How can 25mph be the most efficient speed? You aren't travelling fast enough to get anywhere!! 56mph is to the best of my knowledge the most economical speed for a road going vehicle. You only need a very modest amount of bhp to propel a car at that speed. Any slower and you are not covering enough distance to give good economy. You have only 2 things to overcome on a level road, wind resistance and rolling resistance. So It would stand to reason that these laws of physics would apply to all road vehicles. Try it anyway (I suggest 60mph as 56mph results in irate HGV drivers!!)
60mph on motorways for 200 miles (occasional queue) got me 66mpg the other day - considerably higher than normal. It's far harder to test out slower speeds though, as on any roads where you're not stopping and starting, you'd be going faster than 25mph.
Anyway, I'm not saying it's the most efficient speed - because, as mentioned below (and indirectly in my last post), that would be labouring the engine/needing silly throttle to maintain speed on inclines.
no, not at all. Labouring the engine will make it a lot worse on mpg.
But at what point are you no longer labouring the engine? Is it (as I presume) the
point at which the engine produces sufficient torque that it can maintain speed up inclines without needing much more gas
?
The easiest way to explain (without sounding condescending.. I hope) would be to imagine what Miles per Gallon means. To get a high mpg figure, you'd have to cover the most distance using a gallon of fuel. Simple enough.
However, if you're poodling along at 25mph, you might not be using much fuel, but you're not going anywhere either.
If you travelled at 50mph, the increase in the fuel consumption in 99.9% of cases would be a lot less than the increase in speed (distance covered using that fuel).
How do you know that though? I'm not saying you're wrong, just that what is there to say that you use less than twice as much fuel going at 50 rather than 25?
I think the efficiency of the engine at certain speeds may be a factor? Not sure. It's a balance thing anyway, there's a point when more fuel is used up than the proportionate increase in speed due to air resistance etc. (if that doesn't make any sense, I'm tired!) A lot of car manufacturers seem to give the guideline of 56mph, maybe they try gearing the cars for this?
They may give that guideline because people might drive along the motorway at silly low speeds aiming for low fuel consumption? I note that 56mph is the speed limiter on most HGVs (albeit they have more accurate speedos, showing as c.60mph on most car speedos).
Remember of course there's not only air resistance and rolling resistance to overcome but internal resistance related to the engine rpm, and all those resistances are lower at slower speeds (assuming same gear). There is indeed a point when there's more fuel used up than the proportionate increase in speed, it's just hard to measure - and I suspect it changes depending on the gradient being driven up.
Perhaps someone with a real-time display of mpg could test this out? (i.e. when driving in a 30, go to 5th gear, adjust to maintain constant speed, give it 30 seconds, note the reading; do the same at 40, 50, 60)
Not that I've got a great deal of faith in them, the one in my parent's car seems to record about 10mpg higher than they're actually getting
but they should be alright for relative comparisons.