From Faq
- Correcting Speedometer Error
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The cheap method is to swap out the Speedo drive gear (located on the passenger side of the tcase) with one that has a different tooth count. This method will not usually get you 'dead-on' but can get you darn close depending on how far off your speedo is. You will need to find out exactly what your % of error is either up or down before choosing another gear.
The best way is to use a GPS unit to find out your true speed as it relates to what your speedo shows. the other way is to have someone with an accurate speedo pace you.
Once you figure the percentage of error you can figure out if swapping gears will help much or at all. price from trdparts4u.com is around $20
gears available:
- 28t pt# 33403-39585
- 30t pt# 33403-39365
- 31t pt# 33403-39345
- 32t pt# 33403-39425
- 33t pt# 33403-39415
these pt#s are for the Gen 2 & 3 trucks, ie, '98-'04 but can be made to work with Gen 1 trucks, '95.5-'97 (*)
each tooth is good for about a 3% change
In the example above for going from 31" to 33" tires w/o regearing, the 6.45% error could be corrected with going down 2 teeth since the speedo is reading slower than actual.
If the truck in question has a 30t (standard for v6 trucks) gear, go to a 28t.
If you are reading faster than actual, a 32t gear would get you back to near-normal.
As you can surmise this is an imperfect solution and will not work at all if your error % if too high and/or your OE drive gear is of such a tooth count that going up or down won't correct the amount of error. for example: If you have a 2.7 with a 32t gear and your speedo is reading 10% faster than actual you would need at least a 35t gear to even get close...and there ain't one. Also, factory speedos are only accurate to around 5% so each truck may be a little different. This is why you must calculate your error % first.
Swapping gears takes about 5 minutes if you go slowly. Removing the gear will cause a little oil to escape but it's not much...just be ready with a rag to catch the dribble.
[1]
(*)On the Gen 1 trucks you will need to press the plastic gears from each shaft and press on the new gear to your OE shaft. It is not hard at all.
writeup: [2]
This will get you "close" but not perfect. OE speedos are only accurate within 5% so remember that if you ever get a ticket ;)
Most v6 trucks have 30t gears. The 2.7s can have 5 different gears depending on model, diff gears, tranny, etc. you will need to remove your gear and read the stamped # on the plastic gear to determine what you have. Even running your VIN# will not be accurate. According to my VIN# print out I was supposed to have a 30t on my 2.7 5sp with 4.30 diff gears...I had a 32t in actuality.
- When the gear swap won't cut it
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The only way to get your speedo perfect is to install one of three units, the Dakota Digital, [3], ($80): the Yellow Box [4], $110; or the Superlift ($160-180 ), [5]
these can be 'dialed in' with a set of switches to get your speedo reading dead on.
Install is a little complicated and requires some electrical skills.
