Cars that are economical at 80-90mph? Hypothetically ;)
Discussion
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
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
Ok, so this is all hypothetical, as of course no-one goes over 70mph on the UK motorways
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
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.
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.
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 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
The same car manages over 1000km on 36L of diesel in regular driving, not that you have to fill it very often
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..).
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. 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 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.
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.
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.
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