Whats the optimum speed for maximum fuel efficiency? Put the calculator away. Don’t bother with those fancy torque/power graphs. Just follow a few basic rules of thumb.
Keep a constant speed
Newton was correct: maintaining a constant speed uses less fuel than constantly changing your speed. A constant 65mph is better than fluctuating between 63mph and 67mph. Cruise control (or the speed limiter function) can help you achieve this.
Going slower than the lorries means you have to slow down and then speed up again to re-establish your braking distance every time one overtakes and pulls back in front of you. Lorries are legally required to have a physical speed limiter device restricting then to 58mph. If you do 60mph, you are more likely to be able to keep a constant speed by ensuring the big wagons stay behind you.
On a three lane motorway, plan ahead so that you don’t find yourself having to slow down behind a lorry in lane 2 while waiting for a safe overtaking gap in lane 3. Maybe in these circumstances a steady 65mph may be better until the wagon-congestion had passed, when you could slow down again to the magic 60mph.
Time is money
A 100 mile journey at 60mph takes only 1 hour and 40 minutes. At 70mph the same journey is only 15 minutes quicker but uses significantly more fuel. Some figures from my own car:
I regularly make a journey from the Midlands to North Wales; a round trip of 240 miles on the M6, M56 and A55. Apart from a short section around the England/Wales border, the speed limit is 70mph all the way. If I maintain maximum speed, the journey uses about 20 litres of fuel: at 2018 prices that’s about £25.
The same journey at 60mph uses about 12 litres: a saving of about £10. The time difference is only 20 minutes each way.
To reduce your fuel bill, and enjoy the thrill of lowering your car emissions, aim for a speed which is low but where you can keep it constant. On a road shared with lorries, 60 is a good balance. Oh, and just leave 15 minutes earlier.