Motoring fuel efficiency

[Dieselman]

I was just reading about the E300 Hybrid and it mentions about the optional wheels making a difference to mpg. i was aware of this, but seeing it pointed out made me think more about it.

It combines the 201bhp 2.1-litre engine from the E250 CDI with a 27bhp electric motor and a seven-speed automatic gearbox to average 67.3mpg and just 109g/km of CO2 (in saloon form).

The estate is almost as efficient, managing 64.2mpg and 116g/km, although all these figures are possible only with the no-cost optional 16-inch alloys.

The standard-fit 17-inch wheels increase the saloon’s emissions to 111g/km and its average consumption to 65.7mpg, while the estate's stats are revised to 119g/km and 62.8mpg.

So going up 1" in diameter makes 2mpg difference. It would be interesting to have some comparative performance figures as well.

Is anybody changing their wheels and is prepared to do some timed runs before and after?

I've put this in General discussion as I thought other items might be brought into the topic.

What do people find makes a quantifiable difference to mpg or performance?



Has anyone tried an E300 Hybrid yet, if so, what did you think?

 

[IanW]

Around 2mpg better in summer than winter.

Ian.

 

[Stratman]

Did the larger wheels have the same width tyres as the smaller ones?

 

[Bellow]

So going up 1" in diameter makes 2mpg difference. It would be interesting to have some comparative performance figures as well.

Lee has similar with his Discovery. Surprises me as the widths (IIRC) were the same and while conventional wisdom says the larger the diameter the lower the rolling resistance, the overall diameters don't change due to differing tyre aspect ratios. My guess is that it's down to tyre construction differences between low and higher profile tyres.
Smart fortwos are similarly differntly CO2 rated depending on tyre size - width though.

What do people find makes a quantifiable difference to mpg or performance?

Headwinds, hills, my right foot for reducing mpg.
Anything that helps carry cornering speed for increasing mpg.

Has anyone tried an E300 Hybrid yet, if so, what did you think?

Nope, but Autocar rated its refinement. The Volvo V60 Hybrid is an interesting beast. Sub 6s to 60mph and 100 and something mpg (reckoned to use half the fuel its similarly engined D5 sibling uses). I can see it catching on for those who can spare the £50k price tag and can utilise the plug-in part of its hybridicity.

 

[Deleted member 37751]

Lee has similar with his Discovery. Surprises me as the widths (IIRC) were the same and while conventional wisdom says the larger the diameter the lower the rolling resistance, the overall diameters don't change due to differing tyre aspect ratios. My guess is that it's down to tyre construction differences between low and higher profile tyres.

The wheels I took off were 255/55/19, the current set are 285/35/22.

The consumption has changed by an average of 5mpg less (20-25 as opposed to the 25-30 before)

 

[wemorgan]

I suspect the gradient of the roads increases my fuel consumption by 10-15%.

Am I correct in thinking that the European fuel economy tests do not take in to account road gradient?

 

[Paul/]

I guess aerodynamic efficiency of different wheel designs could make a measurable difference. Though the E hybrid options look pretty similar.

 

[Colin_b]

Lee has similar with his Discovery. Surprises me as the widths (IIRC) were the same and while conventional wisdom says the larger the diameter the lower the rolling resistance, the overall diameters don't change due to differing tyre aspect ratios. My guess is that it's down to tyre construction differences between low and higher profile tyres.

This is my understanding, the bigger the diameter, the lower the rolling resistance, though the effect is minimal with pneumatic tyres. Assuming tyre width and construction remain the same.

Tyre width has a more marked effect. Optional wheels are taller and wider, usually.

 

[wemorgan]

Aren't larger diameter wheels typically heavier?

 

[Paul/]

Aren't larger diameter wheels typically heavier?

...so the question might be, does the lower profile tyre save more weight than the larger diam wheel adds.

 

[Bellow]

The wheels I took off were 255/55/19, the current set are 285/35/22.

The consumption has changed by an average of 5mpg less (20-25 as opposed to the 25-30 before)

The width will be playing a part then Lee. I hadn't realised you'd upped the width by 30mm.

I suspect the gradient of the roads increases my fuel consumption by 10-15%.

I never get the mpg that other fortwo drivers on the fora claim - hilly Scotland vs flat motorways has to be a part of that.

Am I correct in thinking that the European fuel economy tests do not take in to account road gradient?

I don't think they do - or take any real life driving into account either.

I guess aerodynamic efficiency of different wheel designs could make a measurable difference. Though the E hybrid options look pretty similar.

The base (or eco version)A-Class has aero wheel trims apparently as they do make a difference - but we all want big and pretty alloy sculptures to wrap a ribbon of rubber around. Personally, I'd rather wheels were enclosed. (The banging you can hear is car 'designers' falling off their bar stools at the suggestion, but an XK120 looks just fine with rear spats. Similarly, who looks at a Lamborghini Miura or Ferrari Daytona and thinks 'what that car really needs is 30'' rims with 15% profile tyres?)

 

[developer]

the current set are 285/35/22

You pimp.

 

[Dieselman]

...so the question might be, does the lower profile tyre save more weight than the larger diam wheel adds.

Last I knew, steel or aluminium weighed more than air.

Larger diameter wheels use more energy to change velocity or direction due to weighing more and also the mass being further out from the centre.


I'm quite interested in the E300-Hybrid mainly for fuel consumption reasons, but also it's meant to be more refined than the 250 Cdi as it completely decouples the engine from the drivetrain as it has no torque converter, but a clutch.

How do people find the E250 for refinement?

 

[Bellow]

Last I knew, steel or aluminium weighed more than air.

Larger diameter wheels use more energy to change velocity or direction due to weighing more and also the mass being further out from the centre.

Feedback from those running big rims predominately in cities vs those predominately using M-ways might tell the effect of the rotating inertia.

I'm quite interested in the E300-Hybrid mainly for fuel consumption reasons, but also it's meant to be more refined than the 250 Cdi as it completely decouples the engine from the drivetrain as it has no torque converter, but a clutch.

How do people find the E250 for refinement?

Wasn't there a recent thread on this? Or did it not answer your queries DM?
Refinement seems to be the hybrid's trump card judging by the road assessments in Autocar.

 

[Paul/]

Last I knew, steel or aluminium weighed more than air.

Larger diameter wheels use more energy to change velocity or direction due to weighing more and also the mass being further out from the centre.

??? Perhaps you misread "does the lower profile tyre save more weight than the larger diam wheel adds"...

I assumed that a larger diam wheel is heavier... and was questioning whether a correspondingly lower profile tyre would be lighter (than the standard tyre) and if so would the total weight of wheel and tyre be more or less than for the smaller diam wheel pairing.

 

[Dieselman]

??? Perhaps you misread "does the lower profile tyre save more weight than the larger diam wheel adds"...

I assumed that a larger diam wheel is heavier... and was questioning whether a correspondingly lower profile tyre would be lighter (than the standard tyre) and if so would the total weight of wheel and tyre be more or less than for the smaller diam wheel pairing.

I was being a little sarcastic...

The tyre mass hardly decreases with reduced sidewall as it's only thin rubber and 99% air.
The wheel mass not only increases, but the heaviest part of the wheel, the rim, moves further out so increases the rotational mass effect.

 

[Colin_b]

Aren't larger diameter wheels typically heavier?

Yes, but does an additional 20kg on an 1800kg vehicle make 2% difference in fuel consumption? How about a 70kg passenger? More energy to accelerate , certainly. Don't know....

 

[Dieselman]

Wasn't there a recent thread on this? Or did it not answer your queries DM?
Refinement seems to be the hybrid's trump card judging by the road assessments in Autocar.

I currently have a 6 pot diesel and am interested to hear whether the 4 cylinder is refined enough.

Reports from actual drivers on refinement and fuel economy would interest me greatly as I can see one of these being on the shopping list.

 

[Dieselman]

Yes, but does an additional 20kg on an 1800kg vehicle make 2% difference in fuel consumption? How about a 70kg passenger? More energy to accelerate , certainly. Don't know....

Being rotational mass, the wheels have an effect greater than the increase in basic mass.
Every change of velocity or direction needs energy.

 

[Bellow]

Yes, but does an additional 20kg on an 1800kg vehicle make 2% difference in fuel consumption? How about a 70kg passenger? More energy to accelerate , certainly. Don't know....

Polar moment of inertia is what of relevence here.
All rotating elements from crankshaft to wheel/tyre have an inertia distinct from the inertia associated with merely accelerating/decelerating them up the road. In essence, tyres, wheels, driveshafts, gearshafts, clutches and cranks are all smaller scale flywheels. They need to be punted up the road and rotated.