MPG Decrease

[BradJohnson23]

Long time lurker, finally joined. Thanks for all the great info and questions answered. I have an '08 RT w/7k miles. What kind of drop in mpg are y'all seeing with cooler temps and winter blend gas? I went from 48-50 mpg to 42-44mpg. I check my tire psi before each ride. I did have the 6k service completed about the same time as the temps started dropping. By the way, this is my first BMW and the more I ride, the more I like...Brad

[DonB]

Shitty oxygenated winter fuel will easily take 10%.

[Twisties]

Welcome on board the board!

 

That drop is probably normal. Are you running heated grips or seats or other heated gear? That really takes a hit. Otherwise, fuel economy just seems to drop in winter.

[markgoodrich]

hmmm, does the Dallas area HAVE 'winter' gas? In Texas, I think it varies from one region to another.

 

It's been pretty windy lately, that'll do it, all by itself.

[Arizona_oldguy]

The mpg on my '06 R 1200RT had been dropping since last spring, from @50 + to something closer to 43/44. The consumption figure the computer gives me is pretty close to the calculation I get when I do a fill up.

 

On Wednesday JohnLT helped me balance the throttle bodies. the adjustment took almost a full rotation on the cable adjuster. The engine runs much smoother, especially at around 4K RPM, and my mpg jumped immediately back to around 50. I was really surprised.

 

Glenn

[keithb]

Hello Brad and welcome to the board. My inlaws are in Bedford and one of my wife's best friends lives in McKinney as well. Small world eh? This is a good place to hang out. My RT is averaging 45-50 mpg consistently unless I get on it a bit.

[T__]

Long time lurker, finally joined. Thanks for all the great info and questions answered. I have an '08 RT w/7k miles. What kind of drop in mpg are y'all seeing with cooler temps and winter blend gas? I went from 48-50 mpg to 42-44mpg. I check my tire psi before each ride. I did have the 6k service completed about the same time as the temps started dropping. By the way, this is my first BMW and the more I ride, the more I like...Brad

 

Brad, lots of contributing factors..

 

*Thicker engine & trans oil for much longer into the ride..

*Thicker gear oil in final drive for much longer into the ride..

*Thicker grease in the wheel bearings..

*Fuel calibration stays in cold enrichment for much longer after cold start..

*Maybe more engine warm up,, 0 mpg during warm up, but still using fuel..

*Rider usually bulked up more in cold weather so more wind resistance..

*Cooler denser air to push through..

*More highly volatile light ends in winter gasoline (less BTU’s per gallon)

*Possibly more engine power required for generator due to heated equipment on bike being used..

*Maybe windshield being set higher to block cold air..

*Cold roads can hold tire temps lower therefore higher rolling resistance..

*Possibly shorter trips,, or at least more stopping for breaks..

 

Twisty

 

 

 

[johnlt]

Excellent summary Twisty, all make sense but I had not thought of several so thanks for info.

[tlc]

I too have an '08 RT, and I've noticed a similar decrease in fuel mileage -- from 47-50 to about 43-45.

[Bradley_Gillie]

Whew! I've been noticing the same decrease in mpg. Glad to know I'm in good company.

 

Brad, send me a PM if you'd like to meet up for a ride sometime.

[Highway41]

My MPG decreased from mid 50s during the inital break-in, to high 40s in local riding, to low 40s loaded and heading west regardless of season.

Bill

[Sea Beast]

Welcome on board the board!

 

That drop is probably normal. Are you running heated grips or seats or other heated gear? That really takes a hit. Otherwise, fuel economy just seems to drop in winter.

 

Last week I read something in a magazine (Motorcyclist?) about the drop in mileage in the winter, and it made me curious about the impact on mileage of heated gear.

 

So I looked up the energy content of a gallon of gas, power usage for various electric devices, typical gasoline engine efficiency, etc, and while I don't have my calculations here to reference, I concluded that even with an electric vest going in addition to the grips and seat, heated gear shouldn't hurt RT gas mileage by more than 1 mpg, tops. And typically much less.

 

The basic calculation based on energy content goes like this: a gallon of gas has about 36kWh of energy (according to wikipedia). A typical electric vest uses about 35W. So a gallon of gas has enough energy to power a vest for about 1000 hours. Since we typically use more than a gallon of gas each hour (whenever MPH>MPG), the electric vest represents less than 1/1000 of the fuel use. Of course, engines aren't perfectly efficient, so each watt of vest energy actually requires several times that amount in fuel energy. So I multiplied by 4 (to represent 25% efficiency), and conclude that the vest is still less than 1/250 of the gas usage. Thus, even with a few such electric devices going, they should represent 1% or less of your fuel usage.

 

So now I crank up the grip and seat heat with no worries about mileage impact.

 

-Brett

 

[JayW]

 

So now I crank up the grip and seat heat with no worries about mileage impact.

 

-Brett

 

I like it. I have always assumed that the MPG penalty for using electric gear would be very low, but had not really thought through the math. All other things being equal, I would also think that a bigger more powerful engine would have a lower associated % drop in MPG than a smaller engine for a given additional load.

 

Jay

[T__]

Brett, the fuel economy loss percentage from about anything on my list would be pretty low but even assuming ½ % per item (some probably higher & some lower) that would be 6% right there..

 

Nothing really outstanding but it all adds up in cold weather..

 

Twisty

 

[Elhoff]

I get the decrease also. I assume it is mostly the oxygenated winter gas here in CA. But I also notice that the bike seems to run smoother in cold weather. Engine temp is lower at the end of my 35 mile commute also. Amazing how much difference 70 degrees can make

[BradJohnson23]

Thanks for the replies. 75f here yesterday and back to mpg I have been used to for this bike. Now I know. I am just happy any reflash/adjustments done during the 6k service didn't effect the decrease in mpg I was seeing. Thanks again.. Brad

[motoguy128]

Don't forget that the very effecient air cooling effect of the cylinder heads will work against you in lower temepratures. It may result in unessesary losses of combustion energy.

 

You'll probably find that at speed, the thermostat for the oil cooler is closed. Most any time it's closed, you're losing some energy to "overcooling" the cylinders.

 

Remember it takes a finite amount of energy just to warm up the 150lb chunk of metal that is the engine and transmission to normal operating temperatures. Soem of that is waste heat, but some of it is lost horsepower.

[ScottT]

Wow. This thread confirms my suspicion. I think there is more ethanol added to the fuel causing the drop in mpg. Or something. At any rate, something has happened in the fuel to cause the drop in mpg cause I'm now reading about others experiencing the same thing I have. Damn government.

[Mister Tee]

Don't forget that the very effecient air cooling effect of the cylinder heads will work against you in lower temepratures. It may result in unessesary losses of combustion energy.

 

You'll probably find that at speed, the thermostat for the oil cooler is closed. Most any time it's closed, you're losing some energy to "overcooling" the cylinders.

 

Remember it takes a finite amount of energy just to warm up the 150lb chunk of metal that is the engine and transmission to normal operating temperatures. Soem of that is waste heat, but some of it is lost horsepower.

 

Also, the combustion process itself is less efficient at lower temperatures. In thermodynamics courses, we would have to make plots of the various thermodynamic cycles, including the Otto cycle, which models the typical gasoline engine. In every case, net useful energy decreases as the temperature of the cycle is lowered. In a perfect world, if we could have fuels, engine materials and lubricants that weren't effected by extreme heat, we wouldn't cool engines at all.

 

Several years ago, Mercedes made an experimental ceramic engine that ran quite efficiently, except that the limiting factor for heat was detonation, even using diesel fuel. Although the motor itself could withstand substantially higher temperatures than a gasoline engine would see, they couldn't run it much hotter than a standard gasoline engine without destroying it through detonation.

 

[davesbmw]

I just wish it would get above 7 degrees so I could find out if my 06 Rt does it also

 

[Heck]

Well, personally I would be tickled to avg 42-45mpg.....my 07 GS usually averages 35-38 mpg. Welcome to Kalifornia.

 

[Ricoshay]

Well, personally I would be tickled to avg 42-45mpg.....my 07 GS usually averages 35-38 mpg. Welcome to Kalifornia.

 

Me too. I've seen a low of 33 and a high of 50 mpg. Both on cruise control on Interstates. Average is barely 40 mpg.

 

[Bruce H]

I have an 07 GSA and am a easy driver, I can only get 40-42MPG. seems like it should get better. my wife's honda CAR gets 38 with all that extra steel.

[gdouglas]

Not only has my mileage on my 1200RT dropped from 48mpg down to 42 lately, but our Honda Fit car mileage has dropped 8-10%, too. A lot of people I work with are noticing a drop in their vehilce mileage, too.

 

Must be the lower energy content of the government subsidized ethanol in the gas.

 

Now, how are the big 3 American automakers supposed to conform to those pesky CAFE fuel mileage standards that increment up every so often with this "tainted" fuel?