The diesel vs petrol economy debate

Ricochet J

Ricochet J

Ricochet J

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Joined
29 Jun 2004
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I'm after replies from people who have experience with driving both types of engines on long distance motorway journeys. I'm not bothered about urban city driving but you're more than welcome to comment on it.

How economical is a diesel compared to a like-for-like petrol engine on long motorway journeys?

I understand this is an open ended question in which I'll recieve a variety of answers. :)
 
Doing 60mph to 70mph you'll find on average both petrol and derv engines will get there 'combined' MPG figure the companies quote.

It does differ company to company and engine to engine however.

'Normal' driving you'll tend to find both engines get the 'urban' figure, even if this does include some motorway driving and general A and B roads.
 
I have made long distance journeys in the following cars and acheived the following figures:

a) E39 530i Sport: 37mpg on a long distance trip, 34mpg when not really driving with economy in mind, 31mpg on my regular 300 mile round trip mixed A road/dual carriageway/overtaking run
b) E39 530d SE: 44mpg on a long distance trip, 39-40mpg when not driving with economy in mind, 37 mpg on my regular 300 mile round trip mixed A road/dual carriageway/overtaking run
c) E87 118d M Sport: 51mpg on a long distance trip, 45mpg on my regular 300 mile round trip mixed A road/dual carriageway/overtaking run
d) E87 120d M Sport: 50mpg on a long distance trip
e) E91 318d SE: 50mpg on a long distance trip, 42 mpg on my regular 300 mile round trip mixed A road/dual carriageway/overtaking run
f) E91 320d M Sport: 41mpg on my regular 300 mile round trip mixed A road/dual carriageway/overtaking run.

Both E39's I was able to pretty get the 'combined' figure on a cross country run (my 300 mile round trip - Plymouth to Southampton and return).

I was not able to acheive this on ANY of the newer cars. Infact on longer distance runs I struggled to acheive even the 'urban' fuel consumption figures. This seems to be the case with every single new gen BMW diesel I drive. The figures are very very hard to acheive.

I beleive that in some point over the last 4-5 years the manufacturers suddenly got very good at delivering outstanding economy figures on the test cycle, whereas before that the cars delivered the quoted economy in the real world..
 
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I used to put £50 a week in my 1.8 pertol vectra.
I now put £20 a week in my 1.7 diesel astra

same daily mileage.
£120pm saving :)
 
Too many people get caught up in the fuel and tax savings and forget that modern commonrail diesels can be quite fragile and are complex. I know Ford charge more to service their diesel models, not sure if this rings true with other makes.
 
Diesels make great company cars. All I have to pay for is private mileage and I get 50mpg out of my 2.0 Mondeo "Econetic" on a motorway drive.

That said, my GF's LPG S13 Nissan 200SX is still cheaper to run in terms of fuel costs.
 
Oh lord do we really need another diesel thread, these only ever go one way.

Anyway, on a run from Exeter to essex down the 303 I'd see 52ish from the Mondeo and 35 from the Saab. Doing the same trip on saturday in the Honda and I reckon it would be about 40.

In terms of 'like for like' - how do you define that? Bhp? Peak torque? 0-60? Purchase price?

The closest I can think of is the Mondeo vs the Accord - similar performance, similar bhp, similar size and abount 10mpg difference in any given situation
 
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My 120d gets 40-42mpg, as high as 49 when I drove down to Cornwall.

In comparison, guy at works 120i gets 36-37mpg daily, and about 41-42 on long motorway trips
 
The only comparison I have is my LCR and my 330d, both of which are outputting slightly over 200BHP, both of which are turbo'd vehicles.

On a long motorway jaunt, without actually trying, I typically see low to mid 40's from the 330d, and low to mid 20's from the LCR. If I were to really try my utmost to be economical, the LCR might hit high 30's, and the 330d low 50's, but that would take far more effort than it's worth.

Manchester > Val Thorens (French Alps) > Manchester, netted me 44MPG average from the 330d.

Manchester > Southampton > Manchester, netted me 28MPG average from the LCR.

I know you said you weren't really bothered, but A roads will see high 20's/low 30's from the LCR and high 30's/low 40's from the 330d. Likewise, urban consumption in the real world is mid-high 30's from the 330d and low 20's or high teens from the LCR.
 
moved from a 206 1.4 getting 45MPG
to a golf 1.9 TDI where i get 70MPG on the same run a massive saving for me.

servicing wise the golf is a little more expensive with the oil but only by £10 for 5lt so nothing in comparison, i do the servicing my self as well.

both with steady driving on a 160mile trip with about 70% motorway
 
[TW]Fox;16926158 said:
Both E39's I was able to pretty get the 'combined' figure on a cross country run (my 300 mile round trip - Plymouth to Southampton and return).

I was not able to acheive this on ANY of the newer cars. Infact on longer distance runs I struggled to acheive even the 'urban' fuel consumption figures. This seems to be the case with every single new gen BMW diesel I drive. The figures are very very hard to acheive.

I beleive that in some point over the last 4-5 years the manufacturers suddenly got very good at delivering outstanding economy figures on the test cycle, whereas before that the cars delivered the quoted economy in the real world..
Click to expand...

I appreciate that manufacturers figures are more fiction than anything else these days, I wonder if your findings are due in part to the cars you've experienced being brand new courtesy cars? Dad was only seeing mid 30s from his a6 but that went up to nearly 50 after a few thousand miles

bad_ass_neil said:
moved from a 206 1.4 getting 45MPG
to a golf 1.9 TDI where i get 70MPG on the same run a massive saving for me.

servicing wise the golf is a little more expensive with the oil but only by £10 for 5lt so nothing in comparison, i do the servicing my self as well.

both with steady driving on a 160mile trip with about 70% motorway
Click to expand...

I think you have a faulty trip computer
 
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no_1_dave said:
I used to put £50 a week in my 1.8 pertol vectra.
I now put £20 a week in my 1.7 diesel astra

same daily mileage.
£120pm saving :)
Click to expand...

Does the Astra really achieve 2.5x the economy of the Vectra? Seeing as the Vectra can probably achieve around 30mpg that would make a somewhat unbelievable 75mpg from the Astra.
 
I get 60mpg out of my Derv Polow on a run but it does have a 1.9 derv lump in it & weighs about as much as a paper bag. Round town it probably drops to 50'ish but it uses so little i don't really notice, Chuck 20 quid in & it lasts for Ages. :cool:
 
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