MPG & Speed

Yesterday I ran a tank of gas through the 95 RT, at 78-80 MPH on the Interstate, and it got 33.5 MPH. Today I ran a tank through at 73-75 MPH, and got 38 MPG. Hard to believe a five MPH drop yields a 4.5 MPG increase, but that's what the math says. Both days, no head wind or tail wind, to speak of, and speed and mileages were per the GPS. Guess I am gonna have to get used to puttering along, when on the Interstate. Especially since gas is likely to go up again.

If I recall

It has been a long time since I sat in physics class but I seem to remember air resistance may go up logarithmically not in a linear way. So, resistance at 80mph is much higher than at 70mph. I will also guess mpg follows the same rules. It makes sense as resistance is much higher at higher speeds. St.

Drag coefficient goes up with the square of the speed (I think...).

Factor in engine heat at higher speed.

Engine Heat

20774 said:

Factor in engine heat at higher speed.

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Not sure engine heat was a factortboday, it was 36 when I left, and 35 upon return, 240 miles later. Temp peaked briefly at 42, but NO sunshine. Still, a great day for a ride. I have ordered a dipstick temp gauge bc I'm curious about engine temps. Had one on my last Harley, but had to raise the seat to view it. With the Airhead, I'l be able to lean over and check it anytime during daylight. I just like gadgets like that.
The last two days of riding on the Interstate really racks up mileage quickly, 415 total, yesterday and today. I usually stay on rural roads, mostly in 2nd thru 4th gear, much more fun. Hit the Interstate for a break in routine, but not ready to go back to small roads.

Interesting, on the small roads the RT usually gets 40-42 MPG.

Temp and wind direction also figure in. I remember getting 33 mpg on the three bikes I had in the 90's -- an R100, 900SS, and 650 Hawk. All the same mpg in everyday riding. (yes, that is a non sequitur)

With a newer model with MPG readout it is interesting to ride along a flat road and adjust windshield height, put you feet out in highway pegs, or stick and arm out and watch the MPG change.

Whoops!

My mistake, as pointed out, drag goes up exponentially, not logarithmically. That is what I get for not keeping up with physics and math. Now, I am going to study the law of thermodynamics in my cup of coffee, LOL. St.

And your rear tire has to push much harder to overcome drag at higher speeds so your rear tire will wear out sooner at higher speeds.

Harry

It has been my experience that bikes with larger engines have less variation in MPG than bikes with smaller engines. Fuel mileage on my K75s varied by a few MPG depending on speed. My R1150R and Voni's R1100RSs varied even less. Our G310GSs vary a great deal more, by as much as 15 MPG between cruising at 60 and cruising at 75. YMMV literally.

On my bikes, 45-50mph is the sweet spot. I can average as much as 58 mpg 5th gear at 51mph.

Slower Speeds

rangerreece said:

On my bikes, 45-50mph is the sweet spot. I can average as much as 58 mpg 5th gear at 51mph.

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My RT gets best MPG at similar speeds puttering around on KY back roads, even though much time is spent in lower gears. However, riding on the Interstate at 45-50 MPH will get you killed.

jhall said:

My RT gets best MPG at similar speeds puttering around on KY back roads, even though much time is spent in lower gears. However, riding on the Interstate at 45-50 MPH will get you killed.

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What’s an interstate?

jhall said:

Not sure engine heat was a factortboday, it was 36 when I left, and 35 upon return, 240 miles later. Temp peaked briefly at 42, but NO sunshine. Still, a great day for a ride. I have ordered a dipstick temp gauge bc I'm curious about engine temps. Had one on my last Harley, but had to raise the seat to view it. With the Airhead, I'l be able to lean over and check it anytime during daylight. I just like gadgets like that.
The last two days of riding on the Interstate really racks up mileage quickly, 415 total, yesterday and today. I usually stay on rural roads, mostly in 2nd thru 4th gear, much more fun. Hit the Interstate for a break in routine, but not ready to go back to small roads.

Interesting, on the small roads the RT usually gets 40-42 MPG.

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I used to have a dipstick thermometer on an air cooled car
so helpful !

I did not know they made them for Airheads
where did you get it from Jeff ?

thank you for your time
brant

eBay Thermometer

brant914 said:

I used to have a dipstick thermometer on an air cooled car
so helpful !

I did not know they made them for Airheads
where did you get it from Jeff ?

thank you for your time
brant

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Hi Brant,
I got the thermometer off eBay. Cost about $55 including tax, with free shipping.
Joe H

jhall said:

Hi Brant,
I got the thermometer off eBay. Cost about $55 including tax, with free shipping.
Joe H

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Thank you Joe!

and I apologize for calling you the wrong name
brant

In WW2 those pilots had a saying, " The slower it turns the less it burns" when going for max range. They could control engine speed via the propeller control and also manifold pressure via, throttle, they would go for lowest RPM and highest manifold pressure without blowing up the engine due to detonation, using 130 octane fuel helped alot in this configuration.
I havent run any numbers but I'd imagine my 1978 R80/7 would get the max MPG in 5th gear going as slow as I could without detonation or low oil pressure problems, maybe 11-1200 RPM?
This would be lowest fuel burn and least air resistance configuration.
Nick
1978 R80/7

In the helicopter industry, aerodynamically speaking, we basically have two critical speeds (three if you include VNE); max endurance/max rate of climb airspeed. For Kiowa warriors it was 55KIAS, for the Lakota, 65KIAS. And... basically what we are attempting to derive in this conversation, max range, that sweet spot aerodynamically speaking that will give you the lowest fuel burn rate and the max speed without efficiency trade off.

My ‘05 RT, and my ‘16 RT have similar fuel burn rate characteristics, probably due to the similarities in engine design, and faring design.

My absolute best burn rate is 51 MPH in fifth gear. Max range is close to that, 55-60MPH in 6th gear.

The real key to efficient fuel economy once you figure out these two speeds actually has more to do with route planning. The least amount of stops and starts, hills, rapid accelerations, passing, and even wind can make a huge difference in range between fill ups.

93 octane ethanol free gas, and reducing flat plate drag (side cases, top cases, riding posture, wind screen adjustment) go a long way to improving milage, as well as liberal use of the cruise control.

The knowledge gained by a rider regarding fuel economy can pay big dividends when on a long trip, on an across the world adventure, or an iron butt competition.

nickrides said:

In WW2 those pilots had a saying, " The slower it turns the less it burns" when going for max range. They could control engine speed via the propeller control and also manifold pressure via, throttle, they would go for lowest RPM and highest manifold pressure without blowing up the engine due to detonation, using 130 octane fuel helped alot in this configuration.

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This is a neat fact- thanks for sharing, Nick!

My R80/7 is the least efficient of my bikes, but not by much (averages ~40mpg... about the same as the K75 and a couple less than the RS... and way less than my Giant ). Swapping the original 37:11 final drive to the current 34:11 didn't make much difference to fuel efficiency as I recall, but it does feel better overall. On the other hand, fuels have changed some in 30+ years. With E10 being the norm now, I'd guess the engine is burning more gas per mile than it did on E0, so maybe that modestly higher-geared final drive did make a difference!

mpg

m R80 gets about 38 to 42 mpg because of lower gear ratio in rear drive that is back roads and interstate