speedometer recalibration

[freds master]

Can someone tell me how to recalibrate my speedo after installing a bigger size tire on my 1999 c3500 6.5 turbo truck. I am off about 5 mph at 60 mph. I stopped at a dealer and he tells me it can't be done. Is this true? The fellow didn't impress me to much but I didn't want to push the issue with him.

[slagona]

The '99 Feature Articles has the procedure for doing this. It's fairly simple - especially if you know binary numbers. The VSSB is located behind your glove box under the ECM. It has 14 jumpers of which the right most 7 compose a binary number. Changing the jumpers (or installing dip switches and setting the dip's) will alter your spedo/odometer. I used dip switches as I plan on several tire changes (already have had 3 different size tires on the suburban) before I settle in on a size. I used 2 seven bit switches. You have to shave a bit off of one end on each switch to make them fit, but it works.

At your percentage of error, you need to increase (or decrease) your binary number by about 3. If your Spedo is reading too fast (60 is actually going 55), decrease the number, otherwise increase it.

The leftmost 7 bits of the 14 bit number are the one's complement of the rightmost bits - BUT you MUST go from right to left when setting the complement bits (reading the rightmost bits from left to right).

For example, the following is a legal 14 bit setting:

1101000 1110100

In the above example, the rightmost 7 bits are the number 116 in binary. My truck is currently set at 95, but was just trying to give a clear example of how the leftmost 7 relate to the rightmost seven.

If you're confused by this - don't even try it - buy the featured articles to help you.

--Scott

[freds master]

Thanks for the reply, I do appreciate it. I will be needing to keep a log book for work, and it would be nice to be accurate. Thanks again, Richie smile.gif

[freds master]

Thanks again for the posted reply Scott. I do understand it fully except for on thing, What is the orientation of the part to locate right and left and front and back. Example, With the circuit board out of the little plastic case, are you looking at the resistors and where is the wiring connector located, toward your body or away from it?? Once I know that it will be a simple fix. Thanks again. Richie