Speed/Gas Mileage sweet spot?

[k9gromit]

Just got an '11 R1200RT and am loving it. Wondering what others have found is the sweet spot for their speed/mileage on the Camhead motor. I'm talking specifically highway mileage here, specifically what one might do if doing a Saddlesore 1K, Bunburner or other LD ride...

 

For example: Do you ride 80mph (somewhere that's legal, of course) but get 30mpg and stop for gas every 180 miles or do you ride 65 and get 40mpg with a range of 240 miles? Does the extra speed make up for time lost in gas stops? I know that answers are individual but wanted to hear what others have found...

 

 

[4wheeldog]

I cannot speak to your specific MC, but I have managed to get 53 to 56mpg on a couple of occasions holding a steady 70mph, no stops for 300 miles plus. This on an R1150RT.

I find elevation makes a larger difference than you might think......The difference between sea level and 7000' is well over 10%. (Air speed at altitude is lower than groundspeed, since the air is thinner).

[Selden]

Air resistance is lower at higher altitudes. Also, with thinner air, the FI requires less fuel to achieve the same air/fuel ratio, which should leave to better gas mileage.

[David13]

I just ride at my comfort level.

Mine gets good gas mileage. You will probably find yours does too.

dc

[Polo]

I cannot speak to your specific MC, but I have managed to get 53 to 56mpg on a couple of occasions holding a steady 70mph, no stops for 300 miles plus. This on an R1150RT.

I find elevation makes a larger difference than you might think......The difference between sea level and 7000' is well over 10%. (Air speed at altitude is lower than groundspeed, since the air is thinner).

 

53??? I'm getting about 40, but at varying speeds with some hooning thrown in.

[upflying]

Air resistance is lower at higher altitudes. Also, with thinner air, the FI requires less fuel to achieve the same air/fuel ratio, which should leave to better gas mileage.

But you have to use more throttle at high altitude to achieve the same performance at sea level. MPG benefit at high altitude probably won't happen.

I read somewhere about 45 mph is the sweet spot of speed/mpg/aerodynamic drag.

[leikam]

The bike's sweet spot is going to be in top gear at low engine RPM. The low engine speed means less gas used per time; the high gear means the most distance traveled per engine revolution.

 

But what makes a long distance ride is your physical comfort and mental focus. For me, that means variety and stopping when I feel like it even if the bike doesn't need it.

[Dave_in_TX]

Air resistance is lower at higher altitudes. Also, with thinner air, the FI requires less fuel to achieve the same air/fuel ratio, which should leave to better gas mileage.

But you have to use more throttle at high altitude to achieve the same performance at sea level. MPG benefit at high altitude probably won't happen.

I read somewhere about 45 mph is the sweet spot of speed/mpg/aerodynamic drag.

 

I have consistently gotten better mileage at higher altitudes. I suspect that lower drag dues to the thinner air results in less power needed to maintain speed.

[cali_beemer]

my 05 RT would get its best fuel economy in 6th at 70 mph indicated (realistically closer to 65 by GPS). I could get 53-56 mpgs.... using cruise control down hwy 99 in california at near sea lever or withing a few hundred feet of it.

[upflying]

Air resistance is lower at higher altitudes. Also, with thinner air, the FI requires less fuel to achieve the same air/fuel ratio, which should leave to better gas mileage.

But you have to use more throttle at high altitude to achieve the same performance at sea level. MPG benefit at high altitude probably won't happen.

I read somewhere about 45 mph is the sweet spot of speed/mpg/aerodynamic drag.

 

I have consistently gotten better mileage at higher altitudes. I suspect that lower drag dues to the thinner air results in less power needed to maintain speed.

I'm not saying you are wrong but we better get some physics experts to verify what you are saying.

From a pilot perspective, you need more throttle (non-turbocharged) to maintain speed at high altitude. The lower density of the air does not have a significant enough effect on aircraft drag to affect throttle opening. Indicated airspeed does not change regardless of altitude although true airspeed does increase.

[David13]

Upflying quite possibly knows what he is talking about there. As his name suggests flying, so maybe he knows.

I have seen that the optimum spot is usually quite slow. In super mileage tests, they roll quite slow to get the best mileage. But I like to go my own speed. Not 45 or 55. Unless I'm on a slow road.

I know I got average 53mpg on my Colorado (high altitude) ride last September. Good enough for me and I did a lot at 80 to 83 or so.

dc

[k9gromit]

I have seen that the optimum spot is usually quite slow. In super mileage tests, they roll quite slow to get the best mileage. But I like to go my own speed. Not 45 or 55. Unless I'm on a slow road.

 

dc

 

Yes, I should have been more specific in my original post...I know I can bring my gas mileage to an absolute maximum at a realtively low speed. What I was looking for was folks real-world, getting somewhere in a reasonable time frame, expereinces with balancing speed and fuel consumption.

 

I'm not even that interested in the topic from a fuel consumption *alone* point of view...I'm thinking in terms of riding long distances efficently for "Iron Butt" style activities...

[ScottT]

I used to get around 47 mpg all around here at home but it's dropped to about 42 mpg. The reason? Gas... as in there's now that crap ethenol in it. Even at 42 my range is still at least 300 miles if I stuff it full... 8 gallons if I fill bast the filler neck. At home I generally don't because you don't need to go 300+ on a tank. Highway mileage varies according to speed of course. 65 to 70 will yeild 50+ with no ethenol... about 45 with the crap. MPG at altitude is fantastic as stated here. Mid to upper 50s at 5000 and up feet. I've seen as high as 65mpg. I'd say the sweet spot while still being able to make decent time is 65 to 70. BTW, my RT has never dipped below 40 mpg

[malcolmblalock]

My 09 RT does very well at altitude. I've seen over 60mpg when riding distances at higher elevations, and down in the mid 40s at a higher speed (ie 80mph or so) and lower altitudes.

 

I also agree that ethanol content makes a big difference; it seems to lower mpg by about 10%.

[OlGeezer]

I have seen that the optimum spot is usually quite slow. In super mileage tests, they roll quite slow to get the best mileage. But I like to go my own speed. Not 45 or 55. Unless I'm on a slow road.

 

dc

 

Yes, I should have been more specific in my original post...I know I can bring my gas mileage to an absolute maximum at a realtively low speed. What I was looking for was folks real-world, getting somewhere in a reasonable time frame, expereinces with balancing speed and fuel consumption.

 

I'm not even that interested in the topic from a fuel consumption *alone* point of view...I'm thinking in terms of riding long distances efficently for "Iron Butt" style activities...

 

9 mph over the speed limit.

 

[Dave_in_TX]

Air resistance is lower at higher altitudes. Also, with thinner air, the FI requires less fuel to achieve the same air/fuel ratio, which should leave to better gas mileage.

But you have to use more throttle at high altitude to achieve the same performance at sea level. MPG benefit at high altitude probably won't happen.

I read somewhere about 45 mph is the sweet spot of speed/mpg/aerodynamic drag.

 

I have consistently gotten better mileage at higher altitudes. I suspect that lower drag dues to the thinner air results in less power needed to maintain speed.

I'm not saying you are wrong but we better get some physics experts to verify what you are saying.

From a pilot perspective, you need more throttle (non-turbocharged) to maintain speed at high altitude. The lower density of the air does not have a significant enough effect on aircraft drag to affect throttle opening. Indicated airspeed does not change regardless of altitude although true airspeed does increase.

 

Unlike a ground vehicle, an aircraft not only has to propel itself forward but it has to also maintain lift by moving sufficient air under the wings. The sircraft is using more power because its true air speed is faster. It has to travel faster in order to get sufficient lift

[GRB60]

To get back to the original question, I think 70 is a speed that gives good (in my case, over 50MPG)mileage and gets you where you're going. At 80MPH you'll get noticeably less MPG. 60 is too slow.

[BerndM]

On my particular 05 RT, it appears that a steady 4000rpm, 72mph, works really well.

I routinely get 50-51mpg average between freeway and surface riding here in SoCal.

 

[Joe Frickin' Friday]

The bike's sweet spot is going to be in top gear at low engine RPM. The low engine speed means less gas used per time; the high gear means the most distance traveled per engine revolution.

 

Lower RPM means fewer combustion cycles per unit time, but in order to maintain a given speed, you will need to open the throttle more to maintain the same crankshaft power output, meaning more fuel is used per combustion cycle. In the absence of any change in engine efficiency, the net result is the same fuel consumption, regardless of what gear one selects.

 

However, engine efficiency does vary with RPM, and it also varies with throttle setting. An engine's mechanical friction varies with RPM - lower RPM means less friction. That friction also varies with load - open the throttle more, and you get more friction. For a given vehicle speed, when you select a higher gear, the RPM goes down, but the load (throttle) goes up, and those two friction effects tend to cancel out. However, opening the throttle has another effect: the engine isn't struggling to breathe past a restrictive throttle plate. The engine wastes less power trying to breathe, and so the net result is that for a given vehicle speed, engine efficiency tends to increase as you select higher gears. This is why cars that aren't sport-oriented tend to have a really tall overdrive gear, so you can get the best fuel economy possible on the highway. Some cars that are sport-oriented but would otherwise incur a harsh gas guzzler tax also employ a tall overdrive gear to improve the results on the EPA's highway fuel economy test.

 

OK, that covers how to get best engine efficiency for a given power demand: choose the highest gear you can run in without lugging the engine. What about reducing power demand?

 

Aerodynamic drag is proportional to the density of the air through which the vehicle travels. Aerodynamic drag isn't the only thing requiring power; you also give up some power to the transmission, final drive, and the rolling resistance of the tires (this is why your tires are nice and warm). At cruise, you may be losing about 10 percent of your crankshaft power in the gearbox and in the final drive, and maybe another ten percent to rolling resistance - meaning aerodynamic drag is responsible for about 80 percent of your engine's power requirement. So if you compare MPG at sea level with MPG in (for example) Denver, where the air density is about 83 percent that of sea level, you might expect to see about 16 percent improvement in fuel economy under similar driving conditions: if you were getting 45 MPG cruising the DC beletway, then in Denver you might expect 52 MPG when cruising on level ground at the same speed.

 

Aerodynamic drag is also proportional to the square of vehicle speed. Double your speed, and your aero drag will quadruple.. So if you get 50 MPG when you're cruising at 60 MPH in top gear, why don't you only get 12.5 MPG when you're rocketing along at 120 MPH? The answer is that at 120, you've got the throttle just about wide-open, and the engine is much more efficient than it was at 60 MPH. You may be delivering 4X the crankshaft energy per unit distance (8X the energy per unit time!), but the engine's efficiency is way up; your fuel economy at 120MPH won't be 50, but it'll be a lot higher than 12.5.

 

The OP's case is more complicated because it brings in travel logistics and time management. Go fast and stop to refuel more often, or go slow and take fewer fuel stops? My personal preference for cross-country travel - Ironbutt or otherwise - is to haul ass, somewhere around 5-10 MPH above the highway speed limit in whatever state I'm traveling through. at 80 MPH, my '09 RT gets about 43 MPG, which is enough to go 250 miles between fillups (leaving a comfortable reserve fuel quantity). Your camhead probably gets similar mileage, resulting in similar range. If you dared to travel as slow as 60 MPH on the interstate highway during your Ironbutt ride, your fuel economy might improve to 50 or so, increasing your tank range from 250 miles to about 300 miles; that would mean two fillups instead of three over the course of a SS1K. You'd save a ten-minute gas stop, but your slower speed means your actual time at cruise would increase by about four hours.

 

Bottom line? If you want to finish your Ironbutt event (or any other long-distance ride) ASAP, go as fast as you dare, mileage be damned.

[4wheeldog]

3.5 hours is about my personal tank limit between empties, which equates to 250/280 miles, so going slower does nothing to decrease the number of stops per day. Just sayin'

 

If you think 60mph will let you ride longer, install a "Motorman's friend" or better yet, a relief tube.

[leikam]

Thanks for filling in the technical details, Mitch.

[k9gromit]

If you dared to travel as slow as 60 MPH on the interstate highway during your Ironbutt ride, your fuel economy might improve to 50 or so, increasing your tank range from 250 miles to about 300 miles; that would mean two fillups instead of three over the course of a SS1K. You'd save a ten-minute gas stop, but your slower speed means your actual time at cruise would increase by about four hours.

 

 

After doing some math on the back of a napkin, that was the result of my calculations as well... Roadside performance evaluations and fatigue are not factored into my equations.

[g_frey]

I get about 46 mpg at 65 to 70 mph on my 1150RT. What many new sport touring bikes don't seemed worried about is mpg. Some get less than 32 mpg apparently which is car territory on my old 65 VW bug. While I have never run out of gas there have been many times that I scraped by. This is one reason that I will not upgrade; I already have the perfect bike.