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Is it possible to improve the fuel consumption (decrease it) in my van by changing the rear differential ratio? I would like to optimize it for low-speed driving about 40 km/h (25 mph).

Is it true that the rear differential ratio is optimized for a specific speed, and if your most commonly used speed is different, you can decrease your fuel consumption a bit by changing the ratio?

About the van:

  • Mercedes-Benz 308D year 1992, model 602.316
  • OM601.940 4-cylinder 58 kW (79 hp) 2299 dm3 diesel engine
  • Maximum engine torque: 157 Nm @ 2000..2800 RPM
  • Standard rear differential ratio: 4.875

Gears ratios (total drive train ratio in parentheses):

  • 1-st gear: 4.695 (total 22.888)
  • 2-nd gear: 2.401 (total 11.705)
  • 3-rd gear: 1.436 (total 7.000)
  • 4-th gear: 1.000 (total 4.875)
  • 5-th gear: 0.806 (total 3.929)

I read somewhere that for this engine, the most efficient RPM is the lowest one you can run without the engine struggling.

Could changing the rear differential ratio have an effect on fuel economy of this van around 30-50 km/h (18-31 mph), and if so should I try decreasing it or increasing, and to what value?

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up vote 6 down vote accepted

The short answer is that yes, you can change your differential to optimize fuel efficiency. CAVEAT: it is almost certainly not worth it.

Here's a very high level discussion of why there are better ways to achieve the same goal (better fuel efficiency):

Think about how the air-fuel mixture in the engine is managed: for each revolution of the engine, a unit of air is sucked into the cylinder. A unit of fuel is added to this air according to the ratio set by the fuel regulator (an engine computer, a carb or functional equivalent). This combination of air and fuel is then going to be compressed and combusted.

However, that squirt of fuel was your entire fuel consumption: that's the only time that fuel leaves the tank on its way to being ignited. If you want to manage your fuel consumption, you want to reduce the number of times that injection happens and also keep the amount of each injection down.

So, you have two things that you can manage, one of which was the topic of your question:

Gear ratio: a high gear ratio allows you to cruise at lower engine revs (your real goal). This reduces the number of fuel injections per second. The gear ratio can be changed by modifying the vehicle with a higher final drive but it is significantly more convenient to just select a higher gear. Barring that, you can also purchase larger diameter tires. Try driving in 5th gear (an overdrive gear from your notes above): is the engine speed close to your comment of "one you can run without the engine struggling"? If so, you are done.

Throttle position: an open throttle allows more air into the cylinder. As a result, more fuel is added to preserve the mixture. The least throttle required to move is almost always the most efficient throttle position. This is also why leaving the vehicle in gear during decelleration is more efficient that moving to neutral: the vacuum generated will result in even lower fuel consumption than the idle setting.

Practical example: in Top Gear season 4, Jeremy Clarkson drives a turbo diesel Audi from London to Edinburgh and back on a single tank of fuel. That episode summarizes almost everything you need to do as a driver to maximize efficiency. Some of them are a bit extreme (e.g., never turn the heater on? I'll pass) but others are practical and applicable (e.g., management of momentum and situational awaareness).

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Smaller throttle openings may be more efficient at steady speeds, but when accelerating they can be less efficient due to pumping losses (energy spent overcoming the air resistance of the partially-closed throttle). That aside, wouldn't the throttle position be the same whether the car were in gear or in neutral, assuming the driver leaves his foot off the accelerator? –  William Cline Apr 12 '11 at 21:45
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@William, that's one of the great things about this field. These are measurable quantities, not debate topics. Modern cars provide this sort of thing via the OBD connection / ECU datastream (the 92 diesel mentioned above may be borderline convenient). Grassroots Motorsports grassrootsmotorsports.com routinely advertises units that connect to an ECU and transmit to an iphone for this sort of monitoring. Small throttle openings and engine braking are both clear winners in fuel efficiency. –  Bob Cross Apr 13 '11 at 12:26
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