Tire size and speedometer correction

[landshark]

A thought just occured to me, if I go to 31" tires the speedometer will be off a bit. How do you correct this? In my 94 i can do it through the gauge computer.. i assume for the 76 its replacing the speedo gears that go into the transmission? i might as well take care of this while i have everything out...

[Viperwolf1]

Just a change of the gear on the end of the speedo cable. Easy to do after everything is together. I recommend driving it with GPS to determine the actual error in speed first. You'll need a gear with either 1 or 2 less teeth. Math will help determine the closest match.

[landshark]

got it thanks!

[Jesus_man]

Ought to look something like this:


Actually, I can't confirm that. That's what my Atlas has and I think it was design after a different t-case.

EDIT AGAIN:

according to the BC Broncos site, they are plastic:

http://www.bcbroncos.com/dana20.html

[hockeydad4-22]

Yup - and just an FYI - Colorado mustangs unlimited off Peoria and 37th has them in stock (same as the ones in my son's C-4 (66 stang). Actually it is fairly impressive how many small parts are shared between our rigs and the classic mustangs.

[Viperwolf1]

All D20's use the manual trans version of the speedo gear. It's shorter than the auto trans version.

[sbolt19]

Guess. That's what I did until I changed the gears in the axles, then I went to bigger tires & now I am guessing again.

[Viperwolf1]

It's simple. If the driveshaft is turning 10% slower (tires 10% taller) you need the speedo cable to turn 10% faster. Gear needs 10% fewer teeth to turn the cable 10% faster.

[sbolt19]

Naw, what I actually have done in the past, and it is pretty close right now even, so it doesn't worry me, is I gauged it off of mile markers, did some quick math in the ole bean and figured out that I was roughly 6-10% over what I am really going. My rig was originally set up with 3.50 gears, and when I had 31s I was over a little. Then I changed the speedo gear when I went to 33s and it corrected it. Then I went with 4.10/4.11 gears and it under corrected and now with 35s it is pretty dern close.