I may have the 12 pulse unit, (not sure since I am at the office), but hopefully this will get you close. Here it is, from my build blog:
On a perfect October, day we took the car out on the highway for the first time and checked the indicated speed with a GPS. When traveling at a true speed of 60 MPH, the speedometer indicated 110. The speedometer was easily removed, thanks to the knurled knobs, and the DIP switches adjusted so the following were open: 3, 4, 10, and 12. This resulted in an indicated speed within a couple of MPH of the true speed indicated on the GPS. The speedometer read nice and steady with no fluctuations or ‘spikes’ of the needle.
I have the same wheels and tires. Let us know if this is in the ball park on your car.