Motorway Efficiency?

Pricey

Pricey

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Just completed first set of long journeys - Yorkshire to Sussex and back through half term.

Way down, had cruise set at 65mph and got 4.3m/kWh. Charged once back to 80something%.

Way back, decided to see what happened with cruise at 70mph. But we were only getting 3.5m/kWh which meant my maths was all wrong and we crawled into Wetherby services with 3% battery needing a second top up to get home…

Is the difference between 65 and 70 really that significant?
It was slightly cooler as we drove back in the evening and the journey down was heavier traffic…perhaps it was just the accumulation of all the factors.

Any thoughts from more experienced long distance drivers?
 
Well, speed is a "square law" function so just based on those two speeds (65 and 70mph) you'd expect the wind resistance to create about 15% more drag at the higher speed. If wind drag is the only factor (and obviously it isn't) then your range would be reduced by this amount at the higher speed relative to the lower one in a like-for-like comparison. Headwinds will also add (or subtract) to this, so a 10mph prevailing headwind whilst travelling at 70mph would be equivalent to travelling at 80mph. In this case your drag would increase by a massive 50% when travelling at 70mph into a 10mph headwind compared to travelling at 65mph with no headwind.

My view is that heavier traffic which you experienced on the way down would have reduced your average speed overall and that would have helped your range figure. EVs are very efficient in traffic or at slow speeds (as long as you're not making maximum use of heating/aircon etc).

So, in summary I would say the 20% or so reduction in range that you saw on your return journey is reasonably consistent with expectations.

I typically drive at around 40-60mph on local roads and consistently get 4.5-4.8miles/kWhr, which I have to say I'm very chuffed with :)
 
Heavier traffic also reduces drag because the vehicle in front will give you a bit of an aerodynamic tow. I like to play a little game on the motorway where I cruise at about 63mph until I'm overtaken by a van which I will then follow until they of I turn off. Then I'll slow again. You'll find efficiency at a constant 70mph with an open road in front of you is more like 3.2 miles per kWh. Most journeys don't actually allow that.
 
Petriix said:
Heavier traffic also reduces drag because the vehicle in front will give you a bit of an aerodynamic tow. I like to play a little game on the motorway where I cruise at about 63mph until I'm overtaken by a van which I will then follow until they of I turn off. Then I'll slow again. You'll find efficiency at a constant 70mph with an open road in front of you is more like 3.2 miles per kWh. Most journeys don't actually allow that.
Click to expand...
So, get yourself behind a 32-tonner doing 56mph and you'll probably end up with more charge than you started with by the time you leave the motorway.

An interesting question......If you follow a vehicle and it results in you gaining aerodynamic "efficiency", then the other vehicle must surely see an increase in fuel consumption to compensate (i.e. an increase in drag)? Otherwise where does the energy come from? If the van driver realised that he wouldn't be so happy to have someone tailgating him :)
 
"Is the difference between 65 and 70 really that significant?
It was slightly cooler as we drove back in the evening and the journey down was heavier traffic…perhaps it was just the accumulation of all the factors."

And there is also quite some difference in power use between 60 and 65mph.

But I favour 65mph, as it keeps you ahead of trucks. At 60mph, you barely overtake them...and cause problems with traffic behind..
 
Adee said:
So, get yourself behind a 32-tonner doing 56mph and you'll probably end up with more charge than you started with by the time you leave the motorway.

An interesting question......If you follow a vehicle and it results in you gaining aerodynamic "efficiency", then the other vehicle must surely see an increase in fuel consumption to compensate (i.e. an increase in drag)? Otherwise where does the energy come from? If the van driver realised that he wouldn't be so happy to have someone tailgating him :)
Click to expand...
I suspect that it's actually ever so slightly better to have someone following you because they will partly be helping to push along the air that you are sucking behind you. But it's basically zero.

The energy isn't gained, it just doesn't need to be spent by the following vehicle because the air is already moving as caused by the energy expended by the front vehicle.

Obviously the effect increases the closer you get, at the cost of diminishing safety. But I find that being 2s behind a big van still offers a decent improvement over being out on my own.
 
Petriix said:
Heavier traffic also reduces drag because the vehicle in front will give you a bit of an aerodynamic tow. I like to play a little game on the motorway where I cruise at about 63mph until I'm overtaken by a van which I will then follow until they of I turn off. Then I'll slow again. You'll find efficiency at a constant 70mph with an open road in front of you is more like 3.2 miles per kWh. Most journeys don't actually allow that.
Click to expand...
That’s really helpful, I hadn’t thought of the aerodynamic impact of heavier traffic too. Driving back on M11 and A1 was very quiet - apart from the near-constant squelching of all kinds of bugs on the windscreen 🤢

It’s a strange notion that I might be glad of heavier traffic though! I think I’ll just do my maths based on a lower efficiency and time my stop better than I did on this journey…!
 
mj224 said:
"Is the difference between 65 and 70 really that significant?
It was slightly cooler as we drove back in the evening and the journey down was heavier traffic…perhaps it was just the accumulation of all the factors."

And there is also quite some difference in power use between 60 and 65mph.

But I favour 65mph, as it keeps you ahead of trucks. At 60mph, you barely overtake them...and cause problems with traffic behind..
Click to expand...
Yes - driving at 65 was perfectly pleasant. You still overtake a surprising number of other cars and lorries aren’t a problem. It even felt much calmer compared to the previous 75-80 I’d have done in my old car… I’m not sure I’d enjoy 60 as much!
 
Adee said:
Well, speed is a "square law" function so just based on those two speeds (65 and 70mph) you'd expect the wind resistance to create about 15% more drag at the higher speed. If wind drag is the only factor (and obviously it isn't) then your range would be reduced by this amount at the higher speed relative to the lower one in a like-for-like comparison. Headwinds will also add (or subtract) to this, so a 10mph prevailing headwind whilst travelling at 70mph would be equivalent to travelling at 80mph. In this case your drag would increase by a massive 50% when travelling at 70mph into a 10mph headwind compared to travelling at 65mph with no headwind.

My view is that heavier traffic which you experienced on the way down would have reduced your average speed overall and that would have helped your range figure. EVs are very efficient in traffic or at slow speeds (as long as you're not making maximum use of heating/aircon etc).

So, in summary I would say the 20% or so reduction in range that you saw on your return journey is reasonably consistent with expectations.

I typically drive at around 40-60mph on local roads and consistently get 4.5-4.8miles/kWhr, which I have to say I'm very chuffed with :)
Click to expand...
That’s a great explanation - thanks! Now to see if I can find a tailwind and push for 5m/kWh+ 🤣
 
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