Do hot ambient temps have a negative effect on FE?

[whatmaycome14]

I've recently noticed about an 8mpg drop in my highway fuel economy and immediately thought something was wrong with my car. Without having time to really diagnose what the cause of the problem is, I have not taken any action too figure it I out.

After two weeks of 80-95 degree ambient temps, I hadn't noticed a problem with my coolant temps or IAT.

Today it's only 75 degrees outside and my hwy mpg is back up to nearly 60mpg. I haven't changed anything on the car...

I'm confused because I would've thought that hotter air outside would've been better.

[2000mc]

on my cars i've always seen better mileage with higher temps, until i touch the ac button. which if you're pushing 60mpg, would look like a bigger hit than other vehicles take, because mpg makes for apples to oranges comparisons. ....i dont even know what my l/100km is despite knowing its a better system

[PaleMelanesian]

No.

The best mileage I ever got in my old Civic was in August.

I charted that car's every single trip for temperature and mpg, and it's a linear progression with no sign of tapering off, even up to 110F.

[jamesqf]

Same here: without using A/C, the warmer it is outside, the better the fuel economy.

[NeilBlanchard]

Hot weather has two benefits to higher efficiency: quicker warm up time, and lower air density means that aerodynamic drag is significantly lower. My car coasts far better the warmer it is - I can almost tell the temperature by how far I coast. Also, higher humidity also lowers the density of the air (counterintuitively).

[backpacker3]

It sounds like there might be trouble brewing for your car. You might want check and see if anything is breaking or broken.

[2000mc]

in your fuel log, looks like one of the last mods before your best tank was changes on the belly pan, so it might be worth making a check that everything is tight, or might not bend or bow down at 60mph. grill block setup for the temps? (fan running excessively?)

[PaleMelanesian]

Originally Posted by NeilBlanchard

Hot weather has two benefits to higher efficiency: quicker warm up time, and lower air density means that aerodynamic drag is significantly lower. My car coasts far better the warmer it is - I can almost tell the temperature by how far I coast. Also, higher humidity also lowers the density of the air (counterintuitively).

My data shows the same for humidity. Foggy mornings are the best.

Not only quicker warmup and lower air density for drag. Also tires have lower rolling resistance, all the grease and fluids start out flowing better, and the lower air density basically "detunes" the engine to be less powerful.

[basjoos]

Higher air temps have exactly the same effect on aero drag and engine performance as increasing the altitude of your driving location. I once sat down and ran out the calculations that the air temperature at my 500 foot above sea level location would have to reach 170 degrees F to equal the same aero drag and engine performance that someone would get driving around in the Rocky Mountains at 10,000 feet above sea level. The term is called "density-altitude" in aviation.

[XYZ]

This may or may not be relevant to your observation, but it's a fact worth mentioning.

The ambient temperature (and the corresponding time of day) does matter when refueling the vehicle. Gasoline expands when it is heated. This is why it is advantageous to fuel your tank when the ambient temp is cooler, which is usually in the morning. Avoid having to refuel when it's a hot day, if possible.