[W126 Coupe] Speedo inaccuracy

[Gerry Van Zandt]

I agree with Dan.  When I set up the speedo calibration and  
adjustment box, I was very careful to measure against a measured-mile  
calibration setup the state of Oregon has here in I-205 south of  
Portland, which is 4-5 miles long (signs by the side of the road).   
That's where I got the fact that the stock calibration was 10-12 MPH  
"off" with the new 3.07 rear end.

I simply did the ratio adjustment, programmed the DIP switches in the  
calibration box, and that was that.  I've had numerous "roadside"  
radar setups tell me, at all sorts of speeds (city and highway), what  
my speed was and it was right on with the indicated / modified  
reading influenced by the adjustment box. Never had anything as  
accurate as a GPS, but as long as I'm at 1 percent or less, I'm happy.

Other fellow owners of E500/500Es have told me that their speedos can  
read up to 10 MPH off (GPS measured vs. indicated) at V-max of 155  
MPH.  I think the rule of thumb is that the higher your speed  
(particularly over 100 MPH) the greater the difference between  
indicated and actual speeds.  Heck, my Honda motorcycle is even off  
by around 4-5 MPH at 80+ MPH.  Actually, that's saved me from tickets  
more than once...

Cheers,
Gerry