Many of us who enjoy our GenI Montero/Raider offroad have switched to a larger tire size other than stock. A problem with this that you may or may not have noticed is that the larger diameter tire sizes throw the speedometer off. The disparity between actual traveled speed and displayed speed grows in proportion to the larger diameter of the tire.
I first noticed this a few years ago when an officer issued me a ticket for a speed that was 3mph over what I thought I was doing. Even though I knew I was speeding, the speed written on the ticket bothered me as being inaccurate. Fast forward 2.5yrs later to my next speeding ticket and I was in disbelief at the speed the officer said he had clocked me doing. This caused me to take action and look for a possible solution that has plagued most of us larger tire users for some time.
Actually, I made the discovery quite by accident as I was looking for reasons my speedometer needle was bouncing (different tech story). I pulled off the speedometer drive gear assembly to learn how it works. It’s really pretty simple: a nylon gear is turned by a screw on the output shaft.
The rotation of this gear (at a specific ratio) is transferred via cable directly up to the back of your speedometer gauge where it’s plugged in. After pulling out the speedometer drive gear casing and cleaning it off, I noticed an ancient Summarian text that the Mitsu scribes had left behind.
Actually, it was a series of numbers on the outside each preceded by a small triangle: ^20-22, ^23-25, ^29-31, ^26-28. After pondering this for awhile, I discovered that the numbers on the outside of the speedometer casing corresponded to the number of gear teeth on the speedometer drive gear. Voila! For most GenI Montero owners, you’ll have a final ring and pinion drive gear ratio of 4.625 and stock tire size is 225 or 235/75R15. For this gearing and tire size ratio, a 26 toothed speedo gear is used to achieve the correct speed display on the speedometer. So, as the theory goes, as tire size increases, speedometer gear size needs to decrease to accommodate the difference. Correcting for this is as simple as swapping out the small gear on the speedo casing with another one! Lucky for everyone, differing gear sizes are available straight from the parts counter at your local Mitsubishi or Dodge dealer!
As it turns out, Kevin Carbone had been working on the same issue almost 3000mi away, and had taken a more academic approach to a solution.
A good piece of information is that the factory speedometer calibration, at 60 mph its input shaft speed will be 1026 RPM. This is printed on the face of the speedometer on my 1987.
Since I had put taller tires on I was curious how far off my speedometer would be. To calculate the how fast the speedometer drive is rotating we need a few pieces of information.
1: Tire diameter and RPM at 60 MPH
For 31" tires @ 60 mph the tire will turn 651 rpm. There will always be some slight inaccuracies from tire deflection, slippage and tread wear but using the tires height gets you close enough.
Example of the calculation to determine your speed gear for a 31" tire (if you don’t know the tires height you can always get a tape measure and verify it:
31" /12" =2.58 ft.
2.58 x 3.14 (pie) = 8.11 ft circumference of a 31" tall tire. So with each rotation of the tire you move 8.11 feet.
There are 5280 feet in a mile. 5280/8.11 = 651 tire revolutions per mile.
Since 60 mph is a mile per min the tire turns 651 rpm.
2: Gear ratios
We need to know how fast the drive shaft is spinning. With 4.62 gears the drive shaft RPM will be 4.62 x 651= 3008 rpm
The factory gears for my speedometer have a 26 tooth speedometer gear and a 8 tooth drive gear (main shaft).
That gives a reduction ratio of:
26/8 speedo reduction (3.25 reduction ratio). so 3008 / 3.25 = 925 rpm at the speedometer (speedo is calibrated to 1026 rpm = 60 mph).
How far off is the speedo? 1026 / 925 = 1.109.
At an indicated 60 mph you will be going:
1.109 x 60 =66.6 mph
Your odometer will read low by the same factor. So if it indicates you went 100 miles you actually went 110.9 miles.
So lets calculate what the speedo drive gear should be...
3008 rpm (actual drive shaft rpm at 60 mph) / 1026 rpm (what the speedo wants at 60 mph) = 2.93 (reduction ratio).
Since the 8 tooth shaft gear is fixed the only variable we have is to adjust the number of teeth on the driven gear: where x is the teeth needed on the driven gear.
X /8 = 2.93. We will never get an exact correct ratio since you the driven gear increments one tooth at a time.
The choices are:
So 24 / 8 = 3 to 1 reduction ratio
Or 23 / 8 = 2.875 to 1 reduction ratio
Yup a 24 tooth gear is as close as we are going to get.
Also the speedo has a reed switch to send the vehicle speed to the ECU. The switch puts out 4 pulses per speedo revolution. So at 60 mph the frequency would be 68.4 hz.
MD705466 - Gear, Speedometer Drive (N=26): For 4.625 gearing and stock (225/235) tires
MD705464 - Gear, Speedometer Drive (N=24): For 4.625 gearing and 31" tires
MD705463 - Gear, Speedometer Drive (N=23): For 4.625 gearing and 32" tires
MD705462 - Gear, Speedometer Drive (N=22): For 4.625 gearing and 33" tires
MD705461 - Gear, Speedometer Drive (N=21): For 4.625 gearing and 35" tires
MF472536 - Spring Pin, Speedometer Drive Gear (3x16mm)
MD701788 - O-ring, Speedometer Drive Gear (10.3mm)
MF520403 - O-ring, Speedometer Drive Gear Casing (35mm)
3/8” drive socket wrench
12mm socket for 3/8” socket wrench
Adjustable locking pliers
Assortment of small punches and picks
Small plastic cup
Once the speedo gear casing is out…
After the new speedo gear is assembled…
Line up the mating mark on the outside of the transfer case with the appropriate number of gear teeth for the new gear displayed on the outside of the speedo casing. Press the speedo assembly into place.
Replace the holding clip and bolt it down. Replace the speedo cable and sleeve. Drive away and test it out.
You can test the accuracy of the new speedo gear by trailing a friend in any unmodified vehicle. Follow at a predetermined speed and see what your speedometer gauge reading is as compared to theirs. Any further fine adjustment of the speedometer can be done by your local performance shop.