Full tank -- reset trip meter. Fill up and divide miles driven by gallons. Pretty accurate.
I'll follow the trip meter with a GPS...but they are pretty accurate.

This is with the stock 15" wheels.

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2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k

You need to check your odometer over a measured 25 to 50 miles if you are using the GPS.

If it is that close, it probably will not matter. When you get into errors of 10% to 20% it starts to add up.

As an example... My current MB has a speedometer that reads right on the money according to GPS. BUT, over a measured mile there is a 20% error in the "odometer". IE: If I figure my mileage by these indicated figures pump/odometer = 21 MPG. If I correct if for my wrong odometer gear, my actual MPG is 26 and almost 27 MPG driving like a bat out of hell.

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AJ

1985 300D (SOLD)

W124s for USA were not particularly good MPG cars.

It wasn't until the W210 and the 5-speed transmission and the new V6 that things got better.

W124: 3000+ rpm at highway speeds
W210: 2500+ rpm at highway speeds

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Kent Christensen
Albuquerque
'07 GL320CDI, '10 CL550. '01 Porsche Boxster
Two BMW motorcycles