Help with speedometer calibration.

[Ohmario80]

I'm in need of some assistance with correcting my speedometer after adding new tires and wheels using alphaobd. I went from the stock Perelli Scorpion 285/45/22 32.1" tall to Nitto Terra Grappler G2s 265/70/18 32.6" tall. When I made the switch my speedo was 1 mph off. I've done a series of adjustments and can't get closer to 1mph off no matter how much I adjust. I've measured the tires on the truck and they are just under 32". I set it to 2533mm and I'm now off 2mph. I've gone as high as 2604mm that put me 3 mph off to as low as 2508mm that was about 1.5mph off. I've made several adjustments in between to no avail. I believe the stock tires/wheels were set at 2503mm. My OCD is starting to get to me at this point. Any advice would be appreciated.

 

[ya3coub]

Tazer,

I've paid the dealer twice and they recalibrated it 3 times, Tazer within seconds from the first time, it's prefect

 

[HSKR R/T]

I'm in need of some assistance with correcting my speedometer after adding new tires and wheels using alphaobd. I went from the stock Perelli Scorpion 285/45/22 32.1" tall to Nitto Terra Grappler G2s 265/70/18 32.6" tall. When I made the switch my speedo was 1 mph off. I've done a series of adjustments and can't get closer to 1mph off no matter how much I adjust. I've measured the tires on the truck and they are just under 32". I set it to 2533mm and I'm now off 2mph. I've gone as high as 2604mm that put me 3 mph off to as low as 2508mm that was about 1.5mph off. I've made several adjustments in between to no avail. I believe the stock tires/wheels were set at 2503mm. My OCD is starting to get to me at this point. Any advice would be appreciated.

Go lower on the numbers as it was obviously getting closer. Just remember the quoted height for the tire will be different than installed height. Did you try driving it before you made any changes to see how far it was off? Probably not much. And 1-2mph off at 60+ is well within tolerance and could just be the GPS app you are trying to use. Also make sure you are trying to use GPS with phone plugged in to USB on dash.

I have a digital GPS speedometer for my old 1966 d100 because so changed tire sizes and didn't want to mess with trying to find a gear to work. The readout in that is 2mph off compared to GPS on my phone. I'm sure they are both using the same satellites

 

[Scram1500]

You need the rolling diameter. It's usually one inch less than the tire size. Some manufactures will list the rolling diameter on their website, but they usually list revolutions per mile which you can plug into an equation.

Nitto Terra Grappler G2 265/70R18

tiresize.com

The tire you listed states 637 revolutions per mile. The equation is: Revs per mile=63,360 divided by circumference. (63,360 is how many inches in 1 mile).

At 637 revs the tire circumference is 99.466 inches (divide by pi of 3.14) for a rolling tire diameter of 31.7 inches.

 

[CatDaddy66]

Also, just to poke the bear. The amount you are off is always a percentage not a fixed number. The amount you are off will be different at higher speeds versus lower speeds.

 

[John Jensen]

I'm in need of some assistance with correcting my speedometer after adding new tires and wheels using alphaobd. I went from the stock Perelli Scorpion 285/45/22 32.1" tall to Nitto Terra Grappler G2s 265/70/18 32.6" tall. When I made the switch my speedo was 1 mph off. I've done a series of adjustments and can't get closer to 1mph off no matter how much I adjust. I've measured the tires on the truck and they are just under 32". I set it to 2533mm and I'm now off 2mph. I've gone as high as 2604mm that put me 3 mph off to as low as 2508mm that was about 1.5mph off. I've made several adjustments in between to no avail. I believe the stock tires/wheels were set at 2503mm. My OCD is starting to get to me at this point. Any advice would be appreciated.

I don't think you can get better than a 1 mph difference. Too many factors, such as the devices being used, how the satellites line up or how many satellites are connected, or driving speed and roadway conditions. I have done a lot of what you are going through using two different Garmin GPSs, two different cell phones different roadways, etc and could never get it spot on. I'd be very happy with a 1% difference.