"Motorcycle Fuel Mileage"

[Bonz]

Read an article from a more recent IRON BUTT magazine about motorcycle fuel mileage that a freind sent me. REALLY interesting.

The main focus was showing how a lower RPM and taller gearing results in less "throttling loss" by keeping engine vaccum lower than running a higher rpm/shorter gear that results in higher vacuum, all things equal. The idea is the vacuum is a pull on the top of the pistons, and reduces efficiency. As well, the lower rpm results in less friction (fewer piston strokes).

The model the article used was a Gold Wing GL1800, and how known and sound computer models used by auto and cycle manufacturers show running at 60mph with the engine at 1800 rpm in 5th gear (if 5th was geared that tall but it's not) vs. 3000 rpm in 5th gear gave a 21% increase in fuel mileage, from 40.9 mpg up to 49.5 mpg.

The crux of it is that motorcycle manufacturers have always made hay by having bikes with great roll-on performance without having to downshift, and thus virtually every bike is running much higher RPM than needed on the open road. I think we can all agree in the general sense that is accurate. As well, because riders have grown accustomed to this performance, it is unlikely to change anytime soon because how inconvenient would it be to actually downshift if you needed more passing power. The reality is most bikes could have a much taller top gear, and use the lower gears when needed.

Anyhow, I did an experiment...

I just returned from the XS Colorado Rally in Durango on July 4th. 1,025 miles in the 3 1/2 days for me. Spent most of the time on the open road in the 65 mph range on my 80 Special (with the 750 final drive swap) which is turning about 4000 rpm at that speed in 5th. When in the mountains for the group ride, where speeds were in the 45-55 mph range most of the time going from 7000 feet elevation, up to 10,500-11,000 feet, back down, up again, etc I kept the bike in 5th gear the majority of the time, running it around 3000 rpm with a larger throttle opening (relative to using 4th and running around 3400 rpm) due to the lower power output at high elevation.

I used 21.9 gallons of fuel for the entire 1025 miles according to the odometer. Do the math, I pulled 46.8 mpg for the entire trip. That's no BS, a good number of guys watched each time I filled up and it was real. My GPS said the entire trip was 999 miles, so adjust for "actual" miles if the GPS is to be believed (and I do trust it as being more accurate) and the mileage was 45.6 mpg.

I've never seen over 41 mpg on the bike (even after the FD swap), and that was running 50-55 mph on the backroads on a Sunday ride.

To be clear, I didn't lug the engine at any point, kept it at 3000 rpm as my lower thershold for 5th gear pulling the hills, and would go down to 4th when needed, but I definitely did not go to 4th nearly as often as I would have in the past, since I too like the punch I get when keeping her in the higher RPM (but less efficient) range.

So, I'd propose to take a tank or two of gas and run a bit taller gear than you may already be using for a given set of conditions and see what you guys can discover.

Let me know what you guys think, I know where I stand on this as I've done it and it works.

Cheers!

I'll be interested to hear!

[DAVINCI]

Well, BTDT, and it may be due to the altitude difference, but here at sea level I get best mileage in 4th running 4000-4500 rpm regardless of terrain. Best ever was 44.

[motoman]

Interesting findings Howard. Yup, watched that scoot more than you realize and saw the fuel miser it was. Doubt if the one step drop in mains made any real difference, cept maybe "leaner&meaner". The low mileage bike that it is may or may not of helped, as that is nothing more than speculation either. Since my Venturer had un-knowingly 81Special carbs on it with the shorter quick-taper needles, gonna start over with the float settings and stock 110 mains. With its stock final drive, fairing and all three trunks will do a comparison with similar speeds and mountains. Having basicly the same carbs cept for bowl drain locations, will see how the two different set-ups compare with elevations I'll run being the same. Guess any differences I could attribute to the over foot height difference between us as your the only guy I've seen to crawl aboard an XS11 and it looks like a Honda Trail90!

[Ebenezer]

I had similar results this weekend in the Black Hills. Put on almost 600 miles. And as you did (but for no reason at all) spent more time in 5th at a lower RPM rather than 4th, and averaged 42-43 mpg throughout the hills going between 40-50 mph. I was back down to 32-34 mpg through parts of Wyoming on the flat highways going 65-75 mph. I don't know if this proves your point, and I could be wrong, but don't you usually get better gas mileage going 50-55 than you do 65-70?

[crazy steve]

...The crux of it is that motorcycle manufacturers have always made hay by having bikes with great roll-on performance without having to downshift, and thus virtually every bike is running much higher RPM than needed on the open road. I think we can all agree in the general sense that is accurate. As well, because riders have grown accustomed to this performance, it is unlikely to change anytime soon because how inconvenient would it be to actually downshift if you needed more passing power. The reality is most bikes could have a much taller top gear, and use the lower gears when needed...

That doesn't surprise me at all, but as usual, there's trade-offs involved. If fuel economy is all you're after, this is a easy path to that goal. But you will make a sacrifice in terms of performance if all you change is the final drive ratio. And it's not just simply a matter of downshifting to regain that performance. You will take a hit in terms of lower-speed acceleration, and the manufacturers know this. They have two choices to regain that performance; one, increase the 'spread' between gears, so you start with the same effective low gear ratio but end with the higher final ratio. The downside to this is the motor has to operate over a wider rpm range in each gear, and as you get out of the 'sweet spot' in the power curve at one end or the other of that gear, efficiency will suffer. The other solution is to add more intermediate gears (which is why you're seeing 6 and 7 speed gearboxes show up in both bikes and cars) to keep the motor in that sweet spot better.

Now, I'll question that this would work on any bike (or car for that matter) as speed (as opposed to acceleration) is strictly a function of horsepower. It will take a certain amount of power to push a vehicle down the road at a given speed, and if the motor can't deliver that amount at the rpm dictated by the drive ratio, you'll see no gain and more likely a loss. In slower traffic such as stop-and-go, I can see a mileage loss as the motor is working into a bigger load and will need larger throttle openings for the same acceleration.

This will work, but will work best with bikes that have a lot of low-end grunt, which is why Harleys deliver such good mileage.

As usual, you pays yer money and takes yer choice....

[Bonz]

Putting what I read into use was really practical on a mountain ride since I had the choice to go down a gear and run more RPM or leave it in the higher gear at a lower rpm, but with more throttle turned in when it was under load (without lugging it and using 3000 rpm as my lower self-imposed threshold).

It's just that IF bikes had taller gears that would lower RPM in everyday riding regardless of terrain, elevation, etc, we would all enjoy better gas mileage, and with a quick downshift, have all the power and fun we are used to. It's a win-win situation to have both in that case.

Otherwise, there isn't a choice to be made, when the bike is running at 70-75 mph it's in top gear, or even at 50-60 mph, top gear is usually selected as well when commuting, etc, and mileage is what it is.

BUT the scenario where a taller gear and lower rpm for a given road speed where I could realistically have 2 viable gears to choose from was the goal, and running at lower speeds (45-55/60 mph) with higher load by staying in 5th gear in the mountains fit that to a "T".

If I had wicked it up a bit more on the coming and going from Durango (approx. 675 miles of the 1025) , no doubt my mileage would have dropped. There was enough climbing of hills/mountains in the coming and going where I could have zinged up in 4th or left it in 5th that I feel certain the style and mentality I used contributed noticeably to my overall mpg average given that the bike has never gotten over 41 mpg in the past, even when running "traditional" (higher rpm) gear selections in the mountains.

As well, my mpg increase with this method of riding coincides with the gains shown by the Goldwing in the article and it's relative MPG improvement with taller gearing for the same road speed, thus larger throttle opening and less throttling loss (vaccum pulling on the pistons) and friction.

Granted, my RPM drop was 400 rpm by choosing 5th vs. 4th in the mountains (compared to 1200 rpm drop on the referenced Goldwing when cruising at 60 mph) but all engines are different and will respond differently with different rpm loss due to higher gear selection (based on what I saw in the article), but the overall premise seems to be spot-on.

Also there is the variable of bike tune, and some bikes were arguably not as dialed in, probably even at their home elevations, but using my bike as the constant eliminates that and showed me the info I shared in the opening post of this thread has some merit.

Anyway, hope it's good info to digest for whoever reads this thread.

[crazy steve]

Well, I'll throw this out there as a possible cautionary tale about ratio changes...

I'm on a Sportster list also, and one of the riders there had the habit of upshifting at rather low rpms in an effort to get better mileage. Well, he succeeded... sorta. He got great mileage (he was averaging around 50 mpg in-town, and sometimes hit 70 mpg on the highway, and this was with a 1200, not the smaller 883) but he killed the connecting rod crankpin with about 70K on the bike. There was a lot of surmising as to the actual cause, but the general opinion was that the continuous torque load on the pin at low speeds put too much load on it. The other main theory was the oil pump wasn't supplying enough oil at that rpm for the load. It may have been a combination of the two, who knows...

Would this be a problem on a XS? I doubt it, as enough of the FD swaps have been done with no reported problems. But it's something to think about if you're looking to do another bike or run the bike heavily loaded.

[Bonz]

crazy steve,

I totally agree that there is a lower rpm point where the bike would lose efficiency or suffer mechanical strain if this was tested to the nth degree, however within the normal operating parameters of an engines power band, choosing the lower rpm/taller gear showed to make a measurable difference for me. Also, as an edit to address what you shared about the Sportster, 70k miles on any engine, while not extreme, isn't show-room new and if that thing was starved for oil, had extra strain, etc, seems to me it would have shown up a lot sooner than 70k. Just my thoughts though.

We only have the choice with the XS to do an FD swap, or change sprockets on other bikes, or no change at all on most shaft bikes and you are spot-on that it would sacrifice acceleration off the line, affect other areas of performance etc if taken to extremes.

To my mind, it is a simple matter for mfgs. to put a top gear in the tranny as a much lower rpm gear than we currently see on most any bike. That's all it would take and leave the rest as is so we can have the cake and eat it too.

Something the article pointed out was the motorcycle media has a bit to do with that perception of needing the top-gear roll on, and has for decades.

It mentions the new BMW K1600 compared to last years K1300. The new K1600 is faster in every measure of standing start acceleration (0-60, 1/4 mile) but it also turns in really good highway mpg numbers, measurably better than bikes of similar size and purpose, and better than the K1300. BUT it also mentions the RPM is much lower at highway speeds (the K1300 if you have read about it, is geared crazy short) and the writers noted it was not near as good at roll-on as last years K1300. That mentality isn't bad at all, it just keeps people believing that top gear roll-on is somehow a measuring stick for bikes, and people have gone along with that in my opinion.

Ebenezer,

Yeah, lower speed will give better fuel economy, wind resistance goes up exponentially compared to speed. Speed goes from 50 to 75 mph (33% increase) and wind resistance goes up by a much larger percentage. BUT in the article it also said the same Goldwing when run at 90 mph, while turning lower mpg numbers, did not lose mpg by nearly as much as you would think because the engine was now being run at a higher throttle opening which reduced throttling loss, even with the higher engine friction, wind resistance, etc encountered by higher speeds.

I had never heard of throttling loss before, but it makes perfect sense what they are talking about.

Motoman,

It was a blast to ride with you, I really like your bike! Keep playing with it, that's the fun of it all! Yeah, my tall body did pretty well, 'cept my butt... My seat turned into a "WAD", Weapon of Ass Destruction, definitely need to work on that for the next long distance venture. Stock seat looks good, feels good for the short run but is not so good in the reality of taking the bike far and wide.

[crazy steve]

...to address what you shared about the Sportster, 70k miles on any engine, while not extreme, isn't show-room new and if that thing was starved for oil, had extra strain, etc, seems to me it would have shown up a lot sooner than 70k. Just my thoughts though..

Harley bottom-ends are surprisingly tough; the only other 'normal use' crankpin failure I've ever heard about was on another Sporty that had been equipped for touring and ran pretty heavily loaded for most of those miles (don't ask me why; these aren't touring bikes ) and it failed at around 150K. That particular bike had always been serviced at a dealer (woman owned) and even Harley was surprised; they covered most of the repair costs without prompting even though it was long out of warranty. That was a bit of a shocker... Now, if you over-rev them, that will kill the crankpin. You're safe up to 7K, between 7 and 7.5 is no-mans-land, and over 7.5 put a new crankpin on your maintainence list along with changing oil...

Yes, better gearboxes would make a big difference, but they're building these to a perception and price, so what you see is what you get. I spent the money for a Baker 6-speed for my project sporty, which adds an overdrive sixth top gear. You are starting to see this on some of the newer bikes, so somebody is paying attention...

[Bonz]

Good info on the Harleys... The other thing I haven't shared about the article is the acceleration to get up to speed isn't much of a contributor to overall mileage unless you are really gunning it or are really loaded, as you are going through a similar rpm range to do that, regardless of the gearing, but it's the steady state efficiency that is the heart of the matter.

With that said, I didn't lolly gag getting up to speed on the trip I took, just normal acceleration (other than saying "screw the experiment" and letting it rip a few times through the mountains when coming out of corners or up a few straights, or once in a while when leaving a group stop), then paying attention to my "steady state" riding methods.