When I first picked up my rig the speedo was off like 7mph or somethin, so I adjusted the truspeed box once I found out I had one under the dash, computer claimed low 11s
then picked up to 12.9
after correction and I confirmed this hand calculated as well. EGR delete, went up to 13.4
and was fairly dead-on.
After new tires, custom intake elbow+throttleplate delete and stock straight pipe no matter how I drove the truck would get high 14.**
everytime but.....the lieometer never reports better than 13.**.
ONE NIGHT, cruisin across Nebraska, the display actually reported my accurate fuel mileage at 70mph and I seen as high as 15.1
. Fueled in York and those numbers were right on. She has never since left 13.**
Thats why I hand calculate.
I haven't had a chance to check lately with all of the improvements. T-5 oil, straight 4" exhaust, big tune+new FICM and new filters after she went in for headstuds.
My truck is an '04 with a 6" lift with 38" tires. The truck has been that way since I bought it.