Speedometer question?

[Craftsmen]

Can someone help me out? Is the speedometer head in my dash on my 1970 C10 adjustable for calibration? I put 275-60-15 tires on the rear of my truck and my speedometer reads 10 mph slow. I have the Saginaw 3 speed with 3:73 differential Gears. As far as I know everything is original factory except the tire size. I haven't taken the speedo gears out of the tranny to know how many teeth they have. One of our members suggested to just purchase a converter gearbox to correct it. I called to order one and they told me that the speedometer should not consistently stay the ten mph difference at any speed. He said that when you speed up it should be a greater difference. He said I could have an issue with the head, that's why I'm asking about calibration. I'm confused and don't know what the best way to tackle the problem or where to start? Thanks for any information or guidance. I don't want to waste money on gears if it's the head and visa versa.

Just a note, the previous owner also had bigger tires on the back so the speedometer hasn't read correctly since I've Owned the truck. I don't know what size tire came on the truck from factory.

[iambull]

I have the exact same problem. but a little more muddled.

The original tires were 750x16's. that is what i thought the speedo driven gear was set for on mine The originals should have been about 29.75 inches in height.

The PO had 285/75/16's.This made the speedo off making it look faster/further than it was.

I recently put on 225/75/16's which puts the height at about 29.3 inches, pretty close for a match to the original height, so speedo should be good right? Wrong.

since its still off (but now its slower) it seems the PO must have modded the gear to match some earlier tire size in between. OR the gearing is mucked.

Pulling it (the speedo) out this w/e to seal bullet oring leaks and try to figure this out

good luck.
bb

[Craftsmen]

I ended up pulling the tranny out, changing the drive gear to an 8 tooth (orange/tan color ). What a bear by the way on the Saginaw. I got a big education on the inner workings of a manual transmission. I ended up running a 20 tooth (blue gear) for the driven.

With the 20 driven tooth gear according to my gps, the speedometer reads about 2 mph slow at lower speeds but a 65mph the speedo is dead on.

I did try a 19 tooth (white gear) driven gear and at lower speeds the speedo reads accurate but at 50 mph plus it about 5 mph fast on the speedo.

I went back to the 20 tooth driven, I thought it was the best compromise. Now at least my speedometer and odometer is accurate to keep me from getting tickets and to keep accurate mileage for oil changes. A big improvement from what I started with.

Saginaw 3 speed, 3:73 rear end gears, 275/60/15 rear tires.

Just a note, if you pull the bullet with the driven gear out of the tyranny it's best to jack the truck up as high as you can at the rear, this will keep the oil in your tranny. RICK