LBZ: speedo calibration help

[Timmay208]

So for the life of me i cannot get my speedo closer than 3-4mph off my GPS, with efi live i have tried anywhere from like 27 inches to 32 and no matter what i can get close

Maybe i have it backwards in my head but this is how ive always done it

speedo higher than GPS= tire size to small
speedo lower than GPS= tire size to big

right now speedo say at 73 reads about 70 on my GPS, that would tell me that i need to enter a larger tire size right? well after going from 31.25 to 32.00 now its 75 on speedo and 70 on GPS

i have 265's on the truck and normally in the past for other trucks ive done 31.5 and always been within a mph. If i can get within 1mph id be happy

 

[adrenaholc1]

Iirc I just changed the rev. Per mile it should be around 400 its been awhile since I messed with it though.

 

[MuscleTruck03]

use the speedo fine tuning chart in efi, it has a "put your displayed speed here, put your actual speed here" :thumb:

*edit crap just noticed you dont have efi live... in that case Im not sure what to tell ya..

 

[Timmay208]

i did that fine tuning but i put both speeds in and it gave me option to apply but it didnt change anything?

 

[Timmay208]

any other ideas?

 

[JoshH]

The speedo calculator doesn't work. You have to use the revolutions per mile setting. Lower numbers will raise your displayed speed, higher numbers will lower your displayed speed. In other words, let's say you have your revolutions per mile set at 650 right now. If your seeing 75 on your speedo and only 70 on your GPS, you will need to raise that number to something like 700 rev/mile.

 

[Timmay208]

wouldnt putting the revolutions to a higher number make the speedo read higher?

when i enter a bigger tire size the revolutions goes down

 

[JoshH]

wouldnt putting the revolutions to a higher number make the speedo read higher?

when i enter a bigger tire size the revolutions goes down

No, your original post is backward. You said:

speedo higher than GPS= tire size to small
speedo lower than GPS= tire size to big

Here's how it works:

Let's say you have your speedometer calibrated for a stock size tire:

If you put smaller tires on, your speedometer will read higher than actual speed, and you will need to raise the revolutions per mile to correct the speedometer.

 

[trashygasman]

Yeah Josh you contradicted yourself. You're first statement was wrong, but the second one was right. A bigger tire will turn less revs per mile

 

[Timmay208]

Yea i guess im just confused cause in EFI when you raise the revolutions the tire size goes down

Im just used to normal tuners where only thing you adjust is the tire size. With my dads truck 05 with 265's with his bullydog GT i put tire size of about 31.50 and its within 1mph. I did 31.5 in efi live on mine and it was 3-4 mph higher than gps so i changed tire size to 32 and now its 5 higher. Normally you enter a taller tire size the speedo goes down but i guess efi isnt like that

 

[Timmay208]

Yeah Josh you contradicted yourself. You're first statement was wrong, but the second one was right. A bigger tire will turn less revs per mile

yea Bigger tire size should make the speedo read slower right?

 

[trashygasman]

yea Bigger tire size should make the speedo read slower right?

It should? I can't remember which way I went with mine in efi, but I'm less than a 1/2mph off of my gps. I did use the revs per mile though. My tires are 285s and its set at 647 revs

 

[Timmay208]

647 is 31.17 tire size, i have 265 and right now im around 630 revs and im 5 mph higher on speedo vs gps

if i can get within 1 mph id be happy

 

[JoshH]

Yeah Josh you contradicted yourself. You're first statement was wrong, but the second one was right. A bigger tire will turn less revs per mile

Where did I contradict myself? I looked back over what I said, and it is all correct.

Yea i guess im just confused cause in EFI when you raise the revolutions the tire size goes down

Yes, when you raise the revolutions per mile, the tire size goes down. If your speedometer is reading too fast, you have the tire size set too large, and you need to make it smaller (aka raise the revolutions per mile).

 

[Timmay208]

ok i guess thats where ive been going wrong, EFI live is different than a standard tuner when it comes to adjusting tire size

so right now im at 630 revs, how much you think need to adjust up to make up 5mph?

Thanks for the insight josh

 

[JoshH]

ok i guess thats where ive been going wrong, EFI live is different than a standard tuner when it comes to adjusting tire size

so right now im at 630 revs, how much you think need to adjust up to make up 5mph?

Thanks for the insight josh

How is it different from other tuners? If the speedometer is reading too fast, you have the tire size set too big no matter what you are using to adjust tire size.

Anyway, if you are showing 75 on your speedo and only 70 on your GPS with 630 revolutions/mile right now, I would suggest you try 675 and see what that does for you.

 

[Timmay208]

with say a bullydog tuner, if your speedo is reading faster than GPS that means you have tire size to small and you need to make it bigger. If speedo is reading faster than gps means its to big you need to lower it

 

[JoshH]

with say a bullydog tuner, if your speedo is reading faster than GPS that means you have tire size to small and you need to make it bigger. If speedo is reading faster than gps means its to big you need to lower it

You tired?

 

[Timmay208]

ahah sorry playing xbox so i yea attn span isnt there much. i ment if speedo is reading slower than gps its to big and need to lower it

 

[JoshH]

ahah sorry playing xbox so i yea attn span isnt there much. i ment if speedo is reading slower than gps its to big and need to lower it

You have it backwards. It doesn't matter what you are using to correct it. That's just the way it is. If your speedo is reading slower than the GPS, your tires are set too small.

Here's an example. When my truck was stock and I put 285s on it, my speedo ran about 5 MPH slower than my actual speed because I put larger tires on it and it was set to the smaller stock size tires. I had to increase my tire size or lower the revolutions per mile to correct it. When I put my 265s on it, it was reading about 2 MPH faster than than my GPS (when the ECM was set to the 285 size). I had to fix it by lowering the tire size or increasing the number of revolutions per mile.