Speedo Gear

View of new speedo gear next to a quarter.

How Fast Are You Really Going?

Okay, so you've finally added taller tires and lowered your axle gears and now you hop in your Jeep for the first time and notice you're doing 70 MPH through town!   This can't be right?  Rest assured, it isn't.  This will also be the case if you've installed taller tires and didn't change your axle gears, only your speedometer is reading SLOWER than you are really going!  Now what?  It's time to re-calibrate that speedometer.

In order to do so, you will need to change the speedometer gear in your transfer case.  This is an easy job to do with a 1/2" wrench and maybe a screwdriver.  Below you will see how it's done, but first you need to figure the size speedometer gear you'll need.  Then, of course, you'll have to get one.  Mine cost about $14 from a local 4x4 shop in town.

Speedo Gear Chart

Axle Gear Ratio

Tire Diameter

35" 33" 32" 31" 30" 29" 28" 27"
4.56:1 35 36 37 37 38 - - - - - -
4.10:1 33 34 35 36 37 38 39 41
3.73:1 31 32 32 33 34 34 35 38
3.55:1 30 31 31 32 33 33 34 37
3.07:1 26 26 27 27 - - 28 29 31

The tire diameters shown here are the advertised size. Many tire makers build their tires differently, so a 35-inch tire from one company to another may vary as much as a full inch.

Numbers in the table refer to speedometer gear tooth count and are not part numbers.

How It's Done...

First you have to find it! First you have to find it!  The speedometer gear is hiding below the speedometer cable at the transfer case.
Remove the speedometer cable. Remove the speedometer cable.  It will either just screw off (with your fingers) or, in my case with the speed sensor installed for the Mopar MPI fuel injection conversion, require a 1" wrench to remove the nut.  Remove the cable and place it up out of the way.
Remove the small retainer bolt and bracket. Using a half-inch wrench (or a small socket is quicker) remove the one small retainer bolt below where the cable attached.  Remove the retainer bracket and clean the parts.
You may need to loosen the assembly... maybe not. You may have to help loosen the assembly.  This is where the screwdriver may help.  Do not try to pry out the assembly, just shift the grooves slightly to break some of the grime and build-up loose.
Removing the speedo gear. The whole assembly just pulls out, it is NOT threaded.  Have a rag handy.
The gear/shaft just slides out of the housing. The old gear (the green one) will just slide out of the gear index housing.
View of gear size index. Side note:  If you take a close look at the gear index housing, you'll note that the center gear shaft hole is off-set (not centered) and there are three ranges of gear sizes embossed on the housing.  There is also a small index groove below each of the three size ranges.  Note the range that fits the gear size you are going to use.
Setting the retainer with the index groove. With the appropriate index groove facing down, the retainer bracket's "hooks" will fit the outside grooves.  This will allow the gear cog to correctly fit against the drive gear inside the transfer case.
SG-installed - fuzzy.jpg (44677 bytes) Then insert the new gear, housing, and retainer bracket back into the transfer case and secure with the small bolt.  Attach the speedometer cable back to the housing and you're finished!