Mercedes-Benz - E class (124) 84-95 Mech.

Mercedes-Benz - 124 Mechanical Odometer Gear
( view PDF instructions ) 
There are potentially two gears that can go bad. First is the gear on the side of the speedometer. If your gear is metal then you will not need this gear. If it is plastic inspect closely where the shaft comes through the gear as there will generally be a hairline crack and the gear must fit tight. Most common are 18 or 25 tooth count.
$25.00 Select a style:  M10 tooth count M11 tooth count M12 tooth count M13 tooth count M14 tooth count M15 tooth count M16 tooth count M17 tooth count M18 tooth count M19 tooth count M21 tooth count M25 tooth count M29 tooth count

Mercedes-Benz - 124 Odometer Lead Gear
( view PDF instructions ) 
Second is the lead gear on the same rail. Check to make sure it is tight on the shaft. Using a small screwdriver, see if it spins on the shaft. If it does, you should replace this gear as well. There are three sizes. All you need to do is measure the overall thickness. Sizes are nominal
$25.00 Select a style:  1/8 = 3.175mm 3/16 = 4.7625mm 1/4 = 6.35mm