I want to change my deuce's speedometer adapter (the 90 degree elbow between the transfer case and the speedometer cable) so that my speedometer would read closer to accurate with the new 11.00R20's - without paying a shop to re-gear the stock adapter.
However, I had to get out the shovel and dig, and even a phone call to the Kinder/Gentler Ron (
73m819), as often it was just thrown around "get a 5-ton speedometer adapter". That was never very helpful as there are many different 5-tons out there that also come with 11.00, 14.00, or other super single tires. I thought I'd compile my findings for all, and for my own selfish ease of future reference.
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M44 Series (2.5 ton)
Cross links to other helpful posts:
http://www.steelsoldiers.com/showth...-cable-adapter&p=310557&viewfull=1#post310557
http://www.steelsoldiers.com/showth...-recalibration&p=342098&viewfull=1#post342098
In the Deuce "TM9-2320-209-20P Organization level - Maintenance M35 Multifuel repair parts and special tools" it lists the following (images partially stolen from cranetrucks' post)
There are 2 types available:
For the 9.00 tires
- NSN: 2520-00-562-0421
- Part Number: (P/N): 7061216 (Also called MFG SKU)
- Ratio: 1:187 (0.187)
For the 11.00 tires
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M39 Series (5 ton)
In the "TM9-2320-211-20P Organizational level Parts and special tool lists M39 Series 6x6 5 Ton" it lists the following:
Looking up each item #14 from the above list:
- Federal Stock Number: 2520-715-3866
- P/N: 8331043
- NSN: 2520-00-715-3866 (Note: I used the P/N listed above and it cross-referenced to the NSN on PartTarget.com)
- Ratio: 1:250 (0.250)
- Federal Stock Number: 2520-740-9623
- P/N: 99673R91
- NSN: 3010-00-740-9623 (Note: I used the P/N listed above and it cross-referenced to the NSN on PartTarget.com)
- Ratio: 1:222 (0.222)
- TM Notes: "use with 1100 x 20 Tires"
- Federal Stock Number: 2520-740-9624
- NOTE: Even though this says it's item #14, the TM says "ADAPTER ASSEMBLY:SPEEDOMETER SHAFT" so I'm not sure if it's a part we're interested in.
- P/N: 99674R91
- Part number did NOT cross reference on Part Target, so I guessed NSN 2520-00-740-9624
- Ratio: 1:235 (0.235)
- Federal Stock Number: 2520-692-6185
- NOTE: Even though this says it's item #14, the TM says "ADAPTER ASSEMBLY:SPEEDOMETER SHAFT (DRIVE RATIO 26 TO 1)" so I'm not sure if it's a part we're interested in.
- P/N: 649AH10-26-1
- Part number did NOT cross reference on Part Target, so I guessed NSN 2520-00-692-6185
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M809 Series (5 ton)
For the record, this is why people hate looking at the TMs
(It also doesn't help in the TM section there are 2 different posts for both M809 and M939 series)
In the "TM9-2320-260-34-2 Direct Support and General Support Maintenance M809 series 2 of 2" it lists the following:
If you compare all the #7's in the parts list with the -24P-2 below, the first one is not listed in the -24P-2 below. So looking it up:
Looking at "TM 9-2320-260-24P-2 - Parts Manual M809 Series Trucks"
The figure is the same, but the parts list has a little different information:
Looking up the NSNs on PartTarget.com
http://www.parttarget.com/3010-00-7...23.html/-E6AA2B9B-9D34-408E-85D4-41C837C7A739
http://www.parttarget.com/2520-00-7...ption=nsn&originalsearchtext=2520-00-715-3866
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M939 Series (5 ton)
In the "TM9-2320-272-24P-2 Unit, Direct and General support parts and special tools manual for the M939 series truck. 2 of 2. Feb 1999" it lists the following:
Looking up item #11 from the parts list:
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Conclusion #1:
For the M35A2 axle gear ratio, the 1:200 speedometer adapter (NSN
2520-00-404-3098 ) is what I would want, though I wouldn't expect the speedometer reading to still be 100% accurate as actual tire diameter even between different 11:00's is different (NDT vs Goodyear vs Michelin etc...), but it would be very close.
An alternative is to go for a 5-ton 1:222 Speedometer adapter from the M39 or M809 series (NSN
2520-00-740-9623) though speedometer readings would still be a little off, but should closer than the deuce's stock 1:187 speedometer adapter because of the 5-ton axle gear ratio of 1:6.44 vs the deuce of 1:6.72.
I shall leave it as an exercise for the reader to calculate percent error differences.
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Update: 09/29/2015
In a different thread the question was posed:
I'm running 395s, I would think a different ratio would get my speedo closer. What are the benefits? Where do you find the part?
Changing the speedometer adapter will result in a closer speedometer reading and thus, more accurate mileage count on the speedometer. You can "do the math" to calculate the difference between your displayed speedometer reading versus your actual.
Various vendors have speedometer adapters. The 1:200 is hard to find. At the time of this writing I have only contacted 2 places and I was quoted:
$85.00 Big Mike's Motor Pool
~$100 Boyce Equipment (they sounded like they weren't real firm on the price and would probably "price match")
Further Update: Boyce Equipment was confused on what I wanted and only had 1:222 adapters. Big Mike's Motor Pool had an item with the right part number (which I ordered) but it turned out to be a
tachometer. The NSN for the 1:200
does say "Speedometer-Tachometer" so you better have the place physically LOOK at the item to ensure it is a speedometer adapter and not a tachometer! At the time of this writing (12/05/2015), I plan to perhaps pursue a business that can re-gear the stock adapter to said 1:200 ratio. If that does occur, you'll have to look later in this thread (reaching the date limit of updating this post).
I believe "The Math" is just a ratio/percentage change/comparison. For reference, here is an image often used here on SteelSoldiers for different tire sizes:
For example:
The stock M35A2 runs 9.00R20's and are about 40.5" in diameter. The stock 0.187 ratio speedometer adapter reads, for our purposes, relatively accurate. My tires are Goodyear G177s 11.00R20. According to Goodyear's website they are 43.7" in diameter.
Do "The Math":
NOTE:
There is error in these calculations as it assumes the NDTs and stock 1:187 adapter are 100% accurate, which is surely not the case as even the tire's diameter changes as tread wears.
43.7 divided by 40.5 = 1.0790
This means that the Goodyear's are 7.9% taller than the 9.00R20 NDTs.
200 divided by 187 = 1.0695
This means that the 1:200 adapter turns 6.95%
faster than the stock 1:187 adapter.
This means that a truck running Goodyear G177 11.00R20's with a 1:200 speedometer adapter will *still* have an error of 7.9% - 6.95% =
0.9494%, or that what is seen on the speedometer will still be 0.9494% under actual. If the speedometer reads 50 mph the
actual speed will be about 1.009494 times faster, or ~50.47 MPH. Personally, I consider a 1% error negligible.
Lets do the same for the 395/85R20's as listed in the chart:
46 divided by 40.5 = 1.1358
This means that the 395's are 13.58% taller than the 9.00R20 NDTs.
You would then need a speedometer adapter that turns at about 13.5%
faster than the stock 1:187.
Lets work it backwards here:
Let x equal the exact ratio of an adapter needed such that 395's read as accurately as the 9.00R20 NDTs.
x divided by 187 = 1.1358
x = 1.1358 * 187
x = 212.395
This means, for a, relatively, accurate speedometer reading, you need to have a speedometer with about a 1:212 ratio (certain businesses can re-gear speedometers/adapters, but I'm sure it will be a bit pricey). If you put the 1:200 ratio speedometer adapter in your truck it would still read lower than your actual speed.
There are other ratio options from various 5-ton trucks with higher speedometer ratios, but we cannot just use those numbers directly. Remember, the 2.5 ton Rockwell axles have a 1:6.72 gear ratio, while the 5-ton Rockwell axles have a 1:6.44 gear ratio so we have to take
that into consideration too!
Back to the drawing board:
6.44 divided by 6.72 = 0.9583
This means that the 5 ton axles turn at 95.83% the speed of the deuce axle. This means they turn (1 - 0.9583) 4.17%
slower because they are geared
higher.
Note: Needing to down convert the 5-ton's adapter ratio's for the deuce is supported by the fact that the M39 Series TM does state the use of the 1:222 adapter with 11.00R20s, while the M44 Series TM (the deuce's) states to use the 1:200 adapter. The 5-ton adapter wouldn't need to spin faster if it's axles weren't geared higher.
Incomplete information: There is the possibility of some complete information (or something I missed) as the 5 ton 1:222 adapter spins 11% faster than the deuce's 1:200 adapter and that difference does not appear to be fully accounted for by the axle gearing alone (about a 4% difference). However, I still feel that the results are worthwhile and useful.
What if the various 5-ton speedometer adapters were placed in the deuce, what would be their equivalent reading?
Assuming the speedometer adapters universally fit into the deuce transfer case, the quoted ratios would need to be converted to account for the difference in the axle gear ratio.
The 5-tons have multiple speedometer adapters:
1:222
1:235
1:250
1:1.130 (M939 Series - I'm not convinced these will fit in a deuce transfer case - I'm also not sure what "1.130" means in relation to the how the TMs treat "1:222" to equal "0.222")
We need to multiply each of the above by 0.9583
1:222 -> 212.75
1:235 -> ~225.208
1:250 -> ~239.583
1:1.130 -> ~1.083
Now we can start to get excited!
From the math we did above, a deuce running 395's we needed a speedometer adapter with a 1:212.395 ratio. We just found out that the 5-ton 1:222 speedometer adapter in a deuce will have an equivalent ratio of 212.75.
What is the approximate error as seen on speedometer on the dash?
As calculated previously:
46 divided by 40.5 = 1.1358
This means that the 395's are 13.58% taller than the 9.00R20 NDTs.
212.75 divided by 187 = 1.1377
This means that the 5-ton 1:222 speedometer adapter, in a deuce, would turn 13.77%
faster than the stock 1:187 adapter.
This means that a deuce running 395/85R20's with a 5-ton 1:222 speedometer adapter will have an error of 13.58% - 13.77% =
-0.1898%, or that what is seen on the speedometer will be 0.1898%
over actual. If the speedometer reads 50 mph the
actual speed will be about 0.9981 times that, or ~49.91 MPH. That is one accurate match!
What if the 5-ton 1:222 speedometer adapter was used on a deuce, running Goodyear G177 11.00R20s? How far off would the speedometer gauge read then?
As calculated previously:
43.7 divided by 40.5 = 1.0790
This means that the Goodyear's are 7.9% taller than the 9.00R20 NDTs.
212.75 divided by 187 = 1.1377
This means that the 5-ton 1:222 speedometer adapter, in a deuce, would turn 13.77%
faster than the stock 1:187 adapter.
This means that a deuce running Goodyear G177 11.00R20s with a 5-ton 1:222 speedometer adapter will have an error of 7.9% - 13.77% =
-5.87%, or that what is seen on the speedometer will be 5.87%
over actual. If the speedometer reads 50 mph the
actual speed will be about 0.9413 times that, or ~47.07 MPH.
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Conclusion #2
I believe this means that an M39 or M809 series 5 ton 1:222 speedometer adapter is a
perfect fit for a deuce running 395s with only about a 0.2% error (over)! Also, a deuce with the Goodyear G177 11.00R20s using a 1:200 speedometer adapter is only about a 1% error (under)! Both those seem great to me. Also, for Goodyear G177 11.00R20 tires, the 1:222 adapter is a
slightly closer match than the stock adapter with about a 5.87% error (over reading) vs 7.9% error (under reading) which is about a 2.03% improvement on gauge accuracy.
Note: I would love for someone with a good GPS who's made any of these conversions to report how close my rough calculations are to reality.
Warning: This assumes that 5-ton speedometers fit without issue in the deuce transfer case and on the deuce speedometer cable. I have no experience to speak on the matter whether or not they do as I've only taken other's word for it. Update 11/22/2015:
Jeepsinker has
confirmed that the M809 Series 5-ton adapters do fit the deuce transfer case.
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