So I finally have a working speedometer and odometer after piecing one together from 3 different speedo heads. My speedo is off but that isn't an issue as I had the needle off plenty of times and just haven't got it positioned correctly. The problem is for every 20 miles travelled my odometer reads 19. The odometer works separately from the speedo on these Denso heads. It's not a big deal but I've invested several full days in this so I might as well continue to refine it. I've been checking it with a GPS speedometer app.
Truck has a 37 tooth speedo gear, so does anyone know how much 1 tooth will change the odometer? I have a 35 tooth gear but not a 36. I'd like to do this once and set the speedo needle if possible.
Thanks for any help.

Commonly one tooth is 2 to 3 mph.

I am not concerned about the MPH, I'm concerned with the odometer calibration. Once I get that correct I can set the needle wherever I want.

What size tires on the rear?

So you are roughly 5 percent off. I would be looking at 2 teeth smaller.

Stock size 235/85R16 they are roughly 31-31.5" diameter.

I'll give that a shot thanks.

dividing 37 by 35 is 1.057. so the speedo cable will turn 5.7% faster with a 35 tooth pinion. whether you need a 5.7% change or not depends on how much increase/decrease the odometer needs.

So that sounds pretty close. A 36 tooth gear would only be about 2.8% then.
That is what I was looking for, thank you.

going from 37 to 36 is a little less than 3%. there's about a 5% difference between 19 and 20. you probably can't hit it exact but being with in 1% is pretty good for the old speedos. if folks would do the math on this stuff a lot of guess work could be tossed out. the hard ones to do are the '65 and earlier were there are only 6 pinions to choose from and there's about a 5% spread between them all.

I'm a big fan of actually doing the math before swapping pinions willy-nilly. Measure the actual rollout of the tire, divide by pi to determine the tire rolling diameter and do the math from there. This will get the odometer as accurate as possible within the range of available pinions. To get the odometer dead-nuts accurate usually requires a secondary reduction unit.

The 35 tooth gear worked almost perfectly. It is one tenth off in 10 miles, so 1 mile extra in a 100 miles. Got the needle set at right about 1 MPH fast at 60 MPH. It's more accurate than my all factory stock '92 truck.
Thanks again guys.

My truck doesn't use a cable, it uses the 3 wire speed sensor from the '97 truck the trans came out of. The wiring and cluster is all '93 Ram Charger and it's all run off a 2001 JTEC PCM, but thanks anyway.

I figured 35 would get you there. You are well within the correction of error on a non certified speedometer.

Yep, I'm happy with the result. Thanks again.