Interesting MPG observation with the convertible

[captjeff19]

Let me start by saying this is not a discussion for the economy - or lack there of - of the car. I didn't by a Focus and this car will always get better fuel economy than my '09 F150 4x4 that it replaced. Just posting an observation.

I have the GT Premium convertible. Last weekend my wife and I took off for a drive to nowhere. Before leaving we topped of the tank and reset the trip info. With the top down, we drove for about 90 miles at which point the trip computer showed the 20.9 mpg after some in-town driving but mostly 65-70 mph highway set with the cruise control. Just for kicks on the way home, I reset the trip computer, reversed the route but this time with the top up (AC on both ways) and got 25 mpg for the 90 miles home at the same cruise controlled speed.

Could the aerodynamics of top up vs down really make 15-20% difference in fuel consuption at those speeds? All other factors were basically the same i.e., temperature, weight, wind, etc.

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[Chameleon]

Nope. IMO. You were probably getting on the gas more on the 1st trip. Plus it was mixed with "in town" driving.

 

[cbrookre]

Let me start by saying this is not a discussion for the economy - or lack there of - of the car. I didn't by a Focus and this car will always get better fuel economy than my '09 F150 4x4 that it replaced. Just posting an observation.

I have the GT Premium convertible. Last weekend my wife and I took off for a drive to nowhere. Before leaving we topped of the tank and reset the trip info. With the top down, we drove for about 90 miles at which point the trip computer showed the 20.9 mpg after some in-town driving but mostly 65-70 mph highway set with the cruise control. Just for kicks on the way home, I reset the trip computer, reversed the route but this time with the top up (AC on both ways) and got 25 mpg for the 90 miles home at the same cruise controlled speed.

Could the aerodynamics of top up vs down really make 15-20% difference in fuel consuption at those speeds? All other factors were basically the same i.e., temperature, weight, wind, etc.

I was actually curious about this too, and (being an engineer) was planning to run some experiments. I will be picking my car up today so will let you know in a few days what I find...

Chris

 

[captjeff19]

Nope. IMO. You were probably getting on the gas more on the 1st trip. Plus it was mixed with "in town" driving.

Did you miss the part about the wife being with me??? I was not getting on the gas. :lol:

The route was nearly identicle in both directions as far a the mix of in-town vs highway speeds.

 

[Chameleon]

Did you miss the part about the wife being with me??? I was not getting on the gas. :lol:

Gotcha. Well its hard to not get on it even with the wife riding with me. Her=:eyebulge:

 

[GB John]

Top down should make that much of a difference. Aerodynamic drag is a huge factor in highway fuel mileage. I'm going on a trip in July with two bicycles on the back of my GT (fastback). I'm wondering how much that will sap my mileage.

 

[slowhand99]

Could the aerodynamics of top up vs down really make 15-20% difference in fuel consuption at those speeds? All other factors were basically the same i.e., temperature, weight, wind, etc.

Interesting. It's bound to cause a degree of turbulence. A small amount of uphill vs downhill makes a big difference and the wind can't have been the same both ways, unless there was no wind. As you say - we don't buy them for economy. Had the wife had her hair done?

 

[dcrypter]

Top down should make that much of a difference. Aerodynamic drag is a huge factor in highway fuel mileage. I'm going on a trip in July with two bicycles on the back of my GT (fastback). I'm wondering how much that will sap my mileage.

Yuuuup, I'm not sure why anyone would be surprised by getting significantly less mileage with the top down. If you happened to be driving only city miles and under 55~ mph then you probably wouldn't notice much of a difference but above that drag starts coming into the picture in a big way.

 

[Double]

Wind direction ? I get about a 20% difference in fuel consumption with trips going both ways.

And what about elevation ?

 

[emh87]

You should run Google Tracks next time, it will keep track of everything from speed to elevation. It might be the trip back was more "down hill".

 

[73MachI]

Absolutely. Top down can dramatically increase drag and have a negative impact upon your mileage at higher speeds. It basically generates a large negative pressure cell in the passenger compartment. The turbulence and noise is an indication of this.

 

[Chris95GT]

You should run Google Tracks next time, it will keep track of everything from speed to elevation. It might be the trip back was more "down hill".

Exactly what I was thinking. Our weekly trip to the in-laws for dinner always uses more gas going there than coming back using the same route, despite no difference in driving pattern.

 

[Rogues Gambit]

Tell ya what though, top down on a beautiful day/night is worth the hit in mpg's

 

[captjeff19]

[QUOTE

And what about elevation ?[/QUOTE]

It's Central Floriday... no hills to speak of. Wind was null all day.

I'll just chalk it up to the "negative pressure" comment.

Like I said in the opening, not a concern... just an observation.

Compared to my boat, the GT gets 500% better mileage... practically a Tesla.

 

[Raoh]

I believe above 30 or 40, windows open or top down is less fuel efficient than AC. The gap will only become larger as the speed increases. There's a lot of extra drag added as people have stated.

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