warm weather and MPG - HOT weather and MPG

[katiep]

So yes, we get better FE in the summer and it decreases in the winter. But, when it's Africa hot outside, does FE suffer? Does it get outside the range of helpful for FE? My car has started feeling like it's working a bit hard since the weather got above 95 degrees. It's also never in a covered place. I park in the shade when I can get it, but that's not often, during the day. I would guess the radiator fan is running a lot, but I'm not sure. I'm planning on putting a light in with the relay one of these days.

[PaleMelanesian]

I haven't noticed an upper limit.

[Old Mechanic]

91.5 MPG on my 99 Rebel today. Actually cool here today with a high of 81. Going down to 64 tonight. Beats last years max of 109, but it ain't over by a long shot.

regards
Mech

[Duffman]

I would think any loss in fuel economy as you get in the super hot region would be due to increased A/C usage. All fluids get thinner as they get hotter and the engine pumping losses are less in the heat. The air you are pushing should be thinner too.

[Old Tele man]

...also, the hotter it is to start with, the "quicker" everything (coolant, oil, etc.) gets up to 'normal' operating temperature(s) and so you reach max.mpg sooner.

[PaleMelanesian]

Indeed. If I drive home for lunch, the back-to-work trip is usually +10 mpg from normal due to being warmed up.

[JRMichler]

No upper limit in my truck. I can get 30 MPG to and from work when temperatures are in the mid 80's or higher.

[larrybuck]

I don't see an upper limit either, except, as you mentioned that the fan would run more.

Usually the biggest problem is that the driver gets overheated, cranky; and not as attentive, with less patience to drive their best.

I'm still waiting for summer here! We've had maybe 3 days in the 80's at all so far. Sometimes, it's still a wait for the 70's!!!!

[graydonengineering]

Originally Posted by Duffman

I would think any loss in fuel economy as you get in the super hot region would be due to increased A/C usage. All fluids get thinner as they get hotter and the engine pumping losses are less in the heat. The air you are pushing should be thinner too.

I agree, the only negative factors in high temperatures are:
-A/C usage (a huge decrease depending on your car)
-Slightly more rolling resistance due to softer tires
-Possible fan loading if engine temps get too high
-summer blend fule

Overall, between air density and fluid viscosity, heat is better than cold hands down for FE, that is assuming you sweat it out with your AC off and widows up!

[PaleMelanesian]

Correct on the AC and Fan items.

- Summer blend fuel is usually better than winter blend.
- Rolling resistance is better in heat and worse in cold. (The Recumbent Bicycle and Human Powered Vehicle Information Center)