Note that when using the cruise control, you can still change gears and hold the speed setting. I think the "sweet spot" for fuel economy is in the 55-60 mph range. At 55, you can use either 4th or 5th gear... In 4th @ 55 mph, the RPM is about 2800, which is where the engine reaches close to maximum torque. The Sprinter 5 speed tranny is notorious for letting the rpm's drop pretty low before downshifting, often "lugging" the motor a bit. The manual down shirt reduces the lugging (engine load), helps the mileage, and makes for a happy, more clean running engine.
If you are interested in having more info on the inner-workings of the Sprinter at your fingertips, consider getting an OBD (On Board Diagnostics) reader, like a ScanGuage 2. This device allows you to monitor all sorts of engine data, like engine load, exact RPM's, Speed, Water Temperature (and lots more). There is a months worth of reading on the benefits of having one of these... Here's a link to one of many threads on the forum that discusses OBD's.
Hope this helps!
Bob & Cynthia
2017 SunSeeker MBS 2400W
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