- 738
- 16
- 18
- Location
- wynnewood, oklahoma
Approx. 2 years ago, I added 16.00r20 hemtt tires/rims to my M923A0 truck aka "Weeds". Doing so caused my speedometer to read 50 mph indicated, when in reality the ground speed is 60 mph, verified via Garmin GPS. I went from a 41.25" tall tire(stock G177 duals) to a 51.00" tall super single tire.
So here is how I corrected it: The following info pertains to the M939 series and M809 series of trucks. The same adapter can be used on a deuce, but I believe the speedo will spin backwards and not work. Additional mods inside the ratio adapter will be required for a deuce.
I purchased part# AA12020 ratio adapter from Clark Brothers Instrument. www.clarkbrothers.net. This speeds up the output of the cable 20%, making the speedometer read correctly.
The ratio adapter installs on the speedometer cable output on the Rockwell T1138 transfer case. You will need a 1" wrench to install the adapter.
Here are where things get tricky. The AA12020 comes with a couple different drive tang key adapters in order for it to fit different applications. The threads are correct, and they are 7/8"-18. You will need to order 2 more parts to get the ratio adapter box to work with our military trucks. You will need (1) p/n 7709 outputshaft drive slot, which installs inside of the ratio adapter box and is on the output side. You will need (1) p/n 7714 drive tang key, which slips into the input side of the ratio adapter box. 7709 and 7714 are 0.152" diameter drive tang key, which is what our military trucks come with.
Changing the 2 parts is easy, and will require a phillips screwdriver, and a clean shop towel. You will have to press the gear off of the shaft saw slot, and then reinstall it onto the 0.152" one. It is a press fit part and won't require much to seat the gear to the new part. Be sure the gear mesh back together properly during reassembly, do not force anything these are small parts and don't require much force.
Now if you have a A1 or A2 M939 series, your truck came with one of these from the factory.
This particular ratio adapter adds 13.5% to the speedometer, and corrects it for 14.00r20 super singles that the A1/A2 trucks were originally fitted with. The diameter of the 14.00r20's are listed as 49.00" tall, but actual height that I have seen is 48.00"
So if you have added 16.00r20 to your truck, your speedo is 20% slow, and if you have added 14.00r20 to your truck, then your speedo is 13.5% slow. Clark Brothers has a pdf on their website, and can build an adapter with any ratio needed. Check out their website, they have many other military instrument parts and cables for our vehicles.
DISCLAIMER: Just because changing to a taller tire enables our military vehicles to achieve a higher top speed, does NOT mean it is safe to do so. Adding height, whether be it with a lift kit, or simply increasing tire diameter raises the profile of the vehicle, which in turn causes it to handle differently and is inherently more DANGEROUS. Furthermore, the speed rating on most of our tires is 55 MPH. Operating at greater than 55 mph can cause tire failure, and will kill you and others around you. Check your sidewalls for speed rating and operate your vehicle within the limits of your tire rating.
Just because the speedo is re-calibrated and final drive ratio allows a 72 mph top end on my truck, doesn't mean I will be driving that fast!!
So here is how I corrected it: The following info pertains to the M939 series and M809 series of trucks. The same adapter can be used on a deuce, but I believe the speedo will spin backwards and not work. Additional mods inside the ratio adapter will be required for a deuce.
I purchased part# AA12020 ratio adapter from Clark Brothers Instrument. www.clarkbrothers.net. This speeds up the output of the cable 20%, making the speedometer read correctly.
The ratio adapter installs on the speedometer cable output on the Rockwell T1138 transfer case. You will need a 1" wrench to install the adapter.
Here are where things get tricky. The AA12020 comes with a couple different drive tang key adapters in order for it to fit different applications. The threads are correct, and they are 7/8"-18. You will need to order 2 more parts to get the ratio adapter box to work with our military trucks. You will need (1) p/n 7709 outputshaft drive slot, which installs inside of the ratio adapter box and is on the output side. You will need (1) p/n 7714 drive tang key, which slips into the input side of the ratio adapter box. 7709 and 7714 are 0.152" diameter drive tang key, which is what our military trucks come with.
Changing the 2 parts is easy, and will require a phillips screwdriver, and a clean shop towel. You will have to press the gear off of the shaft saw slot, and then reinstall it onto the 0.152" one. It is a press fit part and won't require much to seat the gear to the new part. Be sure the gear mesh back together properly during reassembly, do not force anything these are small parts and don't require much force.
Now if you have a A1 or A2 M939 series, your truck came with one of these from the factory.
This particular ratio adapter adds 13.5% to the speedometer, and corrects it for 14.00r20 super singles that the A1/A2 trucks were originally fitted with. The diameter of the 14.00r20's are listed as 49.00" tall, but actual height that I have seen is 48.00"
So if you have added 16.00r20 to your truck, your speedo is 20% slow, and if you have added 14.00r20 to your truck, then your speedo is 13.5% slow. Clark Brothers has a pdf on their website, and can build an adapter with any ratio needed. Check out their website, they have many other military instrument parts and cables for our vehicles.
DISCLAIMER: Just because changing to a taller tire enables our military vehicles to achieve a higher top speed, does NOT mean it is safe to do so. Adding height, whether be it with a lift kit, or simply increasing tire diameter raises the profile of the vehicle, which in turn causes it to handle differently and is inherently more DANGEROUS. Furthermore, the speed rating on most of our tires is 55 MPH. Operating at greater than 55 mph can cause tire failure, and will kill you and others around you. Check your sidewalls for speed rating and operate your vehicle within the limits of your tire rating.
Just because the speedo is re-calibrated and final drive ratio allows a 72 mph top end on my truck, doesn't mean I will be driving that fast!!
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