Cars are generally geared to run in 4th gear with low revs and thus minimum emissions at a minimum speed for 25 mph.In reality, most cars cannot be driven in full and safe control in 4th gear at less than 27mph.So running at 19 - 20 mph in 2nd or 3rd gear means higher pollution. That, on main A and B roads is potty. Lower gear and slower speeds on a small road, as long as momentum is generally maintainable, is more practicable.Lowest revs and most optimal emissions was around 46- 56mph. Newer engines and 6 speed gearboxes alter that.Many UK spec cars had speedometers that have National Speed limits highlighted in a bolder typeface then the rest of the speeds. So 30, 40, 50, 60 and 70 , being UK wide limits are generally those highlighted.20mph is, on older vehicles, often not clear and often so low on dials that they are not glanceable. Not clearly visible to a normal driving position without moving to read the low end of the scale. That compromises control and alert vision of a a moving vehicle in traffic. Conversely, 140mph at the other end of the scale is also not in the main line of sight and for most ordinary cars a rather optimistic speed - unachievable. It's just to impress and to look cool and sometimes to balance the design of the display. Which have been long designed for those optimum legal speed limits to be easiest to observe at a glance. If you have never noticed this when driving, then it will be no surprise, it's seldom pointed out except by Advanced Driving instructors or professional specialists.Then there are KPH speeds which are often marked and much smaller. Not really visible enough to reference whilst keeping ones eyes where they should be on the road and the mirrors.It is quite amazing how many experienced drivers have never noticed in their vehicles the Nat speed limits slightly highlighted in bold and its the case of 20 and 80 are rendered less noticeable.It varies from make and model. Some have thicker integers, some bolder characters etc.Most older speedos are not very accurate at under 25 as the optimum calibration is set again for what was the national minimum. Nor were they fully reliable. One reason why Traffic division Police cars have (or had) specially calibrated and large display speedometers that were designed to be highly accurate and very clear to read. Modern digital displays are much better in my view, but obviously cannot be retrofitted.With so much mayhem and utter madness of other road users, particularly with scooters overtaking and undertaking, often requiring speed changes to avert or avoid, all drivers have to virtually have eyes in the sides and the backs of their heads.It's imperative to keep ones eyes on the road, not on the speedo and near impossible to gauge 18 to 24 mph. and, it has to be also remembered that 90% of passed licence holders were instructed to gauge their speed at 28mph which was chosen because it is so much harder at lower speeds. How many who ride a bike can tell what speed that are riding at? I can't and for a good while I had a speedo on my bike until it got swiped. But I do know when I'm going too fast to be in total control and law of gravity soon tells me if I'm way too slow.
Raymond Havelock ● 157d