Air conditioing and fuel use

according to mythbusters.... they took two identical, late model trucks. They put 5 gallons of gass in each, drove them around a race track at the same speed. The truck with the A/C on ran out of gas first. The one with the A/C off and windows down ran another 30 laps. something like that.

jack

-- D.A.M. - Mothers Against Dyslexia

see

That is correct as long as your speed is over about 40 MPH. New vehicle designs are wind tunnel tested to be "slippery" - and presumes windows closed.

And keep your tailgate up. Even though it seems like a big wind-blocking panel sticking up at the back of the truck, the cab/bed creates a vortex that behaves like a "bubble" of still air filling the bed. The slipstream passes over this bubble, the bed and the tailgate. Drop the tailgate - no vortex forms and you get disturbed air and more drag.

-- Mike Harris Austin, TX

...and they drove at less than 40 mph. Air resistance follows a cube-square law and is negligible below that speed; becomes rapidly more of a factor above it.

-- Mike Harris Austin, TX

This is true but drag with windows up also increases with speed. On a big car or truck with a big V8 it might be very little difference because the power required to run A/C is a smaller fraction of total power needed but with a smaller car or truck, you will never win unless "maybe" you cruise at 80 MPH or more.

----------------- The SnoMan

Wrong again. Been there done that and down is better. Myth busters is wrong on that one. I have been driiven P/U's for over 35 years and every truck I have owned has gotten better MPG with gate down (or off) on a highway trips. Usually it the range of a 6 to 10% improvement.

----------------- The SnoMan

Mythbusters be damned. Do what makes you happy, but wind tunnel tests say different than your anecdotal evidence - unless you're still driving that '72 Chevy with the rat motor of course.

-- Mike Harris Austin, TX

On Tue, 25 Jul 2006 20:52:33 GMT, SnoMan found these unused words floating about:

... a relatively common occurrence in the desert SouthWest.

The AC compressor is much more efficient today than in years (decades) past. It's true that you can run the AC in current cars and trucks and have virtually no affect on fuel consumption.

I need to test this theory on my 4-banger, as I see a noticeable performance hit with the AC on and have always blamed a low-mileage tank on just that.

Never attempted to do a complete tank test with and w/o, however.

To quote Adam: "I hereby reject your reality and substitute it with my own."

Or the standby: "Are you going to believe me, or your lying eyes?"

-->--

The problem with a wind tunnel is that it is not the real world. I have no doubt that you could find the right velocity with the correct density and humidity that you place the shock wave past the tail gate on a short box truck but that is not the real world and does not factor crosswinds or turbulance for other vehicles. Sit in the back of a truck by tial gate when it is doing 60MPH and feel the wind slamming down on you against the gate. Beleive what you want but you will use less fuel with it down, A few years ago I had to make several 250 mile round trip with my 2000 K3500, same route same weather it got about 1 to 1.5 MPG better keeping it under 70 on all trip. (you could tell it took a bitt less pedal and rolled/coasted easier at speed too.

----------------- The SnoMan

I didn't see the episode where Mythbusters tested the tailgate myths. I don't think they did that in fact, someone got confused with their tests on AC vs windows down.

On Tue, 25 Jul 2006 20:06:25 -0500, "S.Lewis" found these unused words floating about:

_+IF+_the vehicle is a powerful V-8, there'd be little difference in gas guzzling. On a small 6 or a 4, you will notice some/more differences.

On my 2002 Tacoma DC, there's a 1.5 mpg 'penalty' for the A/C. That's on long runs a nearly consistent speed. On shorter use it goes up to nearly 3 mpg. I do get about 22 mpg w/o A/C on fair runs (40 miles or more).

If my car got 20 mpg and gas $3.00 a gallon it would only cost $16.00 more to drive 1,000 miles at 18 mpg anyway.

I roll my windows down and run the a/c anyway. 8))