Speedometer correction

[bbates7995]

I was preparing to change the tire size in my body computer to correct the speedometer and I found that the original size in the computer does not match the tire size that came on my truck from the factory. Any idea why that would be?

OEM tire: LT275/65R18

Computer setting: Dynamic Tire Circumference (All or Rear) (0-65534 mm): 2465.000 mm

OEM tire should have a circumference of 2559 so long as my math is correct. What am I missing??

 

[Darksteel165]

My math comes up to 2557.78mm so close enough.

I wonder if they change it depending on what tires\rims you get?

Where on the BCM did you see that reading? I want to check mine as I have 22s.

 

[bbates7995]

Where on the BCM did you see that reading?

Under Program Tire Size. After you enter a reading it asks you to confirm and displays the current setting. Mine was actually setup with a 17" wheel setting although the truck came with 18's.

 

[bbates7995]

With the new tires, the speedometer was off +2 MPH @ 60 MPH and my gas mileage took a hit also.

I've modified the BCM to correct the speedometer, etc.

OEM tire: LT275/65R18C
New tire: LT275/70R18E

Original Computer setting:
Dynamic Tire Circumference (All or Rear) (0-65534 mm): 2465.000 mm
FS Transforce HT LT265/70R17E A/S

New setting:
Dynamic Tire Circumference (All or Rear) (0-65534 mm): 2569.000 mm

No engine codes or alarms appeared, so no need to clear ABS errors or anything like that.

My speedometer is now -1 MPH off so it looks like I over corrected a bit, but I'm going to change my tire pressure to 35 psi (down from 40 the installers used) to match the load rating per this calculator. I'll test again and see if this corrects the issue before I mess with changing the BCM again.

 

[Darksteel165]

Wacky as hell. I'm going to take a look at mine and set it. Would be crazy if mine was set for 17s when it came with 22s.

Question for you, where are you getting 2569 from?
I'm showing 275/70/R18 are 104.1 inches, or 2644.14MM?

 

[Darksteel165]

Not sure what is up with mine.
Looks like it was setup for 19 inch rims?

These are pictures of my before and after.

It appears their math it wrong, or the website ive been using is incorrect.
305/40R22s are 2520, and mine should be 2560, neither are the marked 2461?
Maybe someone else can chime in, but 100% mine was set incorrectly. It came with P285/45R22 A/S, not Continentals.
Looks like the Continentals might come on a 2500 or a 3500?

I'm thinking I should manually set mine to 2560 and not use their pre-sets.

 

[SKT Customs]

Not sure what is up with mine.
Looks like it was setup for 19 inch rims?

These are pictures of my before and after.
View attachment 147287View attachment 147288
It appears their math it wrong, or the website ive been using is incorrect.
305/40R22s are 2520, and mine should be 2560, neither are the marked 2461?
Maybe someone else can chime in, but 100% mine was set incorrectly. It came with P285/45R22 A/S, not Continentals.
Looks like the Continentals might come on a 2500 or a 3500?

I'm thinking I should manually set mine to 2560 and not use their pre-sets.

Lol that’s weird, yeah that’s the dually size 225/70r19.5.
Edit: that’s the dually size from the 5500! Not a typo, 5500 chassis cab.

 

[HSKR R/T]

As long as the speedometer is correct to GPS, what does it really matter?

As far as the circumference not matching, there is a difference between circumference of the tire by itself and circumference when installed on a wheel and carrying a load. This is why you adjust the number to get your speedo to match GPS speed. Anyone who says they just entered the actual tire size and it was correct probably didn't verify with GPS.

Don't get too hung up on what it shows for tire size and type of tire in computer.

 

[bbates7995]

Wacky as hell. I'm going to take a look at mine and set it. Would be crazy if mine was set for 17s when it came with 22s.

Question for you, where are you getting 2569 from?
I'm showing 275/70/R18 are 104.1 inches, or 2644.14MM?
View attachment 147286

I'm using actual measurements. ie Height of installed tire.

 

[Jimmy07]

I'm using actual measurements. ie Height of installed tire.

If you use the height of the installed tire from ground to top of tire, you’ll still be off.
What would technically be correct is to measure from ground to center of rim x 2 x 3.14 x 25.4 (if your measurements are in inches), and that will be the true rolling circumference in mm.

 

[bbates7995]

So, some added benefits from correcting what was in the computer from the factory, my park sense no longer gives false positives when in reverse causing whiplash! Not sure if its coincidence or not, but so far so good!

 

[bbates7995]

If you use the height of the installed tire from ground to top of tire, you’ll still be off.
What would technically be correct is to measure from ground to center of rim x 2 x 3.14 x 25.4 (if your measurements are in inches), and that will be the true rolling circumference in mm.

Good point. I will check that tomorrow. When I verified my speedometer with GPS, I was 1 MPH off. However, after disconnecting my phone from CarPlay and using just my phone and the waze app, the speedometer was spot on. Not sure why the connection would make a difference, but it does.

 

[Jimmy07]

Good point. I will check that tomorrow. When I verified my speedometer with GPS, I was 1 MPH off. However, after disconnecting my phone from CarPlay and using just my phone and the waze app, the speedometer was spot on. Not sure why the connection would make a difference, but it does.

Ah yes, that too. Your phone can’t be connected to CarPlay because the truck’s gps hijacks the phone and it’s no longer using its native gps.