Mike, I know I am out by a country mile (so to speak) since I just traveled 1,600 mlles and when my speedo showed 90 MPH I was actually only traveling 70 MPH. Also my end milage from my trip (according to the speedo I've traveled over 2300 miles (subtracting end from start milage).
pj, Thanks Peter, I do have it under control. I purchased a new GPS and will make a detail comparison Speedo verses GPS. Then I can calculate the percentage differential between the two and can calculate my fuel consumption something that is not possible right now.
Stan, yes my bad I should have posted it under Instruments ... what was I thinking
Peter, thanks for moving the thread. I do know all about tire size having an impact on the speedo results. My tires are like yours, P185/70R14 Michelin. The speedo was changed in 1992 it might be that the speedo came from a different Mercedes model with different tires or it may have been a refurbished one I have no idea (RM Restoration/Auction have no more file on hand as to what was done to the car in 1992). It's just a huge difference (speedo shows 90 MPH when I actually only go 70 MPH according to the GPS).
My initial hope was to find out how many others have similar problems, may they be cable or connection issues. Or if someone had the speedo re-calibrated and what other issues have surfaced. May have had his serviced etc. ,,, and I appreciate this kind of feedback. I will add a table here once I made my detail road test comparison in 10 mile increments; speedo verses GPS readings in Miles and KM.