Do hot ambient temps have a negative effect on FE?

[TexasCotton]

Originally Posted by 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

My understanding is that winter climates and northern climates have lower or less than desirable fuel economy

[basjoos]

Ambient temperature when refueling doesn't matter if your filling station has underground tanks since, unless they have just been refilled from a tanker truck driving for hours in the hot sun, the gasoline in them will be close to your local ground temperature which doesn't vary from day to night and minimally from summer to winter. The ambient temperature when refueling would matter if your filling station was one of those rare stations that have above ground tanks.

[redpoint5]

I get better fuel economy the hotter it is outside, even if I run the AC.

[PaleMelanesian]

Originally Posted by redpoint5

I get better fuel economy the hotter it is outside, even if I run the AC.

I find that to be true in my Odyssey, but not in my Fit. It makes sense, with high temperatures raising FE but AC lowering it. AC load is a much smaller percent of the Odyssey's large consumption than the thrifty Fit's.

[gone-ot]

Cross-country race planes back in the 1930-40's would fillup with COLD fuel and takeoff in the morning because as the day warmed up, so did the fuel, which then expanded in volume...essentially allowing a 100 gallon fuel tank to contain MORE than 100 gallons.

[whatmaycome14]

Originally Posted by 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?)

I've been thinking that something was messed up. But then the higher mpg today really really confused me. Maybe today is just a fluke as it doesn't correspond with the science of ambient temps.

I need to have a way of monitoring when the fan is running. I'm not sure it would kill my mpg as much as I've noticed, but it certainly would affect it. I haven't noticed my coolant temps spiking, but if the fan were running a lot then that would keep the SG from showing that.

I hadn't thought to check the belly pan. I will.

[jcp123]

If you get a digital readout of your temps, you can correlate that with when the fan runs. I know, for instance, that mine kicks on at 208f and stops when down to 199f.

[XYZ]

Originally Posted by TexasCotton

My understanding is that winter climates and northern climates have lower or less than desirable fuel economy

Yeah, because the ambient temp is lower = longer engine warm ups at higher idle speed = poorer fuel economy.

[XYZ]

Originally Posted by Old Tele man

Cross-country race planes back in the 1930-40's would fillup with COLD fuel and takeoff in the morning because as the day warmed up, so did the fuel, which then expanded in volume...essentially allowing a 100 gallon fuel tank to contain MORE than 100 gallons.

That is my point exactly, and well said.

It's correct, and it makes sense. This is not about the fuel being stored in underground tanks where (I agree) the temperature in the ground is relatively stable. It's about what happens after the gasoline is pumped into the hot fuel tank of your car, that becomes even hotter yet, as the temperature of the day becomes hotter.

[jamesqf]

Originally Posted by XYZ

TIt's correct, and it makes sense. This is not about the fuel being stored in underground tanks where (I agree) the temperature in the ground is relatively stable. It's about what happens after the gasoline is pumped into the hot fuel tank of your car, that becomes even hotter yet, as the temperature of the day becomes hotter.

So what exactly is it that you think happens? The fuel tank is made of thin metal (or these days probably plastic), with a very low heat content. Put cool fuel from an underground tank in it, and the fuel is only going to warm up by a fraction of a degree.

Now as the day goes on, the fuel will warm up and expand, which is why fuel systems have expansion tanks or header space, but you'll still have the same amount of fuel.