Speedo help

I have have a 5.3/T56 in a 92 fullsize.
Allen did the wiring harness and pcm tune, and the truck runs great and is a blast to drive.
When I got the first pcm from Allen, the speedo was slow about 25mph. Talked to Allen and a couple retuns later, I think I'm close, but still have a weird problem with my speedo.

The needle jumps around about 2 or 3 mph all the time. With the cruise on, the cruise works, but the needle increases about 40mph (actual ground speed doesn't increase), and the truck lunges pretty bad).
If I bypass the speed buffer (white box behind the glove box), the needle is smooth, but reads fast about 10-15 mph depending on how fast the truck is actually going, and the cruise dosen't work at all.

Anyone have any ideas why the needle jumps around so badly and why the needle increases so much while using the cruise?


Thanks,
If you need more info, let me know.


Patrick

I bypassed the box all together and haven't installed cruise yet on either 93 or 92 conversions that I have done. Sorry, can't help you there. Maybe I'll get around to it myself eventually. Allen had to manually correct mine with a GPS unit in the truck as we drove around to correct the actual MPH settings. It appears the speedo has a different counts per mile than the 98 and newer models.