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      09-10-2012, 11:59 AM   #1
IamHere
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6.4L/100km or 28MPG?

I downloaded the coupe catalogue from the international site and noticed that the international fuel economy is significantly better than the US fuel rating.

The international catalogue lists the 135i as 6.4 l/100km for Extra-urban. Assuming google is right with the unit conversion this works out to 36.75 mpg. The US brochure lists the 135i at 28mpg Highway.

The Urban/City numbers are almost the same 12.1 l/100km (19.44 mpg) VS. 20 mpg.

Is there a significant difference in the EU version compared to the US that would cause the 8mpg difference?

The spec'ed weights are the same -- EU 1530 kg (3373 lbs) vs US 3373 lbs. The gear ratios are the same.

Is it just a different test method?
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      09-10-2012, 12:22 PM   #2
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I once saw 6.8L/100kph... but you have to be driving damm slow to achieve that! Like less than 80 kph.

The EU has three measurements when it comes to fuel economy. Urban/City, highway and combined. None of them is close to real world numbers.

A 135i will see anywhere from 9L going slow (driving under 110 kph on the highway) to 20L. I normally see 14.5L on average = driving quickly but not really THAT fast (under 180 kph). BUT... if you drive fast... over 210 kph... you can use lots more fuel. 20L, 30L, 40L is possible.

Just this weekend I was driving "fast" and averaged 23L consumption. The most I have used is "only" going 199 kms on 44L 's of SupePlus fuel.
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      09-10-2012, 10:17 PM   #3
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US quoted fuel economy numbers have been reduced about 30% from EPA test numbers to reflect real world driving conditions. I can reset the MPG on my 135i while driving on the freeway and it will get 30 or 31 mpg despite its 25 mpg rating. Of course if I have to slow down for traffic it drops right away. That European number has not been reduced by the EPA fudge factor.
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      09-10-2012, 10:24 PM   #4
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I routinely manage mid to low 30s highway in my 135 - these numbers are relatively achievable (although having a vert helps with this)

My guess is that the EPA numbers are either very conservative or that google is translating into UK gallons (which are larger than US gallons)
If we assume that UK gallons are being used than the EPA numbers translate to about 34 UK MPG - much closer to your source's economy rating.







And because people will be asking about my first point a vert with the top down experiences some fun aerodynamic benefits over a coupe. Namely a re-circulation vortex.

the flow over the top of the vert separates so much that viscous effects on the surface cant take hold. Actually reducing drag!
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      09-10-2012, 10:34 PM   #5
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Brokenvert how did you manage on 30mpg?
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      09-11-2012, 06:45 AM   #6
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Quote:
Originally Posted by BrokenVert View Post

And because people will be asking about my first point a vert with the top down experiences some fun aerodynamic benefits over a coupe. Namely a re-circulation vortex.

the flow over the top of the vert separates so much that viscous effects on the surface cant take hold. Actually reducing drag!
C'mon! It's not the skin friction that causes all the drag, it's the low pressure zone so basically the size of the wake. Surely the size of the wake on a vert with the roof down has got to be double that of the coupe.
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      09-11-2012, 06:59 AM   #7
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Quote:
Originally Posted by BrokenVert View Post
I routinely manage mid to low 30s highway in my 135 - these numbers are relatively achievable (although having a vert helps with this)

My guess is that the EPA numbers are either very conservative or that google is translating into UK gallons (which are larger than US gallons)
If we assume that UK gallons are being used than the EPA numbers translate to about 34 UK MPG - much closer to your source's economy rating.







And because people will be asking about my first point a vert with the top down experiences some fun aerodynamic benefits over a coupe. Namely a re-circulation vortex.

the flow over the top of the vert separates so much that viscous effects on the surface cant take hold. Actually reducing drag!
Isn't the vert heavier thus making it less efficient?
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      09-11-2012, 10:15 AM   #8
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Quote:
Originally Posted by CrustyNoodle View Post
C'mon! It's not the skin friction that causes all the drag, it's the low pressure zone so basically the size of the wake. Surely the size of the wake on a vert with the roof down has got to be double that of the coupe.
At low Re skin friction drag dominates over profile drag. Also the velocity deficit (wake) would be about the same for a vert and a coupe. The verts would be slightly more turbulent - but in the context of flow control a turbulent velocity profile is not necessarily a bad thing.

What makes you think that the verts wake is "double" that of the coupe?

The verts advantage is much the same as the one experienced by pickups. It's called a recirculating vortex (the point that you both seemed to skip last time around)

Since friction is the main source of drag at low Re (and hence low velocities) the flow separation over what would be the roof of a coupe causes the air to bypass actually touching the roof. Reducing skin friction drag!

Quote:
Originally Posted by m7ammed View Post
Isn't the vert heavier thus making it less efficient?
Why at those two points coming up in a talk about constant velocity efficiency? there another concept called inertia that comes into play.

Inertia makes the difference in holding a 3650 pound car and a 3400 pound at speed almost nonexistent - so we can ignore it.


I'll go into more detail when I'm doing this on a computer and not my phone.


Here's another fun fact that blows people's minds - drag doesn't increase with the square of velocity. It's actually a parabolic curve.

Pilots should know the term "flying the backside of the drag curve". Well guess what. In most autos and day to day situations we drive on the backside of the curve. It's means that we can drive faster and create less aerodynamic drag! But only to a point.

I should generate a drag curve for the 135. It'd be fun
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