I drive a convertible daily, and put on about 75 miles a day in a variety of traffic conditions. Here in Miami, I have the option pretty much year round - top open or top closed?? Fun says ride topless (couldn’t resist). But…
Is fuel efficiency better –
A) with the top open, but the AC off? or
B) with the top closed, but the AC running (assume full tilt - coldest setting and highest fan speed)???
Variant - How much does it matter if the side windows are up or down?
To keep it simple, let’s assume that for either choice A or B all other variables are equal - such as traffic conditions, weather, tire pressure, etc. Remember that the aerodyamics of these cars are incredible compared to what they were a few years ago. My car is a 2004 Chrysler Sebring - if that makes a difference… a generic type answer is great.
Helpful info about the side windows - I can ride with the top open, IN THE POURING RAIN, and as long as I put the side windows up, I won’t get wet in the front seat - back seat passengers get soaked… but what the heck?
Mythbusters, in their second attempt to determine which is best, ‘A/C on’ versus ‘windows down’, determined:
Below 50 MPH it is more econimical to drive with the windows down.
Above 50 it is more econimical to drive with the AC on.
Now, the convertible issue complicates it, but my hunch is that it would change the speed at which it becomes economical to drive with the top and windows up and AC on. Because their is more surface turbulence introduced in a top down convertible, you can probably expect to get better mileage with the top up and AC on at a lower speed than Mythbusters quoted.
My educated guess is about 35-40 MPH. Below that, the top is probably best down. Above that, put on the AC.
Which is a good start to an answer in a laboratory. I understand what you are saying. But, when we move this to the real world, we need to take that variable out, as I said in my OP. A typical commute day for me is 20 minutes at an average of 20 mph (moderate traffic) followed by 40 minutes of high speed runs (speed limit only of course). What I am interested in determining is the most fuel efficient approach to the overall run, not just part of it. Besides, once the traffic jam opens up, the last thing I want to do is pull over to the side and raise or lower the top!
My experience with top down no AC is that mileage is about 10% worse than top up with AC. Of course YMMV. [SUB](Someone had to be first)[/SUB]
However, my data maybe very skewed. I’ve probably driven with the top up fewer than 20 times in the last five years and only due to bad weather or a crabby passenger. I say go topless no matter what. That’s what convertibles are for.
Also, convertibles are invariably heavier than the base model of the same vehicle. Not that it matters once you have one. This is due to both structural reinforcements made and the fact that convertibles seem to mandate all options.
If you’re doing 75 miles a day like this, it should be easy to test. Just fill up, do 3-4 days with the top down and see how much gas you used for the milage you drove, then do 3-4 days with the top up and the AC on. Keep a log and do multiple runs to even out things like traffic and wind variances if you have to.
I love your OP intentions, Gringo_Miami, but you are working towards convincing us/you that the best option for economy is to put the top up.
In gets terribly inefficient to drive with the top down, and terribly inconvenient to mix up/down driving.
I drive a convertible, and my anecdotal evidence matches what Mythbusters discovered, except a convertible’s turbulence is magnified, and you don’t get returns on top down driving until speeds really plummet. A car with just some windows down gets a return on windows down driving all the way up to 50. You won’'t see returns on top down driving all the way up to 50…it has to be below 40 with a rag top down (and you spend alot of time above 40. And considering how the energy required to push the vehicle increases exponentially, the case for leaving the top down for your drive falls apart).
You’ve got some decent cruising speeds, for decent am’ts of time. It is safe to say, in a non-laboratory way, that you are best with the top up and AC on.
Ugh…considering how precious little time there is up here in the NE USA, I can’t tell you to put the top up! Keep 'er down, and take advantage of all the sun in F-L-A!!!
My theory is this: Top down driving is good for your soul, in a chicken soup kinda way. Keep the SPF 50 on, and enjoy the ride…de-stress yourself everday, and you’ll save more in medical bills than you ever spent on gasoline!!!
Philster - sorry that I wasn’t clearer! The FUN FACTOR says - drive with the top down, and I do. As much as possible. Heck, if you can’t have a convertible in South Florida…
The original (and current) intent was to see if anyone knew of a study that would show me just how much I am paying for that top down fun via extra gas. Poorly phrased on my part. 25% less efficient? 50%? I may really just do the study for myself, as suggested, and then publish the results here. It will take a few weeks to get a good sampling of data.
[SUB](Someone had to be first)[/SUB]