Send me a note:
darin AT metrompg D-O-T com,
Showdown: testing RPM vs. MPG at a fixed speed
Posted Monday, March 13/06 in Mods & Tests
If you row your own gears (as most fuel economy nerds do), here's a mini-experiment that should be of interest.
I recently did a test run that nicely illustrates the fuel efficiency "costs" of reciprocating mass, accessory drag, and internal engine & transaxle friction at varying engine RPM.
If you ever doubted the economy driving tip that says you should be in the highest practical gear at the lowest possible cruising speed, doubt no more.
Simply, I recorded fuel consumption in four different gears (2nd through 5th) at the same road speed (60 km/h, or about 40 mph) on the same stretch of road.
Route & weather conditions:
The test route was a 1.8 km / 1.1 mile long, very slightly rolling road (maybe a 5-10 ft elevation change).
Air Temperature (ATMP): 18.0 �F / -7.8 C; Feb 28/06; 3:00 - 4:00 PM
Wind Direction (WDIR)...WSW ( 240 deg true )
Wind note: I probably wasn't subjected to the full extent of the wind indicated above. The weather station I get my readings from (located roughly 5-10 km from the test route) is fully exposed in open water on the St Lawrence River, and the 1.8 km stretch of road I used has portions which are tree-lined, and other portions which are protected by some land elevation to windward.
All readings were taken going in the same direction (SW); I looped back around for each run. The car was warmed up prior to this test with about 30 minutes of errand running (in-town driving).
Test speed: 65 km/h / 40.4 mph (+/- 1 mph)
ScanGauge readings (avg MPG function)
For reference: going in the opposite direction (NE) along the same 1.8 km route at the same speed:
(Nice illustration of the aerodynamic penalty/benefit of wind, eh?)
After posting my results, a fellow fuel economy enthusiast did a similar run in his ScanGauge-equipped Scion xB and came up with these figures:
Test speed: 38 mph / 61.2 km/h
Update: March 27, 2006...
Feedback from John G, originally posted at evconvert.com related to my thoughts on getting a taller transmission (and my assumption that the relationship between MPG and RPM is linear):
"Efficiency and engine RPM are not directly related, meaning that a 25% RPM reduction does NOT predict an efficiency improvement of "x%", nor does it predict any improvement at all - although RPM reduction will generally improve efficiency.
Thanks for the comments, John.
Incidentally, I've seen evidence of the "sweet spot" in regular driving (without attempting to test for it properly - though it's on my list): when I switch into top gear, my "instant" fuel economy often continues to improve for a short while as I gradually increase RPM and build up speed.
Not only does that confirm John's point that MPG is not directly related to RPM, it's also evidence that fuel efficiency can be worse if RPM is kept too low.
That said, at least on my car, even when driving the engine at below optimal RPM, choosing a higher gear still results in higher fuel efficiency than the same road speed in a lower gear.
On the question of whether to go with bigger wheels/tires or a taller transmission, Rick writes:
"My vote is for the larger tires. If you check the specs area at tirerack.com...you can get some exact tire weights and so forth. Also wheel weights. Would really like to see you test the larger tire idea. I have a set of 2% oversized tires in storage...waiting until needed to install them."
If only the Metro/Firefly didn't have an uncommon bolt pattern, it would be relatively easy for me to borrow a set of larger rims to try this out. Unfortunately, I'm stuck with bolt pattern not used by Honda, VW, Toyota, etc. But if the opportunity comes up, I'll certainly go for it!
darin AT metrompg D-O-T com, or here