LBZ: speedo calibration help
- Thread starter Timmay208
- Start date
That has not been my experience.
Just to clarify, lets say you have a set of larger than stock size tires, and you load a Bully Dog tuner on it without changing the tire size. You then drive the truck and see that the speedometer is reading slower than the GPS. You would then set the tire size smaller than stock to correct this?
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yes that is correct, and how i did it everytime while i worked there. Like i said i have my dads and uncles within 1mph doing it that way
Well, that makes absolutely no sense, and it completely opposite from what I have personally seen with every tuner I have ever used, including Bully Dog (I work at a diesel shop and we use several different brands).
Yea i dunno man, just in my experience being there 4 years was if the speedo was reading higher than the gps that ment the customer had to small of a tire size causing it to read faster, putting a bigger tire size would be less revolutions causing the speed to read a little slower. I dunno i guess it makes sense in my head
a smaller tire turns more revolutions per mile there for making the speedo read faster than you are really going. I'm not sure how they had it setup in the coding
a smaller tire turns more revolutions per mile there for making the speedo read faster than you are really going. I'm not sure how they had it setup in the coding
on my gt when I put my 35s on from stock with out adjusting my speedo was almost 10mph slower then actual. when I adjusted for the 35s my speedo went up.
what it sounds like to me is your tire size is smaller then what your adjusting for timmay. cuz if u adjusted the tire size up which caused the revs to go down then that causes the speedo to go up. post up the whole dimensions instead of just the 285
what it sounds like to me is your tire size is smaller then what your adjusting for timmay. cuz if u adjusted the tire size up which caused the revs to go down then that causes the speedo to go up. post up the whole dimensions instead of just the 285
so here is all the info for my tire
Section Width: 10.43 in
Rim Diameter: 16 in
Rim Width Range: 7 - 8 in
Overall Diameter: 31.64 in
Sidewall Height: 7.82 in
Radius: 15.82 in
Circumference: 99.39 in
Revs per Mile: 657.3
funny cause when i put 657 into efi live it shows a tire size of 30.70, so that tells me something is up either in efi live or that chart is wrong, blah all i want is my ****in speedo to be right
Section Width: 10.43 in
Rim Diameter: 16 in
Rim Width Range: 7 - 8 in
Overall Diameter: 31.64 in
Sidewall Height: 7.82 in
Radius: 15.82 in
Circumference: 99.39 in
Revs per Mile: 657.3
funny cause when i put 657 into efi live it shows a tire size of 30.70, so that tells me something is up either in efi live or that chart is wrong, blah all i want is my ****in speedo to be right
The way the ECM calculates the speed is by pulses/mile. It knows it will see x number of pulses to go 1 mile.
This number is calculated using the number of pulses by the speed sensor for every rotation of the driveshaft times the axle gear ratio times the number of revolutions per mile of the tires.
I'll use my numbers so you can see how it works. There are 40 pulses/revolution of the driveshaft. The driveshaft turns 3.73 times for every rotation the tire makes, and my tires rotate 640 times for every mile I travel. If you multiply all those numbers together, you get 95,488 pulses/mile.
Now, if I were to put a smaller tire on, I would not travel 1 mile in 95,488 pulses. It would be less (the exact amount would depend on how much smaller the tires are), but my odometer would still think I have traveled 1 mile.
In the same manner, my speedometer would say I am traveling 70 MPH, but the speed on my GPS would be reading less. I would need my pulses/mile to go up in order to correct my speedometer.
The way this is done is by increasing the number of revolutions/mile of the tire (making it smaller). More revolutions/mile makes the ECM look for more pulses/mile before it thinks I have gone 1 mile. This is how the ECM is programmed, and no one, including Bully Dog changes this. That's just how it is. Perhaps you got things mixed up since you left Bully Dog, but what you're saying isn't how it works.
Yeah, that chart is wrong. Well, the revs/mile may be correct, but if it is, then the diameter and circumference is wrong. If the tire actually had a 99.39 in circumference, it would have 637.5 revs/mile.
i fully understand when you add the revs per mile and pulses in there. Bullydog doesnt give a option to change that just tire size. Im gonna test it this weekend on my uncles truck with his GT just to refresh my memory i suppose, it hasnt been long since i worked there and i still sell/install bullydog stuff now. But this is my first time using efi live so im not used to seeing all the other options
I guess ive always just gone by if the tire is smaller it is turning more than a bigger tire causing the speedo to read faster than you are really going
Ummm, I have been saying this entire thread that if you put on a smaller tire the speedometer will read too fast.
yea i never argued that , but why wouldnt the speedo read higher than GPS if you put a smaller tire size in the computer?
say you have entered 31.50 inches in the computer and speedo is reading lower than the GPS, if you went down to say 31.00 wouldnt that make the speedo read higher coming closer to the gps speed?
i was just saying when a customer called in saying the trucks speedo was faster than the GPS, i would have them put a larger tire size in
say they entered 32.00 in the GT and truck speedo was reading 70 and gps was reading 67, i would have them put 32.50-32.75 cause in my mind a larger tire turns less therfor would make the speedo read slower
Now when you pull the revs per mile and pulses into it and after i start entering that in efi live i see that my theory does not work