Based on my experience, I suspect that the answer is: it depends on the car, the engine and the gearing, but in a lot of vehicles, 4th gear is going to be more economical than 3rd at 30 mph.

Basically .... it depends. But allowing an engine to struggle at too low revs is not economic.

Third gear advice in 30 limits has been around for some time. I think it is more about being in control than engine load. The tendency when in the wrong gear with revs on the low side might be to slowly speed up. When in a gear with in the 'right' rev range, control is better and speed control also more stable.

My thoughts anyway!

FTF

Just tried it in the Beemer and it's about 10mpg worse in 3rd than 4th. Of course, having an auto box means I can leave the car to decide what gear it wants to be in.

@Daveyjp

I think that a vehicle at any given road speed in the highest gear possible that the engine can maintain without labouring will result in the lowest possible fuel consumption. It is a well established fact that lower revs=lower fuel consumption.

Of course, at 30mph 4th and 5th or 6th gear can be so tall the engine labours and can compromise vehicle control because you have less acceleration and engine braking available.

I would probably be in 4th at 30 most of the time in my car, if it was a very long clear 30mph limit maybe 5th. Although the engine is happy in 5th at 30mph in my car, I don't like the feeling of reduced control and it feels only just above the point where it would begin labouring. Down a busy street or a residential road etc I would use 3rd. Although I don't tend to fly down a narrow residential street at the full 30mph anyway.

I don't think cycling is an accurate analogy-for your example 'gear 10 at too low speed' requires more torque than you can comfortably or outright produce and you will tire very quickly. A car's engine produces a constant and known amount of torque at any given RPM that basically doesn't vary and doesn't fatigue, and the gearing is matched to this. Your analogy if anything is closer to trying to drive at 10mph in 5th or up a hill with engine RPMS at idle speed or something, then the car will judder and slow and then the engine is getting 'tired very quickly'.

As long as the engine is not being laboured then the taller the gear and the lower the RPMs the less fuel is being used-ignoring issues of control over your speed which may dictate using a lower gear.

Can anyone disprove me?

Cheers.

There is another factor. One of the reasons for the failure of dual mass flywheels is where they are asked to deal with too much vibration. One of the main causes of this is using too few revs & the engine being forced to labour.

If you are running a diesel you will protect you dmf by using a gear that keep the engine in a rev range where it is working efficiently. For most diesels this is not below 1500rpm & for many smaller ones not much below 2000rpm.

How bored were you to find this thread?!

............And THAT is supposed to make it more interesting?!?!?!

The reason I decided to ressurect the thread pertains to the current discussions regarding the Hyundai Rio 1.1CRDi and the increasing discrepency between official fuel consumption figures and 'real world' observed economy.

We can reasonably level criticism at the NEDC, but the biggest variable comes down to driving style. In order to get the engine into its efficient region, you have to use low speed and high torque. The simple fact is that most folks do not to be in this area. You have little engine braking, not much available acceleration, and rather unpleasant vibrations. Unfortunately, this is what it takes to hit the biggest mpg numbers.

The manufacturers have done their bit-the rest is down to the driver.

Virtually always less. There is concern about 'labouring' damaging the engine, but it is more economical. if you want an unbiased view, google "BSFC maps" under 'images'. you'll get a lot of graphs with contours. The numbers represent the amount of work done per unit of fuel-lower is better. You find that virtually all engines reach peak eficiency by 1500-2000 RPM and about 80% load. Note how the numbers rise rapidly towards the bottom of the graph.

The snag with 30-40mph is that you only need about 10-15hp, and that means very low revs to get into the efficient area.

I use my cruise control to control my speed in certain speed limit areas but doesn't work too well if there is a steep-ish decline as the speed is not held.

Higher than I'd have expected - at a steady 30 it gets up to 5 (out of 6). It doesn't cause a problem as it kicks down into 4 or 3 if you accelerate (to go up a hill or when the limit ends).

The gearing may be different from the manual - and my last two cars have been diesel manuals - but with the manual I found that the optimum gear to be in corresponded with 10s of mph - 10 in 1st, 20 in 2nd and so on up to 60 in 6th.

My driving instructor used to encourage me to change up to 5th in a 30mph limit if the stretch ahead was long and clear.