4 in lift, how to calibrate speedo?

[johndough]

1988 Ranger 4x4. Has a 4 in lift but the speedometer just bobs up and down until i hit about 30 or so but even after I hit 30 the speedometer is still about 5 miles off of whatever I'm actually going. How can I fix this? Is this an easy fix? Thank you

 

[gaz]

JohnDough,

To the best of my knowledge, it is a simple mechanical driven speedometer. It works off a plastic gear mounted to the output of my transfer case. I would inspect the connection there 1st, since it is held in place by a simple clip. Turn the drive, see how it feels. You could also pop off the instrument cluster an verify it is fully connected to the gage on back.

 

[ericbphoto]

You might want to disconnect the cable at the transfer case and inspect the gear on the end of it. The gear could be getting worn. They make gears with different #'s of teeth to correct for different sized tires. Are you using oversized tires?

 

[johndough]

You might want to disconnect the cable at the transfer case and inspect the gear on the end of it. The gear could be getting worn. They make gears with different #'s of teeth to correct for different sized tires. Are you using oversized tires?

That's right, duh. I forgot it's not the lift that messes with a speedometer its the tire size. Duh, sorry. I'm running 31x10.50xr15's.

 

[ericbphoto]

That's right, duh. I forgot it's not the lift that messes with a speedometer its the tire size. Duh, sorry. I'm running 31x10.50xr15's.

Those will do it. You probably need a different gear - by one tooth. It's not an exact thing. It just gets you closer.

 

[johndough]

Those will do it. You probably need a different gear - by one tooth. It's not an exact thing. It just gets you closer.

How do I know which size gear will match up and run close to accurate with my tire size?

 

[ericbphoto]

Hopefully someone will doublecheck me on this. But... you're running bigger tires. So, for any given speed in mph, your drive train is running slower than normal and the speedometer cable is rotating slower. So we need to speed up the cable. To do that, you need a gear 1 toothcount lower than what you have now. Remove and examine the gear you have now. Count the teeth and note what color the gear is. They are color coded. Then order a gear with 1 tooth less.

Jeff's Bronco Graveyard sells them as well as other places. I think some people have even ordered sets of the gears on Ebay.

 

[scotts90ranger]

From what AlanD used to say one tooth was roughly 7% which has proved fairly close in my screwing around with stuff...

There are only a couple styles of speedo gear on Ford's, they're easy to find on ebay and stuff... take out the one bolt (I think it's a 7/16" or 3/8" wrench), pull it out of the trans, look at the color and count the teeth for good measure (or they're stamped with the number too...) then get one less tooth...

 

[4x4prepper]

TCI Auto Speedometer Gear Calculator - TCI® Auto

Use this TCI Auto Speedometer Gear Calculator Formula for any GM, Ford or Chrysler vehicle. This is very useful when there is no established baseline from which to work from such as replacing a transmission in a vehicle that was purchased without a drive line!

www.tciauto.com

Ford Ranger/BroncoII speedometer gear change/calibration

www.therangerstation.com

 

[RonD]

Drive train turns at the same speed
A 31" tire rolls farther in one turn than a 28" tire, so when speedometer say 60MPH you are actually going 65+ MPH

The speedometer gear is matched to stock tire size and stock rear axle ratio
Go here: https://www.tciauto.com/speedometer-gear-calculator

Go down to Speedometer Gear Calculator
Use 7 as the Drive Teeth
Do you know your rear axle ratio?
3.73 was the most common, but it MATTERS so don't guess
And then use 31 for tire
then calculate

7
3.73
31
Will come up as 16.9 teeth, so use a 17 tooth driven gear