Cars that are economical at 80-90mph? Hypothetically ;)

Cars that are economical at 80-90mph? Hypothetically ;)

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mrrossi

Original Poster:

127 posts

66 months

Friday 24th October 2014
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Are there any cars that are economical at 80-90mph (hypothetically speaking, of course!)?

Ok, so this is all hypothetical, as of course no-one goes over 70mph on the UK motorways wink

But lets just say... a 2004 Seat Leon 1.9TDi diesel FR 150bhp will do 55mpg sat at 70mph and 2000rpm on the motorway.

At 80-90mph the revs are up to 2500-3000rpm and the economy drops to 45mpg.

I understand this is due to the engine being overstrained/revs being higher.

My question is this: would a bigger engine be more economical at these speeds?

Although I guess the problem is that a bigger engine would be less economical in the first place. For example, I know one car with a 3l diesel that will get 42mpg at 70mph. Because this is a 'stronger' engine and higher geared, so you'd still have low revs at 80-90mph, would that then be less affected? I.e. maybe still get 42mpg, or only a tiny fraction less, at those speeds?

If that's the case... perhaps there is an overlap...

I.e. is there a car, with, say, a 2.5l diesel, that gets 50mpg at 70mph. Perhaps this will still get 49/50mpg at 90mph?? So if you were regularly doing those speeds, it would be MORE ecomonical than the 1.9 diesel.

Or is there too many other factors involved - drag, gearing, weight etc.

Even if that's so, in theory, is this logic correct?

Similarly, I might just say f**k it and go for a 2l petrol 'for kicks'. This would get around 35mpg at 70mph. I'm guessing at 90mpg it would get less than 30mpg, lets say, 28mpg.

But if I went for a 2.5l petrol with higher gearing (because, lets face it, a more powerful car with a higher top speed does have higher gearing, so it won't be revving its socks off at 90, right?) that got 33mpg at 70mpg, might it still achieve 31/32mpg at 90mph?

Fingers on the buzzers.... go smile

Matt Clay

55 posts

58 months

Friday 24th October 2014
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Drag increases exponentially with speed, will require more energy to maintain momentum and therefore burn more fuel.

IanMorewood

4,309 posts

187 months

Friday 24th October 2014
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Still have to overcome increased drag, so unless you have some active aero parts that make you slippery at 80mph then no. Gearing could make a difference of course but probably not enough to find 80 as a sweet spot.

mrrossi

Original Poster:

127 posts

66 months

Friday 24th October 2014
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Could it be then that 80+ is a critical speed when our limits in aerodynamic design kick in, regardless of engine power?

Mound Dawg

1,868 posts

113 months

Friday 24th October 2014
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No, typically the sweet spot of gearing, engine efficiency and drag is more like 35-40 mph. 80 mph just multiplies the drag too much.
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Max_Torque

13,308 posts

156 months

Friday 24th October 2014
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super low CdA!

cptsideways

12,965 posts

191 months

Friday 24th October 2014
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What you need is a car with a Low Cda figure, tall gearing & low inherent fuel consumption.



Mound Dawg

1,868 posts

113 months

Friday 24th October 2014
quotequote all
But even then, the fuel economy is worse at 80 than at 40...

DonkeyApple

32,569 posts

108 months

Friday 24th October 2014
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I had an Overfinch Classic which did 16 mpg at any speed between 60 to 90 and 4.5 mpg in town.

So in relative terms it was at its most economical at 90.

underphil

1,013 posts

149 months

Friday 24th October 2014
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The effect of higher speed of different types of cars' economy can be quite varied:



ExPat2B

1,931 posts

139 months

Friday 24th October 2014
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Lots of experience doing this on the continent.

The short answer is no car is particularly economical at 85. There is approx 30% more air resistance at 85 compared to 70. So you can expect a 30% drop in fuel economy.

Worse than this is the acceleration, to maintain 85 even on the continent you will be forced to slow down for slower cars and vans overtaking lorries. Whilst you are accelerating back up to 85 you will be doing 15mpg or less.

My experience is that diesel cars experience less mpg drop at higher speeds than petrols.

This is why every car on the German autobahn doing 100mph + is a diesel, apart from the rich people in 5.0l v8 who clearly don't care about mpg.

cptsideways

12,965 posts

191 months

Friday 24th October 2014
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According to VW's Lupo 3L graph I posted above the figure at 100mph/160kph is 6L/100km or 47mpg in reality it does 5.4L/100kph flat out at 105ish mph or 54mpg. Tested over many miles biggrin I've never seen a figure worse than 5.6L/100km no matter how you drive it which is quite impressive.

The same car manages over 1000km on 36L of diesel in regular driving, not that you have to fill it very often hehe

mrrossi

Original Poster:

127 posts

66 months

Saturday 25th October 2014
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Thanks all, some interesting points to think about there! What I need then is a super slippery low Cda car... smile

toobusy

60 posts

91 months

Saturday 25th October 2014
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Anecdotally - not using the calculations above - my Polo Bluemotion does about 60 - 65 along A roads with traffic but mid 50s (or worse) at the speeds you referred to. The consumption falls off a cliff, small engine + wind resistance. My 911 (996) does about 26 on A roads but 30 - 32 ish on a long steady run on motorways at those speeds (in another country of course..).

Welshbeef

35,482 posts

137 months

Saturday 25th October 2014
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At 90mph I'm doing 1,900 odd revs in 8th

I've not owned it long enough or spent enough time at that speed to state what mine gets at the higher speeds - but I would assume a very low reving car/very long gearing and ample power -

I'd say Tesla S would be the winner or i3 EV or 535d wink.

grumpy

963 posts

180 months

Saturday 25th October 2014
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4.2L XF 28MPG with the cruise set to 90MPG.

mrrossi

Original Poster:

127 posts

66 months

Saturday 25th October 2014
quotequote all
toobusy said:
My 911 (996) does about 26 on A roads but 30 - 32 ish on a long steady run on motorways at those speeds (in another country of course..).
Chuckle smile

blearyeyedboy

4,741 posts

118 months

Saturday 25th October 2014
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grumpy said:
4.2L XF 28MPG with the cruise set to 90MPG.
I wish I could set the cruise at 90 mpg in any big V8. Grumpy, you've just solved the world's fuel problems. wink

OP, are you asking the wrong question? A Polo Bluemotion may have less of its peak efficiency at 80-90mph when expressed in percentage terms, but will be burning less fuel than the aforementioned Jaguar XF. The question is how much fuel use you wish to tolerate during your Private Road/Autobahn driving wink rather than the percentage of peak fuel economy it produces?

Small cars tend to be taller since longer cars can have their occupants positioned less vertically and still comfortably fit inside. Weight is also less of a concern at constant speed than air resistance (affected by cross-sectional area). I would suggest that something narrow and long with a relatively small engine and long gearing would give you the combination you seek, though you may have to sacrifice acceleration and agility for that.

buggalugs

8,468 posts

176 months

Saturday 25th October 2014
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O/H's Clubman 1.6 Diesel does around 60mpg at a steady 80 on runs down to Cornwall or up to the Lakes. I was very impressed with it.

gizlaroc

12,001 posts

163 months

Saturday 25th October 2014
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My 335i touring auto was surprisingly economical at 85mph. I averaged 78mph in it over 3 hours and saw 33mpg from that run.

The drop off from 60mph to 85mph was around 10%.


The 535d touring was similar too.


The same can't be said of my 2.0tdi A3, that would see 60mpg nearly at 60mph but at 85 would be down to around 39mpg.