LBZ: speedo calibration help

[JoshH]

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

Let me try it another way.

Say you have stock size tires on your truck and your speedometer reads dead nuts on accurate. Your truck is counting driveshaft rotations to determine how fast you are going. When it sees 1700 RPM at the driveshaft, it knows you are going 70 MPH.

Now you put on a lift and some 35" tires. Now that you have put on 35" tires, you are covering way more ground for every rotation of the driveshaft, but your truck doesn't know that. When it sees you have 1700 RPM at the driveshaft, it thinks you are going 70 when in reality you are actually going closer to 80 because you are covering more distance for every rotation.

In order to fix this, you have to tell the truck that you have larger tires, and it now covers more ground with every rotation of the driveshaft. So now it will only look for 1500 RPM to know it is going 70 MPH.

 

[Timmay208]

yup makes total sense that way. Maybe im just thinking to deep or something, ill probably start confusing my self soon haha

 

[Timmay208]

changed it to 660 today and dead nuts with gps, thanks for all the insight josh

 

[Bearce Sr]

I have 265/70R16 efi set at 655 i'm bout 1mile off slow on speedo at 65 gps say 64