Stock gearing for '79 diesel?

[cam.man67]

Ever since I've owned Lester, the speedometer has been horribly inaccurate. At an indicated 90, I'm actually only doing about 60-65, which is roughly 35% off. I'm wondering what the stock gearing was for '79 diesels, and whether the PO swapped axles when the diesel died. There's no mention of a special axle ratio on the SPID.

EDIT: Though I can't be sure, as I haven;t checked, it feels like I have 3.73s currently.

[Keith Seymore]

Attached.

Choices are 2.76 (std), 3.07 or 3.40 are optional.

Some models used an external ratio adapter. Perhaps yours is one of those models and someone has removed it.

K

[cam.man67]

Perfect, thanks for the info!

Assuming the truck came with a 2.76 originally at this point...here's why:

Indicated 75 is actually 55mph (1.36 times faster than actual speed)
Indicated 90 is actually 65mph (1.38 times faster than actual speed)

If that is the ratio we are working with (1.36-1.38:1), then it's reasonable that the axles now has 3.73s (1.36:1, in relationship to a 2.76 rear).

Thanks!

[Corts60]

What size tires are you running? If you have 3.40 with 31" tires, that should yield you around 2400 RPM at 65 roughly.

[cadillac_al]

Somebody either changed the gears or the trans. I have swapped plenty of transmissions that messed up the speedometer. I never bothered changing speedo gears but that's how you fix it.