Was wondering if anyone knew why a 02 Ultra with FI that in 70-80 degree weather only averaged 35 mpg highway would get 45 mpg highway in 55-65 degree weather?
I had been told by a mechanic during the summer that it would probably get worse mpg in cold weather but the opposite happened. Of course there is the winter fuel now but would that lead to an increase in MPG? It seems to run the same regardless of the temp's. It has the S&S upgrade with forged pistons so its a 94 CI ? instead of 88, not sure if it was retuned at the time or not. (didn't own it at the time) No obvious symptoms so far that's it running too lean or rich.

 

I would attribute it to bad math somewhere.
It takes more fuel to run at colder temps to start.
The DA is denser in cold weather...everything is working against better mileage..
The only possibility you are running a better AFR now.

 

There's a scientific explanation for why you use less fuel in warmer weather, and it's to do with air temperature.

As you may know, your engine uses air, as well as gas, in order to generate energy. In fact, it uses huge amounts of the stuff, though only the oxygen is used in combustion. These oxygen molecules combine with carbon in the fuel during combustion, to form--you guessed it, carbon dioxide.

Anyway, one property of air, like many substances, is that it expands when it warms. This makes it less dense--molecules of the various gases are spread further apart. This means that every gulp of air your car is taking during combustion has less oxygen in it at warmer temperatures, and if there's less oxygen, the engine compensates by using less fuel.

This is bad from a power perspective, but good for economy. The engine is combusting less fuel due to the warm air, improving fuel economy. During winter, the reverse is true--denser air encourages the engine to chuck in more fuel, boosting performance and harming economy

randy

 

Don't know my mileage or the science but my butt dyno says my bike runs smoother and better in cooler air. 55-60 seems to be optimal. The denser air carries more oxygen for combustion? Bike runs a few degrees cooler? Maybe I applied auto tune during a cooler time frame? Don't know but it definitely is a happier bike.

 

 

Since the bike is new to you, I would suspect an error in computing MPG by the previous owner or a recent change of which you are unaware. Most of the factors reducing MPG in cold weather have been mentioned, but not mentioned was increased viscosity of lubricants. Engine oil reaches almost the same temperature in cool weather as in warm weather but wheel bearings have no combustion near them to heat them up so they stay cooler in cool weather than in warm so there is more resistance to movement in the grease. The same is true to a lesser extent to the primary case and transmission, as they are both warmed by contact with the engine, but less so in cool weather vs hot weather.

 

Every late fall/early winter my bikes MPG drops so consider yourself lucky. I guess???

 

I usually loose about 3 to 4 MPG once the weather cools in September in Canada.

 

My MPG drops from the upper 50's in the summer, to the low 50's in the colder months. From what I've read that's to be expected mainly because of the winter blended gasoline.

 

Don't know about the bike but my truck gets worse in the winter due to winter blend fuel.