What's new
CK5
Register an account today to become a member! Once signed in, you'll be able to participate on this site by adding your own topics and posts, as well as connect with other members.

Recalibrating my speedometer

de3en16

de3en16

Registered Member
Joined
Feb 20, 2011
Posts
86
Reaction score
0
Location
utah
I need someone to check my math for me before I start on this.

My tires are 285/75R16 with 686 revs/mile.
4.10 gears

I came up with an input ratio of 0.05228659.
if I did right the jumper wires should go 1111010.

I used the info at TBIchips to do this.

Thanks Jim
 
What is your circumference of your tire (in inches)? Like 103"

Did you install the DIP switches?

Using 103" tire circumference, I get:

[(63360)(4.10)(40)] / [(103)(128000)] = 0.78815523

DIP Setting: 1 0 1 0 1 0 1
 
I keep getting 92.361516 for my tires rolling circumference. to get that I am dividing 63,360 by 686.

I like the number you come up with its where I thought I would end up also. I'm going with jumper wires in place of the DIP switch I am having a friend that's electricit engineer do the work for me.

Thanks for the help with this Jeff
 
If you can get the toggle switch thing in, it works great. That way, if you are just slightly off, you can toggle up or down to fine tune.

I changed my axle from 3.73 to 4.10....a few flicks and my speedo is dead on again.

If you are getting 92.3615...., it sounds like you are calculating that.

Take a flexible tape measure and wrap it around your entire tire (that will be your actual tire circumference). Then take that number and substitute it in the formula I spelled out above in the "103" place. That should get you the correct ratio.
 
Yeah I was calculating that out, I see why the tape measure would be better.

Thanks for the help again
 
2 mph off at 60 mph by GPS That's a lot better then 20 mph off at 65. My friend had some DIP switches on hand that fit. This mod wasn't as hard as I thought it would be.
 
Well done! Glad you are back in the game. What also helps is that signal goes to your computer so the engine may run a little better too because it knows the correct speed you are traveling now..... :)
 
Does the ECM control anything else other then lock up for the transmission? The little I did drive it seems a little better. It's just nice to know how fast I'm going instead of guessing by what the TAC says.
 
de3en16 said:
Does the ECM control anything else other then lock up for the transmission? The little I did drive it seems a little better. It's just nice to know how fast I'm going instead of guessing by what the TAC says.
Click to expand...

I think that's it but don't know for sure. It does also send a pulse to you cruise control.

Cruise
ECM
Tranny torque converter
Speedo and Odometer
 
What about in an '82 K20 diesel, TH400 w/ 208 with Yokohama Geolander AT/S 285/70-17 (32.70") and 3.73 from the factory (not sure if they have been changed).

82burbbuildspecs_zpse9c49f6d.jpg



I have been trying to figure out how to correct it, but :dunno:. It is indicating 53-54 on the speedo and the roadway radar says 41.
 
Drive/driven gear swaps. Cable speedometers suck. :)

Google TCI and speedometer gears or something like that. They had/have info on what is needed to correct the cable drive setups.
 
It appears you have this figured out already but i'd like to add that in order to get TRUE circumference it is best to put a chalk mark on your tire and on the ground then push the truck forward until the mark on your tire is facing the ground again after rotating one full rotation THEN measure from the first mark to the second mark. The reason this is the best way to do it is because when your tires are off the ground you lose the "bulge" effectively making the tire slightly larger in circumference than when it's on the ground.
 
You must log in or register to reply here.

Latest Posts

Top Bottom