Speedometer correction with steeper gears

[fullsizerwd]

I have a 90 Grand Marquis with an electronically (VSS) driven speedometer. The plan is to upgrade stock 2.73 gears to 3.55s and add a limited slip diff.

My question is do I correct the speedometer/odometer by installing a different speedo gear just like with earlier cable driven speedometers, or do I need an electronic speedo corrector?

[pantera77]

Yes it's the same as cable driven ones. I used the purple 21 tooth gear, but my speedo reads 5mph slow.

[andymac0035]

Yes it's the same as cable driven ones. I used the purple 21 tooth gear, but my speedo reads 5mph slow.

That seems odd. Is that the factory correct gear for a K-code rear axle car? Since 3.55's were an option, I would have thought you could just go to a dealer, get the right gear and be all set? Or do you have taller than stock tires?

Since I want 3.55's eventually, I'm really curious about this.

[fullsizerwd]

Would an orange 20 teeth gear be more accurate?

[gadget73]

depends on the specific gearing and tire size. check this out:

http://www.sccoa.com/faq/speedgr.html

There are two different driven gears in the transmission, so the speedo gear you need will change depending on the tranny. I've used both a purple and an orange in my car at different points when i had different transmissions in there, and I have factory 3.55 gears. The speedo is very slightly out, I've calculated about 5% at 70 mph, meaning it shows 72-73 when I'm really doing 70. Oddly enough, when the speedo says 85, I'm doing 84. Its most inaccurate in the middle of the scale, which tells me the speedo is a little wonky, and the gearing is pretty balls on.

[andymac0035]

That's interesting.
So, according to that:
All 89' and up AOD's have the 8-tooth drive gear? (as opposed to 7)
8-tooth drive gear + 3.55's + 225's = 20.9 (closest being the 21 tooth purple gear?

Am I right so far?

Also, there is mention of a 23 tooth gear (I assume a 22 as well) but no listing on the chart......

[CaptainKRM]

i used a 21 tooth with my 3.73s and it was spot on with the speedo

[gadget73]

yep, later AOD uses a purple with a 3.55, early uses the orange. Early AOD uses a purple for 3.73

I think the 22 and 23 tooth gears are aftermarket. Should be able to get them through 50resto or one of those types of places.

[88grandmarq]

For 3.55 gears ....

7 Tooth drive gear requires a Tan 19T driven gear
8 Tooth drive gear requires a Purple 21T driven gear

My car has a 7 Tooth speedo drive gear, running a 225/60/16 tire, the 19T driven gear is perfect.

this is the correct formula to use;

drive gear teeth x axle ratio x tire rev. per mile / 1000

Assuming you are running a stock tire size (215/70/15) it would look like this;

8 x 3.55 x 751 / 1000 = 21.32

in this instance you would need a 21T purple gear.

this is a good site to figure out what the tire revolutions per mile are;

http://www.miata.net/garage/tirecalc.html