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Size and aerodynamic efficiency

Even using Hawks combined figures there is a 20% difference. That's quite significant over a years motoring.

Say 10,000 miles at 35mpg = 285 gallons/1300 litres

10,000 miles at 30mpg = 333 gallons/1516 litres.

At £1.27 that's a difference of £275.
Yes, But can you imagine anyone saying 'Oh dear, this big £45,000 4x4 is going to cost me £275 per year more in fuel than a saloon, so I won't buy it.'

No. Nor can I. And that is why we all know this was a cynical stealth tax and not a green tax.

A green tax would charge a 5 litre V8 at least treble what it charged a 2 litre engine. Progressive noticeable pain. (BTW I am not advocating it).
 
A green tax would charge a 5 litre V8 at least treble what it charged a 2 litre engine. Progressive noticeable pain. (BTW I am not advocating it).

Too late...!! It's said now...

Hawk for Chancellor...
 
You're assuming still conditions in your calculations. Things get much worse when driving into a headwind or a cross-wind.
 
Good point, but it would make no difference, as most are only used on the school runs.:D

Ducks.:rolleyes:

Very true. Aerodynamics play very very little part at sub 20mph journeys!
 
I can remember Audi declaring in their publicity when they first introduced the quattro feature that a driven wheel can have less rolling resistance than an undriven one. I know that a four wheel drive system will use more energy because of all the effort needed to rotate all the extra transmission components, but if Audi's claim is correct it shouldn't be by much. Most SUVs have simply ridiculously wide tyres, and they must play a large part in their poor aerodynamic drag.
 
Aerodynamic forces will always be proportional to the square of the speed. If your average speed is very low, then they matter very little. If you have a very high average speed, then aerodynamics takes on a much more significant role. The biggest factor for fuel efficiency that we can change is the way we drive.
A few years ago I had a whole string of Audi A6 Avant comapany cars, only allowed to go to 8000miles prior to change. Each had the same aero, tyre size and usage but had very different fuel economy.
1.9 TDI (110) Mauual 55mpg
2.5 TDI (163) Manual 47mpg
2.5 TDI (163) (4WD) 42mpg
2.5 TDI (163) Mutitronic 2WD 41mpg

I had a total of 8 cars over a couple of years and two of most of the above so the figures are a good average for the same driver over the same route.
I likes the CVT Multitronic, but while the factory fuel figures claimed it varied little from a manual, that was not my experience.:confused:
I have always found it more difficult to get to the factory figures in a Mercedes auto than in most 'stick shift' cars.
 
I have always found it more difficult to get to the factory figures in a Mercedes auto than in most 'stick shift' cars.

I have the same problem, my car is quoted for 8.5 l/100 km and my 30 000 km average is currently 8.3 l/100 km and getting lower. :rolleyes: I don't think I'm going to reach the factory figure. :D

Honestly I assume I have less city traffic than the factory average figure assumes.
 
Even at the legal maximum of 70mph, aerodynamic drag is not a big part of total resistance.

I wouldn't say that...

On a W163 ML270 CDi aerodynamic drag equals rolling resistance at 66mph (at 533N). At 100mph aerodynamic drag is 1224N versus the same 533N for rolling resistance.

On the W203 C32 AMG aerodynamic drag equals rolling resistance at just under 79mph (at 383N). At 100mph aerodynamic drag is 619N versus the same 383N for rolling resistance.
 
At 70mph a typical car will use about 35bhp to maintain its velocity. Well over 50% of this is to overcome aero drag. (Depending on Cd)

A good rule of thumb - I've used about 70bhp being typical to maintain 100mph.

To compare a compact saloon with an SUV:

To maintain 70mph my C32 AMG needs to muster just 29bhp, whereas my ML270 CDi requires a hearty 48bhp.

At a lower 50mph the C needs just 16bhp versus 25bhp in the ML.
 
A good rule of thumb - I've used about 70bhp being typical to maintain 100mph.

To compare a compact saloon with an SUV:

To maintain 70mph my C32 AMG needs to muster just 29bhp, whereas my ML270 CDi requires a hearty 48bhp.

At a lower 50mph the C needs just 16bhp versus 25bhp in the ML.

Makes you wonder whay everyone strives for 200+ Bhp.

oh, I know it's to get the torque..;)

Nice figures there Bobby, I agree with the 70Bhp/100mph, slightly less actually but near.
 
That's about bang on what I was thinking.

70bhp should see about 108 mph.
 

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