View Single Post
Old 05-25-2020, 11:28 AM   #37 (permalink)
Aero Wannabe
COcyclist's Avatar
Join Date: Dec 2007
Location: NW Colo
Posts: 714

TDi - '04 VW Golf
90 day: 55.78 mpg (US)
Thanks: 678
Thanked 206 Times in 160 Posts
Driving without A/C

We are entering summer here in the northern hemisphere. This may be a good time to talk about limited use of A/C for increased mpg. In short, the energy to run the compressor and fans comes from the engine. If you run the A/C a lot your mpg will suffer. What can you do so you don’t suffer from heat in the cabin? In no particular order...

1. Park in a garage or carport if possible so your car is not roasting when you enter it to start your trip.

2. Tint as much of the windows as is legal in your area.

3. If you must park in the sun, use a high quality sunshade in the windshield. A couple of US accessory manufacturers make some that are custom fit to the specific car. You could go a step further and make side and rear insulating shades. I did this on my little camper van and it makes a huge difference. There are plenty of how-to videos on YouTube.

4. When you return to a hot car, use the key in the driver door to hold “unlock” for 5 seconds to roll down all the electric windows at once, to let built up heat out before you drive (if it is hotter inside than out). In town at slow speeds where aerodynamic drag is lower, drive with the windows down for maximum ventilation.

5. On the highway there is a huge drag penalty to driving with the windows down. Roll up windows and use the fresh air vents directed on the parts of your body you want to cool. If you are driving solo, close off or redirect vents on the passenger side. If it is really warm I will open a passenger side window slightly to increase flow through the cabin.

6. Dress in the lightest clothing you can. On the hottest days I wear shorts and flip flops with a thin synthetic wicking shirt in light colors.

7. Prepare “cooling towels” pre-soaked in a small cooler so you can change them out as they warm up and dry out. I drape these around my neck and shoulders to extend my comfort range. I have also experimented with a chiller designed to reduce swelling after an injury. It circulates ice water through a pad that I place between my shoulder blades. I run it on a small inverter plugged into a lighter socket. It works great but requires more preparation than the cooling towels.

If you have passengers it may be better to run the A/C on “recirc” so you are not pulling in hot outside air. I rationalize that my per passenger mpg goes to 100 mpg, using only slightly more fuel to transport twice the cargo. My spouse is unwilling to do the silly hypermiling tricks when it comes to comfort.

8. If you must do extended engine braking on long hills, by all means run the A/C on high to help slow the car. Diesels do not engine brake like gas cars so I take advantage of some extra chilling at no mpg penalty while in DFCO.
60 mpg hwy highest, 50+mpg lifetime
TDi=fast frugal fun

Originally Posted by freebeard View Post
The power needed to push an object through a fluid increases as the cube of the velocity. Mechanical friction increases as the square, so increasing speed requires progressively more power.
  Reply With Quote