Background

In Wales, in September 2023, the urban national speed limit is being reduced from 30mph to 20mph. This move has divided opinion somewhat.
One of the enduring, and incredibly popular myths circulating on social media, is that having to drive your car at higher revs in a lower gear will produce more pollution than driving it at lower revs in a higher gear.

Discussion

This page is here to give me a reliable link to, hopefully, a clear explanation.

Throttle position determines power (and therefore fuel used). RPM is determined by load.
I think the simplest way to picture it is using an aircraft as an example. If you’ve ever had a chance to fly a typical propeller driven aircraft, you’ll have noticed a lever marked throttle and one often marked RPM. The throttle provided the power and the RPM level controls the propeller pitch. With the throttle fixed at one setting the prop can be altered from coarse to fine.
When coarse the prop will take a bigger ‘bite’ from the air, loading the engine more and the rpm will drop. When fine, the prop will take less of a bite and the rpm will rise.
The beauty of this analogy is the fact that the throttle is fixed and unlike a car doesn’t return to closed when you take your hand off it. Changing the propeller pitch makes the RPM vary but the throttle, and therefore the fuel consumption and relative emissions stay the same.

Hence the RPM will vary with load on a constant throttle.

Transferring the example to a motor vehicle, If a car has exactly the correct accelerator pedal position drive at 20mph it can do it at 1500rpm in one gear or 1800rpm in a different gear.
it’s the pedal position that dictates the fuel consumed not the rpm of the engine.

If that doesn’t help think of a car approaching a hill.
Let’s say that our car requires 1500rpm in third gear to maintain 30mph.
That’s just a mechanical constant.

So, when our car, driving at 30mph in third, at 1500rpm with a constant throttle, comes to a hill it is only going to do one thing – slow down

If the throttle is not altered the car will slow (possibly even too much to continue in 3rd gear) and the rpm will reduce.

If the throttle is increased the car will stay at 30mph at 1500rpm but now the fuel consumption will increase.

Said like that, it’s a bit obvious, isn’t it?

Here’s my van speed in gears.

Notice the nasty first gear (it is a commercial) and how overdriven it is.