Did a highway trip yesterday, 140mi each way. First leg was sunny and 60F with displayed 54.0 mpg (doing upwards of 73mpg most of the way). May have had a weak tailwind, and it was uphill just a bit (150ft). The return trip was substantially more consumptive: 46.0 mpg. May have had a slight headwind, but not much. Temps were cooler, but only down to 45F. It was night, so I did run my headlights, and the front defrost had to be used once in a while. The big difference is that the fog was nearly continuous and very heavy at times - I never went over 70mph, and at times had to slow down to near 60. There was no "water" on the roads and I didn't have to use my wipers, but humidity was probably near 100%. Q: how much impact on aerodynamics does airborne moisture have? I don't think an 8mpg swing (15%?) can be explained just by temp differences and headlights - especially since my average speed was slower.
How about Rain Also, high humidity air has more "heat capacity" so it will cool the engine more effectively or looking at it the other way, it will suck more of the energy that would normally warm the engine. I can see this effect on foggy days with my Scangauge - sometimes the car does not reach operating temp (82°C, when the thermostat opens) in a 20 minute drive.
It all depends on whether the moisture is in droplets or vapor. Water vapor is less dense than dry air, so moist air will reduce aerodynamic drag a bit. Water droplets impact the car, so reduce mileage slightly. Fog contains droplets. Rain contains big drops. Moist air has vapor. Tom
As noted in both the title and my description, this was not rain. The roads were effectively "dry" and I did not need to use my wipers. But the air was completely saturated with moisture. That's interesting, and might explain it - although I'm sure the engine was warm enough due to highway speeds, it may have been working harder due to "colder" air taken in. Also, moisture in air displaces other molecules (nitrogen/oxygen) - so that may have made the combustion process less efficient. And the "heat sink" effect may have also affected the rolling resistance by keeping the tires as cold as if it were 30F instead of 45 - that's pretty big at highway speeds also. Added together, that could be significant. Actual rain would have all this plus the mechanical (splash) losses. FYI: filled up today, actual mpg was 48.0. (slightly less error % than most other tanks)
With saturated air there are droplets in the air. If you can see fog it means droplets have started to form, so there will be some small mechanical aspect of pushing water out of the way. Tom
