I am seeing a downward trend in mpg. I've only had my Honda Insight since February (about 12 fuel ups). I live in CA where presently it is very hot (about 91-92 degrees today) so I am running the A/C more frequently, though conservatively.
What are others' experiences with hot weather and its effect on mpg?
Barely any heat up here in the northwest, so I can't comment on the impact of heat.
Running the AC does have an audible impact on the engine RPMs. I do know I get different power from it on steep hills: it's more difficult to maintain speed on some of the I-5's steep hills with AC on than without.
I figure that means my MPG takes a hit, but I don't know how much because I travel such different areas so often.
Mileage in all three of our vehicles rises significantly once it warms up. Our tC, for example, has averaged 31.4mpg during the last two months, with a best tank of 34.1mpg. During the winter it will struggle to do 30mpg on highway road trips and will usually average around 27-28. We even have used the AC a few times recently. Temps are never extreme here though. I think it has more to do with summer gas than the actual air temperature.
Over two years of ownership, we've also noticed a slight but constant upward trend in mpg. The first 10k miles on our car averaged around 26mpg; now at 40k miles our lifetime average is just over 29.
I'll bet the fact that your car uses less gas in general means that using the AC has more effect on mileage. I don't know the specifics of the Insight's AC system, but I'll bet to keep it affordable they gave it a normal AC compressor. So if you're doing 50mpg, and the AC uses just as much power as it does on a Honda Fit, then it will drop the mileage more than it would on a Fit because you're using less gas to move the car but the same gas to power the AC.
I have a turbocharged engine of which i'm assuming runs better on cold air (feels quicker on a cold day) however it's hot here now, i had the AC on the other day and i still got 51 MPG. This is probably due to my inconsistent driving habits, but it just shows that even with AC on a modern car, it doesn't really affect the fuel consumption as my average is only 31 MPG.
Older cars with old AC units are obviously less efficient!
Hot weather makes the gas go farther, however there is some trade off with regards to A/C usage. The last couple of tanks (that I haven't posted yet) reflect me using more fuel, mainly to keep the van cool during a bit of waiting with the kids in the van, while the other half is doing some shopping. Granted this doesn't happen all the time, but I certainly am not going to make the little guys cook in the car with me if I don't half to. So obviously that will affect my mileage.
But if I do mostly highway miles AND use the A/C I still see higher numbers than average. Maybe the Insight has as someone else stated, a different setup that may affect it more. Hard to say. It could also be how its driven, as for example in my case letting it run and keep cool while going nowhere.