MPG sensitive to intake air temp

For my '04 FJR the miles per gallon goes down when it's cooler outside. The computer doesn't seem to adjust all that well. Maybe the coolant temp is a little lower too -- thermostat working properly?. Yet, on my autos the mileage doesn't seem to be affected by outside temps.

Are the auto computers more sophisticated or is there something else in play?

Lot's of possibilities I can think of...and many of them NEPRT candidates.

When it's colder it takes longer for the engine to get to optimal operating temperature. Mileage on shorter rides where the engine cools back down would be lower than if you took a much longer ride.

Colder air is denser air...so more fuel is being used. This is largely offset by the increased power in the more dense charge, but maybe not wholly.

At the same it's colder air resistance is then greater than it's warm and motorcycles have more coefficient of drag than cars.

Another one that I'm guessing you already compensate for, but if not....less air pressure in tires in colder temperatures and increased rolling resistance.

Typical mileage reduction in the winter is about 10% in my experience and what I've read before.

So what'd you do with the windshield as it got colder?

How are you comparing numbers? If you are comparing on relatively short trips, yes, your mpg is going to be affected. Once everything warms up, irregardless of outside temperature, mpg should be fairly close to being the same. When it's cold outside it takes longer for everything to warm up, i.e. water temp. first, engine oil next, then don't forget that the rear gear will will affect mileage somewhat until it warms to operating temperature. And, do you run your windshield at a higher setting when the outside temp is colder; that's going to increase drag as well.

I never move the stock windshield. It's always in the lowest position. I don't ride when it's cold. My lower limit is 48-50 DegF.

It stays in a relatively warm garage -- it reaches 2 bars quickly. My local rides are typically at least 20 - 30 mile jaunts.

With denser air I shouldn't have to open the throttle as much to reach my typical speed.

My first tank of the day always has lower mileage than later when it warms up. I have noticed this many times. I usually start riding at day break.

Winter gas is formulated differently as well.

Colder air with proper fueling requires smaller throttle openings to maintain speed. Smaller throttle openings causes more turbulence in the incoming air and somehow mixes/burns less efficiently, causing you to need to open the throttle just a little bit more to use more air and fuel at the lower efficiency to maintain speed as compared to if the air was warmer.

Some cars run the engine coolant through the air box to warm the air and improve mileage at the loss of maximum available power.

wfooshee said:

Winter gas is formulated differently as well.

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If this is true for your state, then this could account for almost all the difference in mpg. In Calif., the winter gas has ethanol added and ethanol has something like 30% less energy than gasoline.

r1byker said:

In Calif., the winter gas has ethanol added and ethanol has something like 30% less energy than gasoline.

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Used to be true, but I believe most places are now running 10% ethanol year round now.

And yes, it's about 34% less energy content for ethanol. 75,700 BTU (LHV) per gallon of ethanol and 115,000 for gasoline.

as already hinted at it is primarily the cold air/ fuel. while you don't want too hot an intake charge to cold can be about as bad performance wise. with too cold a charge the fuel will not atomize well or stay atomized. humidity increases the effect. the cats end up getting a lot of waste fuel and you will see the nox go up. ford had this problem with their imsa/gtp mustang years ago where it had too much inter-cooler. and in the car it never was as efficient as on the dyno. during practice a piece of cardboard blew into one of the intakes and the performance increased. then the lights came on.

I have noticed on cooler mornings (less that 40 degrees) it takes almost 15 miles to get everything warmed up and bring the instant MPG to normal. Since you said your average rides are 20-30 miles that would be atleast half of your ride. It is almost an 8 MPG difference for me between riding at 1-2 bars and fully warmed up. This isn't as noticable when the ambient temperature is above the mid 50's.

The FJR likes cold weather, runs a little stronger, makes the throttle hand heavier. Maybe that's just me.

Somewhere in the distant past Jestal made a comparison about gas mileage and riding at altitude. I've searched diligently but can't find it. IIRC, the higher gas mileage at higher altitudes was because of less air resistance because of thinner air (no shit). I've seen the same when riding from Houston to Breckinridge. The mass of the bike and rider are the same so it would take the same energy to accelerate it and keep it at a constant speed in both locations. But thinner air allowed for less drag so less power at constant speed. I haven't done the math but could it be that the cold air is dense air having anything to do with it? Yea, a bit of a stretch but maybe?

OK, just looked up some air tables and it shows air at sea level and 40F is about .080#/ft3 and at 70 F it's about .075#/ft3 so about a 6% increase as it gets colder. About the same (only backwards) of going from sea level to about 2000 feet in elevation.

You didn't mention how much of a decrease in MPG you got or what the temperature of "cold" and "warm" were. These are pretty small changes so they could be over ridden by simple observational error. But a thought no less.

For all you anal retentive tire pressure types, when I get REALLY bored here at work I'll gin up a table to show what you should expect when you fill you tires on the top of Mt. Sandia on a cold day and then ride to the lowest point in New Mexico in the heat.

Where's El Toro?