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6MT Fuel consumption (Test IV): Different Speeds and Different Gears
Today's Fuel Consumption Test is a follow up on my 2nd test that explores the finer detail of how efficient the engine is at different RPM in the speed range between 35-50 MPH. I'm going to use the data I logged to make some conclusions and also determine what could be "ideal shift points" (for cruising, not so much acceleration)
The test was conducted on a 2 mile long stretch of straight, nearly flat road, but the actual data was only logged on 1.5 miles. The road is a 0.6% nearly constant grade either way. All runs were done with headlights, defrost on low, and through the course of the runs I lost about 1.7 gallons of fuel, or about 10 lbs (slightly boosting efficiency), and the temperature gradually fell from 27F to 19F (-3C to -7C) slightly hindering efficiency. I would set the cruise before point A, reset my trip on the Scangauge at Point A (top of hill), then note the AVG mpg value at point B. Then I'd drive to the turnaround, turnaround, pick up speed, resume the cruise, then reset the trip again at point B (bottom of hill), then note the MPG at point A, then basically repeat until I had the data I wanted.
Based on previous tests, I predicted that the 1800-2000 RPM range would be the least efficient and the 2400-2500 RPM range would be the most efficient. I think it's pretty safe to say I was correct.
Gear choice for steady state cruising should be calculated depending on your cruising speed and the expected demand on the engine to maintain that speed. I think it goes without saying you should take a higher gear and lower RPM when traveling downhill. But, when is the right time to start downshifting as the hill flattens out and transitions to uphill? What RPM should you try to target if possible?
For steady state cruising at 35 MPH, it is better to take 4th than 6th, unless traveling downhill enough to notice. I'd say probably between 1.5-2% downhill and 6th will overtake 4th for the more efficient gear. Grade is not the only important factor, weight and aero drag should also be taken in to consideration when deciding which gear is best.
Going up a 0.6% grade, It was better to hold 4th even up to 2670 RPM at 43 MPH, although I predict that either 4th or 5th would have been equally suitable at 45. 5th would almost certainly overtake 4th by 48 MPH, but unfortunately I ran out of endurance and cut my testing short so I can only speculate.
Going down a 0.6% on the other hand, 5th had already overtaken 4th by 43 MPH, but not by 40 MPH. So, on a flat, level road, you should cruise in 4th below about 43 MPH, and starting at 42-43, you should shift to 5th for steady cruising. Going up a slight grade is roughly equivalent to the extra drag of driving around corners, so on twisty roads, I'd say stick with 4th until you hit at least 45, but maybe up to 48 if it's really twisty or slightly uphill. Obviously maintaining this sort of speed up significant grades will call for 4th or even 3rd.
6th gear only benefited me in this run in favor of 5th once speed reached 50 MPH, and that was only the downhill section of the road, the uphill section still preferred to cruise at 2650 up the hill instead of 2380. The flat road crossover for efficiency must be between 48 and 50 MPH, with a peak efficiency not much beyond 50 MPH in 6th. This means the 6th gear isn't as bad as I originally thought it was. Going by feel, it seems like 6th gear is the gear of choice for steady cruising on flat ground at any speed above 30 MPH, and still usable down to 25 on slight downhill stretches. You would think that it would cause efficiency to suffer beyond 40 or 45 MPH because there isn't an additional overdrive to keep the RPM even lower, but that isn't remotely a hinderance, at least at the speeds I've discussed so far. In my test, 6th was not the preferred gear until 50 MPH was reached and a downhill was in effect. It goes to show that the engine really is incredibly efficient at the mid 2000s RPM range, and you don't gain anything from the Fit's engine by revving 1500 rpm at 50 mph, even though a K24 Accord engine would absolutely benefit from low revs like that, and a J35 Pilot would absolutely hog gas if it couldn't reach the lower 1000s during steady speeds.
As the vehicle travels faster, the power required to keep it at that speed goes up, so for an engine to propel a vehicle faster and get better fuel efficiency, it must mean that the engine is converting fuel to energy more efficiently at a higher rate than the resistive forces go up. If the MPG were to stay the same for an incremental increase in speed, then you can tell the efficiency of the engine is about to plateau (at least locally). With that logic in place and taking analysis of the data I have compiled from this test and previous tests, I can confidently say that the <1700 RPM zone of the Fit's engine is medium efficiency, only useful for very low power/low load scenarios like traveling downhill.
The 1700-2050 zone of the Fit's engine is low efficiency and should be avoided if possible. If it's possible to downshift or upshift to keep the engine out of that particular region of the rev range, it should be done. For low speed and low load scenarios, you should take the highest gear that the Fit can comfortably cruise at that isn't going the make the engine rev between 1700-2050 RPM. The instant you need a significant amount of power, you should downshift to get above that region, but for optimum efficiency, you should be shifting up to get back into the peak efficiency zone after revving much past 3000.
After 2100 RPM, the Fit's engine rapidly becomes much more efficient, and reaches maximum efficiency between 2400-2600 and then slowly starts tapering off from there. This means that the 6MT Fit is pretty much going to get it's best fuel efficiency at 50-55 MPH on flat ground, slower uphill or downhill (where the grade is insufficient to propel the vehicle under gravity at or faster than 55MPH, otherwise all speeds below "terminal velocity" are equally efficient assuming no gasoline is needed for propulsion).
"Ideal" shift points should be shifting at the earliest RPM that will keep the engine out of the low efficiency band of the rev range. Acceleration should be done at a significant throttle input for best results. Shifting into 2nd at 17 MPH, into 3rd at 26 MPH, into 4th at 35 MPH, into 5th at 43 MPH, and into 6th at 48 (or you can just skip 5th and wait until 48 to shift to 6th) will keep the engine as close to it's sweet spot as possible and keep it out of the low efficiency band. If only a small amount of power is needed, it would be better to keep the engine under 1700 rpm than dwell between 1700-2050 for any significant length of time. It almost sounds absurd, but short shifting at 2200 rpm will cause the engine to spend a great deal of time in the low efficiency band and paradoxically hurt fuel efficiency. Revving to 4k in 1st gear with significant throttle is not only the more thrilling alternative, but unexpectedly the most efficient option as well. You'll dodge the stumbling in 2nd and 3rd gear that frequently happens on a cold L15B1 because the stumbling usually occurs in the low efficiency band anyway. The only downside is all the racket you'll make every time you pull away from a stop.
Cruising at 15 mph - use 3rd or maybe 4th if downhill
Cruising at 20 mph - use 4th or maybe 5th if downhill, use 2nd instead of 3rd for steep hills.
Cruising at 25 mph - use 4th, 5th or 6th depending on grade, and resistive factors like passengers, cargo etc. Avoid using 3rd, still better to use 2nd for steep hills.
Cruising at 30 mph - use 3rd if any significant amount of power is required, 2nd if over 80% throttle is required, otherwise straight to 6th for downhills.
Cruising at 35 mph - use 4th unless downhill, then straight to 6th. Use 3rd for more power, 2nd is reserved for full throttle.
Cruising at 40 mph - use 4th unless downhill, then go with 5th. Only take 6th if you can ride downhill with no gas. 3rd is used for steep hills, more than 75% throttle.
Cruising at 45 mph - use 5th unless noticeably downhill, then go with 6th, use 4th for twisty roads or slight hills. 3rd if 4th can't handle it, 2nd for full throttle.
Cruising at 50 mph - use 6th unless going slightly uphill. This is a hard call to make if the road is unknown. If there are resistive factors, take 5th unless noticeably downhill.
Cruising at 55+ - use 6th unless more power is needed.
A taller 6th gear would really be helpful to give the Fit higher efficiency beyond 55-60 MPH. This wasn't news to anyone before, but now we have context on exactly how bad the Fit really needs a 6th gear. Not as bad as we thought, but being able to keep the engine in the 2500-2700 sweet spot for higher loads would make this a 45 MPG highway car. I'm sure there are people getting 40+ with CVT Fits going steady 65-75 MPH, but us 6MT Fit owners can only dream of being able to rev under 3k past 63 MPH.
Here's a graph for visualization of the data. It's handmade in paint, so enjoy.
Last edited by Chugiak76; Apr 2, 2021 at 06:37 AM.
Am sure it's a common sentiment.....beings that 6th gear is pretty much the same as 5th on the previous Fits.......they should've left 5th gear the same (it's like a half step between gears) and 6th should've been geared higher for the hwy .... 3500 rpm at 70 mph is a lil silly .....but we know that. Thanks for the work you put in, I found it interesting. I live in a small, hilly, twisty/turny city, the Fit is great just scooting around, as how most of my driving is done, anymore. If I'd left the factory muffler on, it might be less noisy at hwy speeds.......my mother in law doesn't understand why I paid for the Tanabe Medallion axleback...most times she's in the car, she says I need a new muffler...LOL .....but, I still have the factory muffler in my garage (1 yr old or so when replaced in '16) so at least I have a muffler to fall back on if the Tanabe ever fails. I love the sound, don't even listen to music much driving anymore, just listen to the sound rowing through the gears. BUT, yeah, a taller 6th would've helped with less engine drone on the hwy and better mpg as well.....but, again.....we know that. Still love it though.......fixing to retire soon, this may be the last car I have as a daily driver, and one my wife retires, we can go down to one car, I think. Hopefulle not an EV or EV/self-driving car. I love3 being in control, haven't had an accident since I was a teenager......great fun
Am sure it's a common sentiment.....beings that 6th gear is pretty much the same as 5th on the previous Fits.......they should've left 5th gear the same (it's like a half step between gears) and 6th should've been geared higher for the hwy .... 3500 rpm at 70 mph is a lil silly .....but we know that. Thanks for the work you put in, I found it interesting. I live in a small, hilly, twisty/turny city, the Fit is great just scooting around, as how most of my driving is done, anymore. If I'd left the factory muffler on, it might be less noisy at hwy speeds.......my mother in law doesn't understand why I paid for the Tanabe Medallion axleback...most times she's in the car, she says I need a new muffler...LOL .....but, I still have the factory muffler in my garage (1 yr old or so when replaced in '16) so at least I have a muffler to fall back on if the Tanabe ever fails. I love the sound, don't even listen to music much driving anymore, just listen to the sound rowing through the gears. BUT, yeah, a taller 6th would've helped with less engine drone on the hwy and better mpg as well.....but, again.....we know that. Still love it though.......fixing to retire soon, this may be the last car I have as a daily driver, and one my wife retires, we can go down to one car, I think. Hopefulle not an EV or EV/self-driving car. I love3 being in control, haven't had an accident since I was a teenager......great fun
i have that exhaust , i don't think it's that loud at all , you want loud ? change the B pipe . loud and droney . was too much , i put the stock one back on , it's in the attic next to the stock axel back
Nah, I don't think it's loud at all, it's "pleasant" to me ......and especially funny since she's told me multiple times of "back in the day" my father-in-law getting repeated tickets for "excessive noise" or something on his cars, dunno if he ran straight pipes or just a loud muffler, but......apparently my FIL fancied himself to be a hot rodder of sorts, and the local police in southeast MI didn't take a liking to it - guess a noisy exhaust is an easy ticket to write. He's like 80, now, starting to go downhill cognitively, wonder if he's remember what he did to his cars. Quite likely, he just has trouble connecting his thoughts to his speech, unfortunately ...... it's rough getting up there. But, yeah, the Tanabe isn't loud, but it sounds better than the factory muffler
