DRB3 Revs per mile

[toh4od]

Hi all! Possibly a dumb question regarding changing revs per mile to adjust tire size using the old DRB3 on an ‘01 2500 4X4 Ram 5.9 Cummins.
Why am I having so much trouble with this programming? Every time I change the number in the abs module the speedo goes out of whack the opposite direction and by a huge amount. Any ideas? Thanks in advance!

 

[BluegrassDiesel]

Are the values on the DRB3 the same as your truck? Assuming your truck is in km/h since youre in Canada?

 

[toh4od]

Truck was 15 km/hr slow on speed, I changed rpm first by a small amount, speedo went to 15km/hr faster, so I changed rpm to middle # and speedo went 10km/hr slow. All with factory gear ratio.