Warm weather...
 

I'm getting record high MPG trips when the weather is warm and dry. My IAT isn't at a record high, though.

We've certainly explored the question before...but with my IAT not up, why am I getting trips at record high MPG?

Lower air density.
Faster warm up.
Less heat loss from engine.
Other fluids have less viscosity.
Less use of lights, heater, and other accessories, as long as you minimize AC use.
Less energy lost to tire flex.

regards
gary

My windows have been open, which probably evens out with less accessory usage. Would the other stuff really add up to that much difference? Engine warm-up time shouldn't be a big deal since my commute is a 38 mile trip each way, and the other stuff seems very minor.

I agree 100% with all these.

I have a 40 mile one way commute and in the summer I average around 42 to 44 mpg per tank. In the winter this drops to around 35-38 mpg. Cold weather really takes it's toll on mileage.

another situation is that you have only been hypermiling for a year (I assume that because you have only been on the site about a year).

you may have perfected your driving technique over the winter. maybe it would be better to say that you are driving more efficiently than you did a year ago.

(there are a few assumtions in that post but it is a possibility if nothing else)

The difference is like night and day. In 58 degrees recently I was getting 36-38mpg on a good day or 33mpg on a bad day. In 78 degrees I'm getting 38 on a terrible day, 42 on a decent day, and today I got 46.5 on the way home.

I forgot a couple of things (probably more buried in the rust ;)).

The coolant exiting your radiator and entering the engine is much warmer in summer than winter, temp differences can easily exceed 60 degrees. The colder coolant in winter absorbs more heat from the engine in the winter. It also slightly reduces the work of the water pump because the more open thermostat in summer reduces the pumps work load slightly.

That's why radiator blocks work well to increase mileage in winter, as well as WAI. To me they are the two most significant causes of worse winter mileage, but the above mentioned causes are also relevant.

regards
gary

As your driving technique improves, all other factors become more significant. If you develop your hypermiling skills to the point where your average engine efficiency improves from 18 to 20%, your mileage improves by 11% from that change alone.

Merely eliminating unnecessary idling can reduce your fuel consumption by 13%. Light timing and turning the engine off when you are going to be sitting for any amount of time beyond a few seconds can make a huge difference.

My daily route is like a game for me. I have managed to time the lights as far away as 9 miles by maintaining the correct average speed, even to the point where I can regularly coast through a left turn at a major intersection many times. Every day I learn new nuances of how I can refine my technique for that exact same route. Cars fly by me and get quite a distance in front of me only to coast by them at the light.

I know exactly where I need to be, within a couple hundred feet to make it through a light a half a mile ahead if it changes to green. Some of the lights I have timed are only on for ten seconds in a 2-3 minute cycle. I know when I start catching the yellow lights, I need to speed up slightly. In a 20 mile 30 minute trip with 22 traffic lights I have, on occasion, only been stopped for less than one minute total. Usually it doesn't work that way but it is always sweet when you get it right and have a little luck.

On another light where I can turn right on red, I know if I wait for the light to change green I will get caught by the next two lights, so if I have to wait for the green to make my turn, there is no reason to go over 20 MPH for the next 4 tenths mile. People are flying by me like I am some kind of Moron and I coast up behind them and never have to slow down as the light changes.

In the Insight that route has given me 82.7 MPG for 20 miles as long as I get a little luck on the few not synchronized lights.

regards
gary

The mystery remains. I had 100% upper and lower grille block installed when I previously read the higher IAT in cooler weather, and have zero grille block installed right now. My IAT yesterday during my 46.5mpg trip was ~95 degrees F; I had >120 IAT with the grille block on cool (but not cold) days.

Any other factors that were missed?

Although I've always noticed about a 5% - 8% difference in winter vs. summer MPG, I've never really known why. My best theory is that the temperature of the fuel leads to better combustion, although obviously all of the above-mentioned items would also contribute.

-BC