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Speedometer adapters, pinions, extensions, oh my!

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  • Transmission / Overdrive: Speedometer adapters, pinions, extensions, oh my!

    Hi there. Have enjoyed searching the archives for a solution to this problem. Got a '51 2R5. California truck, wonderful shape. cleaned it up, rebuilt motor, runs like a champ (pardon the pun) Bought lots of books, lots of spare parts, repaired a few minor things, it's a very fun truck. Got the factory papers from the Museum too. I've verified all the museum papers said the truck has, and here's my dilema. When I first got it running, i threw some radials on it, just to get it to drive around. I noticed the speedo was off, but then again, I expected it. I was using tires that were >2" shorter than they should have been. Then for Christmas I got the right tires: 7.10-15 Universals. The speedometer accuracy improved, but not nearly enough. At 25 mph i'm still 5 mph off (we have a street with a permanent radar display speed sign.) Plus, it feels slow for what the speedometer reads. So, I proved the rear end matched the papers as best I could (4.89, it's a stump puller) by jacking it up, marking the drive shaft and tire with chalk, and started the shaft spinning. Almost 5 times around. Tranny is a 4 speed spur gear T9. Looking at my book, I'm supposed to have a 664522 pinion only, and no adapter. Crawling under the truck, lo and behold, I find the 664522 pinion, a SW 19634 sleeve, a pinion extension shaft, and what appears to be an adapter. What is going on? Even if the rear was changed to a 4.82, it still calls for the same adapter, and the speedo would only be .35 mph off. Previous owner (original owners son and my father in law) is sure nothing was ever changed on the truck, cheap old farmers. Would you think I could just remove the adapter and extension shaft and just put the pinion back in with the speedo cable, and screw it all together? Which begs the question, why would all that extra stuff be there.
    Thanks in advance.

  • #2
    That is a tricky one, without checking the Stude. Parts Catalog myself, I have to wonder which Drive Gear those 2 combos require, because if they are different you can't just remove the Adapter gear box because the Pinion will not match.

    So the Truck was delivered with the optional oversize 7.10 Tires instead of the standard 6.70 Firestones?
    I know that would be the case if a later Truck (60's) had 7.10's on a 1/2 Ton, but not sure about a 1951.
    Second Generation Stude Driver,
    Proud '54 Starliner Owner
    SDC Member Since 1967


    • #3
      Looks like the truck would have an adapter if it originally had 6.50 X 16" tires.


      • #4
        Yes Rich, the truck was delivered with the optional tires. The build sheet indicates two 710x15 tires for the front, and three 710x15 tires for the rear (decimal does not exist on the builders sheet.) When I picked the truck up, the most recent set of tires were H78x15, which is the "later" match for the 7.10's... so it looks like not only was it delivered with those, the only available replacement size was chosen when the tires were replaced (70's? prior to metric tire sizes). My parts book indicates only the pinion, no adapter, and the pinion number in my book matches that which is stamped on the pinion. I thought maybe the gear broke years ago and the local dealer replaced with something that would be "close enough", but that's not the case. The adapter has two different numbers on it, one on one side, one on the other, and i tested it out last night, input shaft of the adapter turns 24 full turns, output shaft turns 25 - which would drive the speedometer faster than it should.

        Dwain, looking at my parts book, for the transmission I have, 6.50x16 would require the same pinion, and no adapter for 6 ply "P" tires, but a different pinion and the use of the adapter for 6 ply "T" tires... If I remove the adapter and use just the pinion extension and sleeve (I have to use the sleeve, the tranny has female threads, and the end of the speedo cable is also female...) it'll bump me up 1.041%, which is going to be closer...

        That leads me to wonder how many different adapters there are? With different numbers on each side it looks like SW would be able to make any gear ratio adapter they would want!


        • #5
          Well, after removing the adapter and fitting the speedometer cable back into the tranny the way it's supposed to go, the odometer is right on, i got a nice app for the iphone and clocked it over and over, 5 miles, right on the money.... but the speedometer is off... anybody know how to adjust that thing?