 Tranny. Now that it's 6mph off (tested numerous times with a stopwatch and milemarkers, also tested slow with police radar), that would indicate that there is a 6T drive gear in there, or my speedometer is off calibration.

It's interesting that the 3.27 geared car had the gray 18T, that's wrong no matter the speedo drive gear.

Are we sure there is no 6T drive gear?

[Blaze86Vic]

There is no 6 tooth. You probably just need to have your speedometer recalibrated.

[PICKUP6772]

This is an easy one. To speed up the speedometer, you need to lose a tooth in the driven gear. Example: If you are running an 18 tooth gear, you need to go down to a 17. Rule of thumb is, each tooth will set the speedo about 5 m.p.h. lose a tooth to speed up, and add a tooth to slow down. Check the gear, and go from there.
Mike

[Tiggie]

I'll try a 17T on there and see what happens. I may just grab a 16T while I am there, too. I'll let you all know which works best.

[gadget73]

I've also been told that if the speedometer cable is worn out and binding, it can also throw the calibration off. Also, are you absolutely 100% certain that the tranny donor car has the original rear gears in that housing? Its possible someone stuck a random boneyard rear in there and its not the same. Just something to keep in mind.