It is one of the features of the ScanGaugeII that I recently bought and put on my wife's 1996 E300D. It plugs into the OBDII port that all 1996 and later cars have. It gives you a bunch of gauge readouts, a trip meter that includes MPG and gallons/hour (and Metric equivalents), and the ability to read and clear error codes.

ScanGaugeII

You also have the ability to program additional readouts into it (but obviously it can display only those things that are available via the car's OBDII port). One nice ability is manifold pressure, aka "boost" (nice for the '98-99 turbo diesels, at least).

It's small enough to velcro onto the top of the steering column in front of the gauge cluster without getting in the way of anything.

It can do "miles 'til empty" sorts of calculations, based on what it has learned from the previous fillup, as well as calculating how much each mile costs, how much of the current tank of fuel you've burned (miles, gallons, dollars).

It's powered from the OBDII port and also has a small internal battery that you can use for checks of the system when the car is turned off, reviewing old data, updating after filling the fuel tank, etc.

List price is $170; I got mine for $160 from an eBay seller with free shipping.

The usual disclaimer: I am not related in any way, except as a customer, to the people who make and/or sell the ScanGaugeII.

Jeremy

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970