Changing tire size using Forscan

[lil_Blue_Ford]

It does all of the things it's supposed to without issue and there are no trouble codes present. It's quite strange

That is strange

 

[RonD]

Only from 1998 thru 2000 was the GEM used to calibrate speedometer

From 1983 thru 1997 there was a gear drive on trans or transfer case that could be changed for calibration, even with electric 1995-1997
In 2001 Ford added an OSS(output shaft speed) sensor to manual and auto transmissions, at that time thru 2011/12 the PCM(computer) was used to calibrate speedometer signal
In a 4WD, computer software re-calibrated for 4low speed, so no transfer case speed sensor was needed, just FYI

 

[holyford86]

Only from 1998 thru 2000 was the GEM used to calibrate speedometer

From 1983 thru 1997 there was a gear drive on trans or transfer case that could be changed for calibration, even with electric 1995-1997
In 2001 Ford added an OSS(output shaft speed) sensor to manual and auto transmissions, at that time thru 2011/12 the PCM(computer) was used to calibrate speedometer signal
In a 4WD, computer software re-calibrated for 4low speed, so no transfer case speed sensor was needed, just FYI

Truck is a 98 4x2 with RABS, only difference from factory configuration is a swapped in 8.8 with the same gear ratio as factory and a limited slip. If it was only me driving it I wouldn't worry about the inaccuracy but family does borrow it on occasion so I was trying to make it right. It's about 8ish mph off from actual speed at highway speeds.

 

[RonD]

Yes, the 8.8" tone ring may have more "teeth" than a 7.5" so would change the pulse count on ABS sensor used for speed in a 1998

If its a must to calibrate, and Forscan doesn't work, then Dakota Digital makes the SGI-100BT, seen here: https://www.dakotadigital.com/index.cfm/page/ptype=product/product_id=1192/mode=prod/prd1192.htm

It allows you to calibrate any electric speedometer for any situation, tire size, ratio, or Ford's +/- 5%, lol

 

[vwnut]

Curious to weather you're using the free version or the extended license.
I'm looking at making the speedometer correction on a 99 but reluctant to invest if it's not capable on the older models.