Hi All-
I have been toying with someday changing the differential ratio on my 220D (because the engine SCREAMS at highway speeds), but I wanted to do an actual analysis to understand the benefits and costs. Hopefully some of this will be interesting/relevant to a few of you. If somebody wants it, I would be happy to recreate this analysis for a different vehicle.

My analysis used data from the OM 617, OM 617A SAE paper 780633, and I scaled it for my smaller OM615 (see first picture). I also needed to estimate the required engine hp at a given vehicle speed. I assumed a flat road and no wind, so power goes to overcoming rolling friction, air drag, and drivetrain losses. At a given vehicle speed and diff ratio, I can determine the engine rpm. Once you know the rpm, you can use the first graph to determine the max power capable of the engine at that vehicle speed.

Obviously if the max power exceeds the required power, the car could accelerate at that point. Once the two curves intersect, the car can no longer accelerate (this is the top speed). MB clearly chose their diff ratio for a particular vehicle to achieve the highest top speed. Maybe this is an obvious choice, but it surprised me. My vehicle has a 3.92 diff, and you can see that curve intersects the required power curve at 80 mph, which is the published top speed of my car.

So the key advantage of going from a 3.92 diff to a 3.69 or 3.46 diff is that at a given speed (say 70 mph), the engine doesn't have to turn as fast. These engines have peak efficiency (bsfc, specifically) at ~2400 rpm, so the lower the rpm the better for fuel efficiency at highway speeds.

And the key disadvantage of going from a 3.92 diff to a 3.69 or 3.46 diff is that the vehicle top speed is reduced (see second graph). So basically, MB really did pick the best diff ratio for my car. If you went higher or lower than the 3.92, the vehicle top speed would be reduced.

In conclusion, because my vehicle top speed is so close normal highway speeds, it would be a bad decision to reduce the diff ratio. For vehicles with a top speed well above normal highway speeds, it clearly could make sense to reduce the diff ratio. Basically, you are trading a lower top speed for better fuel economy at the speeds you actually drive.

So the good news is that I don't need to spend any money/time changing my differential. An evening of playing with Excel saved me a lot of work. The bad news is that I am stuck with a very noisy car at highway speeds.

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