Rev Per Mile - Speedo Calibration

[Accarguy]

I installed 265 75 16 on my 2000 with the stock size was 255 70 16
So I figured out with the Ford IDS Scan Tool that with my new size tire is 658 rev per mile and the stock was 290.
I then used the NGS tool to program the PCM to the new rev per mile of 258.

I am comparing my speedo to my Garmin GPS speed I know it is not 100% exact but it is close.

Prior to program change my speedo seemed to be pretty close at speeds like under 50 but when I was doing 75 it said it was 77.

Now after the program it seems as through it is off at lower speeds and under the actual speeds: 35 is 32, 50 is 48, 75 is 74.
I did do the 10 mile test going around 75mph and that said it was 9.9mi which is pretty close.

So my question is, am I too low with 258?
Should i try like 265 or something?
When I used the online tire calculator it said stock was 271 and new was 238, so i know that is incorrect.

I have read all of the older posts on this, but there is nothing that pertains to my size.

Thanks for the input.

 

[swshawaii]

265/75-16 = 637.6 Revs/Mile (31.6" Diameter)
255/70-16 = 671.4 Revs/Mile (30.1" Diameter)

Use "Tire Size Comparison"
https://tiresize.com/calculator/

 

[Accarguy]

That's is exactly what I calculated too. But the rev per mile in the pcm before I changed it was 690. So I know that calculation is not right.

 

[Accarguy]

I think I am going to try 665 and see what that does.

 

[intelisevil]

When I used my NGS to reprogram for my larger tires (255/70R16) I found an empty parking lot and had an assistant (the wife) tell me when the valve stem on one wheel was pointing straight down. She marked the ground and I drove straight forward four revolutions until the valve stem was pointing down again. Then we measured point to point.

Here was my math:
30'11" / 4 = 92.75"
63360" (1 mile) / 92.75 = 683.126 revolutions per mile.

I punched 683 (actually 00683) into the NGS and my speedo is dead on at 60 MPH according to 2 different GPS units. Speeds below 60 are a bit more optimistic . . .

25 GPS, 30 Speedo
35 GPS, 38 Speedo
45 GPS, 47 Speedo
55 GPS, 56 Speedo

 

[Accarguy]

Right that's where I am at. Thank you for the reply.
I set mine to 670 but it seems to be further off than the 658 because
658 10 mile test was 9.9
670 10 mile test was 9.8
But similar to yours it was more off at lower speeds as I mentioned before. Now at 670 the 75 is 75 according to the gps.

So I have no idea what to do