Dana20 Speedometer Gears?

[Rescue Diver]

Ok, my new toy 79 CJ7 is finally on the road with tags. I just installed the speedometer cable and checked it against the GPS to find it's wayyyy off due to the 33" tires. It's only reading 40mph at 60.

Anyway, I was trying to look up a source for speedometer gears for the Dana 20 and ran across all kinds of conflicting info. One site said you can't even change the speedo gears in a Dana 20 but you can change them in an 81 or newer Dana 300 . Is this correct? If so, I guess I'll have to get an external adapter to screw in between the cable and T-Case.

I found other sites where people said you CAN change them in a 20 but I haven't found anyone that sells the gears.

For the record, I have a 1979 Dana 20 and 3.55 axle gears.

 

[BrockGrimes]

Here ya go:
Control Speedometer Readings - Jp Magazine

Info and links in the pic's to sources for gears

Gears found here:
http://www.bcbroncos.com/dana20.html

Also search for Spicer 20 not Dana 20 .

 

[Rescue Diver]

Here ya go:
Control Speedometer Readings - Jp Magazine

Info and links in the pic's to sources for gears

Gears found here:
B.C. Broncos - 66-77 Early Bronco Dana 20 Transfer Case

Also search for Spicer 20 not Dana 20 .

Thanks!
I'm a little confused. The bcbroncos web site has them listed at great prices, but isn't the Dana 20 supposed to have metal gears? With my speedometer off by a full 3rd, reading 40 at 60, I wonder if the gears can be changed enough internally to correct it? I'm guessing it will require both drive and driven gears to be changed to make that big a difference.

 

[BrockGrimes]

No it should just be the small plastic gear that needs to be changed.

 

[hagar]

no, the driven gear ia a one piece metal gear for his application.
no plastic gears.
count the number of teeth on the gear
one tooth is worth approx 6-7 mph
if you had a 20 tooth, you're looking for a 13 or 14 tooth gear
the gears are no longer made, and the gear will have to come from a used or donor Transfer Case . (1973-1979 Jeeps with Dana 20 )
check the SW number stamped into the shaft
you need to stay in the same number series in order to use the same drive gear inside the Transfer Case ....in other words, not just any old 1 piece 13 or 14 tooth metal gear is going to work; damage will result.
your existing gear should have a number similar to: SW 890237-13 tooth;
staying within the existing series means another gear with numbers similar to: SW 890238-14 tooth, SW 890239-15 tooth, SW 890240-16 tooth
If you are in doubt, post the number on the gear and it can be determined to be correct series or not.

 

[BusaDave9]

I also need a speedometer gear for my Dana 20 . I never fond one and I think you might be better off looking for a Ratio Adapter. It's a tiny gearbox that calibrates the speedometer.

Here is where you can get one:
Speedometer Tachometer Clock Gauge Repair Restoration

 

[Rescue Diver]

no, the driven gear ia a one piece metal gear for his application.
no plastic gears.
count the number of teeth on the gear
one tooth is worth approx 6-7 mph
if you had a 20 tooth, you're looking for a 13 or 14 tooth gear
the gears are no longer made, and the gear will have to come from a used or donor Transfer Case . (1973-1979 Jeeps with Dana 20 )
check the SW number stamped into the shaft
you need to stay in the same number series in order to use the same drive gear inside the Transfer Case ....in other words, not just any old 1 piece 13 or 14 tooth metal gear is going to work; damage will result.
your existing gear should have a number similar to: SW 890237-13 tooth;
staying within the existing series means another gear with numbers similar to: SW 890238-14 tooth, SW 890239-15 tooth, SW 890240-16 tooth
If you are in doubt, post the number on the gear and it can be determined to be correct series or not.

Thanks for all the info. That pretty much matches what I was finding online. From what I gather, places are really proud of their used steel gears and it would be much cheaper to just buy the ratio adapter. I still plan to pull the driven gear to see what I have, just out of curiosity since it'll only take a few minutes.

 

[Rescue Diver]

I also need a speedometer gear for my Dana 20 . I never fond one and I think you might be better off looking for a Ratio Adapter. It's a tiny gearbox that calibrates the speedometer.

Here is where you can get one:
Speedometer Tachometer Clock Gauge Repair Restoration

That's what I'm most likely going to do. We have a local speedometer shop called Metro Speedometer he in KC. I've dealt with them before. The guy that owns the shop can actually make an adapter in about 20 minutes specifically for my application for $75. From what I gather, any shop selling a used driven gear for a Dana 20 will be asking more than that, and then there's always the worry about the new driven gear not meshing well with the old drive gear.

 

[BusaDave9]

Your speedometer is off because you have oversized tires and the factory 3.55 ring and pinion gears. You will never find the right gear to correct your speedometer. Now if you were looking for the oem speedometer gear you might find one since that is what people are taking out of the Dana 20 when they regear. It's only reading 40mph at 60. Thats a big jump. Go with the Ratio Adapter and make careful measurements beforehand. Use mile markers. Write down the odometer when you are at a mile marker. Drive 10 miles and double-check the odometer. With those Ratio Adapters they can be very accurate IF you have accurate measurements for how far off your speedometer is beforehand.

 

[Rescue Diver]

Your speedometer is off because you have oversized tires and the factory 3.55 ring and pinion gears.

Ya think?

Sorry, I couldn't help the sarcasm on something so obvious.

You will never find the right gear to correct your speedometer. Now if you were looking for the oem speedometer gear you might find one since that is what people are taking out of the Dana 20 when they regear. It's only reading 40mph at 60. Thats a big jump. Go with the Ratio Adapter and make careful measurements beforehand. Use mile markers. Write down the odometer when you are at a mile marker. Drive 10 miles and double-check the odometer. With those Ratio Adapters they can be very accurate IF you have accurate measurements for how far off your speedometer is beforehand.

I kind of figured for a jump that big it would take replacing both the drive and driven, but since I already have a local source for the ratio adapter, that's what I've decided to do. I don't need to do the mile marker test. The shop can easily figure it out from my GPS mph reading vs what the speedometer shows. It's easy math in this case because when I'm going 60mph on the Garmin, the speedometer reads exactly 40mph. In other words, I'm going 50% faster than the speedometer says, or you could also say my speedometer reads 1/3 slower than my actual speed.

They'll calculate what gears my adapter needs from that.

 

[BusaDave9]

Ya think?
Sorry, I couldn't help the sarcasm on something so obvious.

No problem. I deserved it.
BTW those GPS's are plenty accurate. That should work out good.