.....the co-efficient of drag..............
Thread Starter
Super Member
Joined: Oct 2014
Posts: 517
Likes: 22
From: California
Mercedes C250 Coupe
.....the co-efficient of drag..............
........and fuel efficiency are big time related.
While I've no MPG complaints at all, I see both .26 and .27 in researching.
...........which is correct? (not a life threatening event - just curious)
2012 C250 Coupe
ez
While I've no MPG complaints at all, I see both .26 and .27 in researching.
...........which is correct? (not a life threatening event - just curious)
2012 C250 Coupe
ez
Junior Member
Joined: Aug 2015
Posts: 23
Likes: 0
From: NW Blighty
2015 Mercedes C220 CDI Coupe
Indeed you are correct, the aerodynamic drag force squares with speed and at steady speed cruise becomes especially significant above 100km/h (62 mph). As for which is the correct Cd factor, I doubt you'll be able to find a definitive answer as there are different methods for determining the result. The difference between 0.26 and 0.27 is well within experimental error and could simply be SE vs Sport versions. Either number is pretty good for a car like the C-class IMHO.
The biggest impact on fuel economy is harsh acceleration and braking.
The biggest impact on fuel economy is harsh acceleration and braking.
Thread Starter
Super Member
Joined: Oct 2014
Posts: 517
Likes: 22
From: California
Mercedes C250 Coupe
Indeed you are correct, the aerodynamic drag force squares with speed and at steady speed cruise becomes especially significant above 100km/h (62 mph). As for which is the correct Cd factor, I doubt you'll be able to find a definitive answer as there are different methods for determining the result. and could simply be SE vs Sport versions. Either number is pretty good for a car like the C-class IMHO.
The biggest impact on fuel economy is harsh acceleration and braking.
The biggest impact on fuel economy is harsh acceleration and braking.
Whether my coupe is .26 or .27 would seem secondary to the calculated MPG that this sailor recorded last Fall on a 450-mile trip - yes, Interstate - to/from Carmel (CA). 34.8 hand calc (not the C250's computer). I was impressed.
ez
Junior Member
Joined: Aug 2015
Posts: 23
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From: NW Blighty
2015 Mercedes C220 CDI Coupe
No worries, always glad to discuss stuff like drag factor and fuel economy.
I should add that I wasn't referring to 'my' experimental error. Instead I was trying to indicate that there is no absolute number for drag factor. It's a calculated number that can be derived using different theoretical or practical methods.
34.8 mpg is very good mileage, but just because it's hand calculated doesn't mean it's an error free result. There is likely to be at least +/- 2% error associated with filling the tank (even using the same pump) and another similar error associated with the the distance travelled measurement. So hand calculated fuel economy can easily be out by 4 or 5 percent even when carried out under controlled conditions (been there, done that
).
Cheers!
I should add that I wasn't referring to 'my' experimental error. Instead I was trying to indicate that there is no absolute number for drag factor. It's a calculated number that can be derived using different theoretical or practical methods.
34.8 mpg is very good mileage, but just because it's hand calculated doesn't mean it's an error free result. There is likely to be at least +/- 2% error associated with filling the tank (even using the same pump) and another similar error associated with the the distance travelled measurement. So hand calculated fuel economy can easily be out by 4 or 5 percent even when carried out under controlled conditions (been there, done that
).Cheers!
Thread Starter
Super Member
Joined: Oct 2014
Posts: 517
Likes: 22
From: California
Mercedes C250 Coupe
The continuing saga of a small C-class coupe and somewhat impressive fuel numbers.............
Hello from 38N 121W............
Your hand calc (miles driven by gallons used) views unexpected (and welcome).
Would it then be a fair statement that the true MPG (corrected for volume/distance) could range 4 to 5 per cent above (or - alas - below) my calc'd 34.8?
Thanks for your positive input here. It would appear your MB burns diesel (like my last ship - only I think we called it ND). Diesels get VERY impressive fuel numbers. I've recorded 54.5 - again not the car's computer - on my work car: a 2-liter diesel wagon.
all the best, ez
Hello from 38N 121W............
Your hand calc (miles driven by gallons used) views unexpected (and welcome).
Would it then be a fair statement that the true MPG (corrected for volume/distance) could range 4 to 5 per cent above (or - alas - below) my calc'd 34.8?
Thanks for your positive input here. It would appear your MB burns diesel (like my last ship - only I think we called it ND). Diesels get VERY impressive fuel numbers. I've recorded 54.5 - again not the car's computer - on my work car: a 2-liter diesel wagon.
all the best, ez
Junior Member
Joined: Aug 2015
Posts: 23
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From: NW Blighty
2015 Mercedes C220 CDI Coupe
Hi,
Yes your hand calculated fuel economy figures could be out by +/- 5%. They won't be +5% on one measurement and -5% the next, ie. a swing of 10%. I would expect repeatability to be within +/- 2.5% in a reasonably well controlled test using the same fuel pump.
As for my MB it is a diesel. So far the trip computer says it's doing about 46 mpg. Not done a hand calculation.
Yes your hand calculated fuel economy figures could be out by +/- 5%. They won't be +5% on one measurement and -5% the next, ie. a swing of 10%. I would expect repeatability to be within +/- 2.5% in a reasonably well controlled test using the same fuel pump.
As for my MB it is a diesel. So far the trip computer says it's doing about 46 mpg. Not done a hand calculation.
