[G50]

Originally Posted by Wills2

Diesel engines are 15-20% more efficient than petrol engines, forget aggressive offsets it's a standard BMW not a race car, the real difference is the fuel, if you had a 530d it'd be pretty much as the 330d.

So a drag coefficient of 0.32 for the f31 against 0.22 for the g30 makes no difference at speed? Just the fuel?

Since we are talking about economy at cruising speeds I'd say it does, and that was my point. The lower the drag coefficient the less the mpg figure will drop off as speed rises for the same engine. At 50mph a f31 330d might have the same economy as a g30 530d (let's forget one is n57 one is b57), but at 80mph the g30 will be more efficient.
Couple to that Pete's response(thanks Pete, interesting stuff) of diesel engines getting less efficient as revs rise and petrol engines more efficient at higher rpm as peak torque is higher, and THAT explains my experience, not stating the obvious that diesel in general is more efficient and some drivel about race cars.
At autobahn cruising speeds the biggest thing that influences mpg is drag coefficient, then I'd say gearing, then fuel type.

On the wheels, if you look at a car head on and can see the wheels "poke" from the sides of the car, where the standard car has wheels that "tuck", then you have increased the drag coefficient of that car, which will make it less fuel efficient as the speed rises than a car with standard wheels.

[MashinBenzin]

@G50 are those drag numbers right, 0.32 and 0.22? The former sounds early 1980s poor and the latter Volkswagen XL1 territory?

Coefficients aside, frontal area has a massive impact. The same CDa on a supermini yields very different results on a SUV. A bigger car always at a disadvantage. As a Discovery driver, I've noticed this

[HighlandPete]

Aerodynamics do make a big difference at speed. Problem is, we don't have much constant speed data around to make comparisons.

I remember the time when the original official fuel test had the 56 & 75mph constant speed consumptions. That showed some very interesting results across different models and engine designs/sizes.

An interesting constant speed comparison from some older German tests. A Golf 2.0 TDI vs. 535d touring. At 50mph about a 17 - 18mpg difference, but by the time the Golf runs out of puff, they are within ~1mpg.

Not the best comparison, but against an 535i touring, the diesel Golf has ~23mpg advantage at 50mph, but at the top end, the Golf is just 5mpg ahead of the big petrol.

[Eddamoo]

Originally Posted by HighlandPete

I ran a VW 1.9 PD TDi, with a BSFC achieving 195g/kW-hr at best point.

I ran a couple of Ibiza's with these engines - they were awesome but very fussy! I remember mine got over 100mpg driving through Mont Blanc

Did you end up fiddling with the cam-timing value? Knowledge of this was only growing when I got rid of mine. A cam belt change without the proper tools could ruin efficiency of these engines as the cam could adjust around 6 degrees! You could literally lose 15%+ efficiency by having the cam set wrong, and this was only written on a silly sticker on the cam cover that didnt last until the average owners change interval!!

[HighlandPete]

Originally Posted by Eddamoo

Did you end up fiddling with the cam-timing value? Knowledge of this was only growing when I got rid of mine. A cam belt change without the proper tools could ruin efficiency of these engines as the cam could adjust around 6 degrees! You could literally lose 15%+ efficiency by having the cam set wrong, and this was only written on a silly sticker on the cam cover that didnt last until the average owners change interval!!

No, never had the TDi's that long. Ran a couple of new B5 Passats. A saloon 110 TDi for a couple of years and 40k miles, then an Estate 115 PD TDi for two years and similar miles.

Both very economical, over 55mpg averages. Both were 60+mpg over any decent distance. One trip south in the 110 TDi, from the Highlands to Warwick Services recorded 69.9mpg.

[G50]

Originally Posted by MashinBenzin

@G50 are those drag numbers right, 0.32 and 0.22? The former sounds early 1980s poor and the latter Volkswagen XL1 territory?

Many sources on the internet state the G30 to be best in class at 0.22, a snip from Wikipedia below as an example of one of them.


The F31(as this is what I owned and was comparing) is a bit harder to find a credible website, with lots of "autostat" type pages putting it at either 0.31 or 0.32. The same wikipedia page as in the snip has the F30 saloon as 0.29, and a bit of research suggests an estate can be around 0.02-0.04 higher than a saloon.

The discovery is 0.33 apparently, and the new defender 0.38.

I think some of these numbers are a bit like the vehicle weight and vehicle power stats, it all depends on which measurement system/standard is used.

[MashinBenzin]

Originally Posted by HighlandPete

Aerodynamics do make a big difference at speed. Problem is, we don't have much constant speed data around to make comparisons.

I remember the time when the original official fuel test had the 56 & 75mph constant speed consumptions. That showed some very interesting results across different models and engine designs/sizes.

An interesting constant speed comparison from some older German tests. A Golf 2.0 TDI vs. 535d touring. At 50mph about a 17 - 18mpg difference, but by the time the Golf runs out of puff, they are within ~1mpg.

Not the best comparison, but against an 535i touring, the diesel Golf has ~23mpg advantage at 50mph, but at the top end, the Golf is just 5mpg ahead of the big petrol.

I always felt I knew where I stood with the 56/75 examples. Rarely attainable but easy to extrapolate from. No doubt test cheat devices would have made mincemeat of them in recent years though!

[MashinBenzin]

Originally Posted by G50

Many sources on the internet state the G30 to be best in class at 0.22, a snip from Wikipedia below as an example of one of them.


The F31(as this is what I owned and was comparing) is a bit harder to find a credible website, with lots of "autostat" type pages putting it at either 0.31 or 0.32. The same wikipedia page as in the snip has the F30 saloon as 0.29, and a bit of research suggests an estate can be around 0.02-0.04 higher than a saloon.

The discovery is 0.33 apparently, and the new defender 0.38.

I think some of these numbers are a bit like the vehicle weight and vehicle power stats, it all depends on which measurement system/standard is used.

That's a great comparison of Defender vs. Discovery. Two big blocks of similar shape, height, glasshouse, with radically different surfacing.

The Disco takes flack for the rear design, but it's exactly the aero rounding of the corners, combined with normal disco height, that's one of the biggest problems. But with a D4 (i.e. Defender) type rear, we see the aero impact.

Given that Discos are bought as all rounders with large proportions of motorway/a-road cruising, plus fleet averages, it's easy to see why LR did this.

The offset number plate is not what I'd fix if I had a pen in the LR design office.

[Stunt.Monkey]

One of the only things I remember from O level physics is E=MV*V

E = energy, M = mass, V = Velocity

So for a constant mass the energy held by the body increases in proportion to the square of the velocity.

The energy can only come from the fuel (assuming flat road)

I also think there is a similar relationship with respect to increasing air resistance as velocity rises.

I am pretty sure this will mean that you would burn more fuel the faster you go and that this holds for comparisons between any two speeds - 70 and 75mph as well as 10 and 15mph.

[NanasBack]

Have another think.

Was it E = M C * C ?

[isleaiw1]

On more important topics, TouringPleb, hows the car now you are a few weeks in?

[Eddamoo]

Originally Posted by Stunt.Monkey

I am pretty sure this will mean that you would burn more fuel the faster you go and that this holds for comparisons between any two speeds - 70 and 75mph as well as 10 and 15mph.

You're right that higher speed = higher drag = more kinetic energy required.

The thing you're missing is the energy is stored in the fuel and it needs converting. The engine and gearbox convert the energy and both are very dynamic in how efficiently they do it (even different gears produce different parasitic losses). If the engine is in its 'sweet spot' that conversion efficiency can overcome the KE increase.

[Stunt.Monkey]

Originally Posted by NanasBack

Have another think.

Was it E = M C * C ?

i have had another think and I was right. Of course your equation is right too, but they are two different things. E=MV*V is the energy in a moving body whereas your E=MC*C is the energy that exits in the body as result of it's mass, totally irrelevant to this thread unless you intend to split every atom in the car - I'm pretty sure atoms don't split and release energy at 75mph.

[HighlandPete]

Originally Posted by Stunt.Monkey

...totally irrelevant to this thread unless you intend to split every atom in the car.

We get close to that in many a topic.

[Wills2]

Originally Posted by MashinBenzin

@G50 are those drag numbers right, 0.32 and 0.22? The former sounds early 1980s poor and the latter Volkswagen XL1 territory?

Coefficients aside, frontal area has a massive impact. The same CDa on a supermini yields very different results on a SUV. A bigger car always at a disadvantage. As a Discovery driver, I've noticed this

I think he over reached in his outrage, like I said diesel engines are more thermally efficient that is the main factor in this comparison.

[Wills2]

Originally Posted by G50

So a drag coefficient of 0.32 for the f31 against 0.22 for the g30 makes no difference at speed? Just the fuel?

lol

[G50]

Originally Posted by Wills2

I think he over reached in his outrage, like I said diesel engines are more thermally efficient that is the main factor in this comparison.

lol