Generally the data displayed by VMSpc is extremely accurate. But an improperly calibrated engine may send speed, distance, and fuel information that is consistently wrong. For example, if you put larger or smaller tires on the coach, the speed and odometer data will clearly be incorrect. To compensate for this kind of error you can enter correction values in the Engine Corrections dialog box. For the odometer, hourmeter, and fuelmeter you can enter both an “Offset” and a “Multiplier”. VMSpc will automatically add the Offset to the indicated meter, then Multiply the result by the multiplier. In addition, the multiplier will be applied as appropriate to the Speed, Cruise Speed, MPG, Fuel Rate, and Rolling MPG gauges. For example, if your engine was reprogrammed and lost its history at the 20,000 mile mark, and you added tires that cause it to understate the speed by 5%, change the Odometer Multiplier to 1.05. Change the Offset to 19,408. (Remember that the Offset is added, then the Multiplier is applied. 19,408 * 1.05 =20,000.)  

In the event that the VMSpc cannot parse the data correctly so as to overcome data bus errors (as is the case with some Cummins ISC and ISL engines) two solutions are in place: "The Check Boxes" (check them both to force more queries for data) and A simpler/ older VMS program to use in the place of v3.x. Here's the link for Version 2.5x here from VMSpc Downloads. Rename the current Silverleaf icon on your desktop to "3.x" Open the link, save it to C:\ or desktop and install, though you need to also rename the 'default installation directory' (VMSpc) to something like VMSpc 2.5. Rename that new desktop icon to "2.5" also. This is just to allow having both programs available. You'll need to copy your trips and gauge over--my_really_great_gauges_screens.scn for example.