Speedometer mystery

76 D100 / 318 / A833-OD / 8 1/4"-3:55 / 275/60/15 (28" dia) on 9" wheels. I used the chart on Brewer's website to determine the speedometer gear I needed as 33 tooth. Brewer's Speedometer gears

I picked this little jewel up in February out of Oregon. Due to transmission gross leaking, title issues, work, & life, I have just got it on the road. Transmission rebuilt and sealed thanks to parts and tech expertise from Brewer's.

Speedometer shows about 9 mph too fast at 60 mph or about 69 mph. Checked against two different phone speed apps. I also jacked the rear up, re-checked the tag on the diff and did a manual check. Yep, 3 1/2 turns of the drive shaft equals one turn of the tires.

Bought and installed a tach yesterday. Sure enough the rpms match what everything else indicates it should. About 1850 rpm at 60 mph. OD is .76 for those that don't know it off the top of your head.

Speedometer was replaced with one from a salvage yard that I lucked upon while there getting parts for my 99 2500. The one in it was sticking horribly around 50 mph and when I pulled it to attempt some lubrication, the needle was so brittle in wound up in 7 pieces.

When I calculate the number of teeth I need per the percent error or the longer set of calcs to get to speedometer cable rpm it indicates that instead of the standard 1001 cable rpm for 60 mph, I need 875 cable rpm or a 38 tooth gear. Chart indicates this is a match for 4:10 gears.

What am I missing? Surely / hopefully something simple I am just overlooking. TIA.

Is the 3/4 lever on the transmission case pointing down

You may have a speedo that needs repair. You can verify what's at fault by doign a mile marker check on a highway. If the odometer reads correct then the speedo itself is at fault. If it checks out OK, There are places that will put together an inline speed corrector that is sometimes easier than playing swap the gears.

Have you checked the odometer over a known distance? Often the odometer will be correct but the speedo won't be due to a faulty mechanism.

CrazyD. Funny you ask that because it was backwards / pointing up when I first bought the pickup which I realized the very first time I drove it and was the first thing fixed. However, That makes no difference in regards to the speedometer. The output shaft and driveshaft spin at a rate per tire diameter, diff ratio, and vehicle speed which the speedometer is driven from. What gear the transmission is in only determines the engine rpms in relation to that speed.

TJP & John,
I have not checked the odometer. Thanks both of you for pointing that out. I had not thought of that. I will be checking that today and report back.

Yes and these speedometers were problematic. Hey did you also lube the speedometer cable? I installed a NOS speedometer in my '79 and it still had some slight wobble, so I installed a NOS cable but lubed it real well first since it was old NOS, and that smoothed it out completely. I found a NOS one on feebay (cable) for pretty cheap, but I think if you just pull the cable from the housing like I did and regrease it probably okay. If none of this works I think I'd just get another speedometer gear with more teeth, kind of guessing at it. IIRC used ones still available pretty inexpensively and that's probably the easiest thing to swap.

I have the same problem on my 73 dart. I swapped in a couple speedos and its still off the same. I used the correct gear for 3.21 and a 26" tire. Tried a gear with +1 tooth and -1 tooth and its not right with any of them. Mine shows 58 mph when its doing 50.
I think these things were never accurate, we just didnt have gps to check them.

Change the speedo gear to get it to read the way you want it to, todays tires are not the same size that our old vehicles came with stock
One other thing to check is the circumference of your current rear tires and then do the math to get the correct height, circumference divided by 3.14, I think 88.7 inches circumference divided by 3.14 =27.376 hieght

Cab_Burge
For tire size I was using 28" diameter as reported on Discount Tire's website where I purchased the tires. Checking circumference with a tape on the spare, same tire / same wheel as on the ground I get 87 5/8" which is 27.89" diameter versus the 28" reported. Negligible change in calculations as it still shows 33 teeth.

TJP & John,
For the odometer check I didn't get to it until this morning. One way trip per google maps of 24 miles. Speedometer app on phone recorded 24.05 miles. Odometer on pickup recorded 24.9 miles. This results in a 3.75% overage error for the odometer versus the 15% too fast error on the speed.

Is there anything that can be done to help or correct these old things? Clean, lubricate? Or just a lost cause and not worth it?

Larry, It has been so long (30 yrs?) since I've lubed a speedometer cable, I can't remember what is best to use. What did you use?

Thanks again for all the reply's and input.

Any speedometer shops nearby? Sounds like the speedo head needs to be calibrated. No reason the speed should be off more than the mileage is.

Just for giggles you may want to try checking the odometer using the highway mile markers vs. google or apps. Would be interesting to see how close they compare. Also factory speedo's were not dead on which is why police cars had the "Certified" speedometers.
3.75% is not all that bad IMO, as at 60 MPH that would = 2.25 MPH or 62 MPH
So I'm going to guess your speedo is the root of your issue
keep us posted

The odometer error is within the range of different pinion tooth counts. The speedo can be re-calibrated but it requires dedicated equipment a speedo repair shop has.

https://deluxespeedometer.com/

also, the "loaded rolling diameter" of a radial tire is WAY different than a bias-ply, and needs to be accounted for.

Above Post , Vehicle WEIGHT Loaded Tire , measure to center of your Axle HUB to Ground . Thats what your RADIAL Tire height is and what Your Speedo sees driving down the Road ! Try this one , a 1969 GTS 383 4 speed , with 3.91s in the rear , with 235 / 60 / 14 s that measure 22.5" tall, MABE 11.25 to the Center axle hub . You bet It's NOT running the recommended speedo gear to drive MILE Marker , to MILE Marker ! But I have speedo Dead nuts perfect using 2 mile markers , or even with Phone GPS !

That's part of the reason why I have F70 14 Bias plies on my Dart , I didn't change the gear though so I'm guessing the speedo is off a bit.

The only way to accurately determine the loaded rolling diameter is to measure the rollout and divide by pi.

that is correct sir.

I used "Fifth Wheel Grease". I love that silky stuff. I've used it on my power window gears and other stuff that doesn't get heated like bearings.

John,
Thanks for that link. I had actually found that shop the night before. Been crazy busy since my last post. As an FYI to all, the link business quoted me $150 to $180 for a calibration.

Larry,
Thanks for the info on grease.