I forgot to mention that I can sell my current car for around £5000 (according to websites, it has 52000 miles).

You said you are looking for 'better economy', without saying if you mean fuel consumption or your overall domestic outlay. If the latter, the biggest change you could make would be to get £5K for your car (if you can) and buy something much cheaper - which clearly may not have the effect intended.

If you mean purely fuel consumption, you aren't likely to save very much with either a petrol car or a diesel, which will use fewer litres, but they cost 10% more than petrol. So as has been said, save by not spending on this occasion. If prices are 'crazy high', that applies when either buying or selling !!

Edited by Andrew-T on 27/04/2022 at 18:14

.... you might be doing 2000rpm in 5th going up a hill but your foot is hard down due to a the lack of torque. Changing down to 4th or 3rd would result in higher revs, but because you are not using so much throttle to maintain the same speed, your mpg will be higher.

Not so. Illustrated by the realtime mpg display in our Peugeot 2008. I have experimented with this in manual mode (the EAT6 in auto does not allow any engine 'labouring'). On change down, even though you might be lighter on the throttle, the mpg reading immediately worsens as the higher revs allow more power to get to the wheels and also to be wasted in engine heat and frictional losses. This obviously uses more fuel, even if the speed remains constant.

It is always best to keep the revs as low as possible without labouring the engine, although I suppose modern fuel and ignition systems prevent the damage that could be caused in the past by overfuelling and pre-ignition. But it's still a good reason to choose auto over manual when buying used as the engine will have been protected from such misuse.

It is always best to keep the revs as low as possible without labouring the engine, ...

John, I think you are describing petrol-engined cars. The effect you describe doesn't (always) happen with my Pug diesel. Climbing a gentle gradient at about 50mph in top (5th) I can change to 4th and use less fuel, probably because I am nearer the peak of the torque curve - which is typically around 2000rpm for a diesel or 3000 for a petrol.

Interesting. Is that demonstrated on a realtime mpg gauge or a theoretical argument? Assuming speed remains constant, the power needed is exactly the same whichever gear is used, but extra power will be needed to spin the engine at higher revs if in a lower gear. Your argument must rely on the assumption that the engine runs more efficiently at higher revs, i.e. power output per ml of fuel, whether petrol, diesel, or LPG, which I find hard to believe. If that were the case, cars would be much lower geared.

People often just want to change car but try to justify with calculation to make it look like rationale decision :-)

If I go for MG, it would cost me around £25 000 to buy, without to mention the interest because of the finance I would need. I would safe £1136 from petrol each year (if price stay 1.60). I would need to drive 27 years to recoup the £30 000 price of the MG. For these years the car will be 200 000 miles driven. And I'm not counting the cost for charging battery at home.

<< If I pedal up a hill in a high gear, the crank will be turning relatively slowly (rpm), but I am using a lot of energy (fuel) to maintain a given speed. If I change to a lower gear, the crank will be turning at a faster speed, but I am using less energy to maintain the same speed. >>

If you integrate your input of effort, is it better to do fewer turns requiring more effort, or the opposite - more turns with less effort ?

And cruising downhill with closed throttle should use zero fuel in a 'modern' diesel !

Edited by Andrew-T on 28/04/2022 at 12:46

Using a bicycle as a comparison, my thinking is this: If I pedal up a hill in a high gear, the crank will be turning relatively slowly (rpm), but I am using a lot of energy (fuel) to maintain a given speed. If I change to a lower gear, the crank will be turning at a faster speed, but I am using less energy to maintain the same speed.

I don't see why this would be any different in a car?

The difference in the two gears you notice is the difference in pedal effort (force).

You also need to consider the speed at which you rotate the pedals, 'work' is defined as force x distance, and power as work per unit of time.

So the lower gear uses less force but the pedals move further in the same time, the 'work' and 'horsepower' will be the same.

I thought that car engines usually give best thermal efficiency at full throttle, or is that not the case?

Using a bicycle as a comparison, my thinking is this: If I pedal up a hill in a high gear, the crank will be turning relatively slowly (rpm), but I am using a lot of energy (fuel) to maintain a given speed. If I change to a lower gear, the crank will be turning at a faster speed, but I am using less energy to maintain the same speed.

I think you are confusing force with energy. The same amount of energy is required to get the bicycle from the bottom to the top of the hill, no matter what gear it is in, and no matter how long it takes. The comparison is complicated by considerations of heat generated by muscular contraction and relaxation and the frictional losses at the joints, which I suppose might be similar to the considerations of what goes on in the joints of an ICE. The more mechanical oscillations there are, the more energy is needed to supply these heat and frictional losses.

<< I'm not suggesting the engine speed has no bearing, but neither do I believe that coming down hill, in gear (but off throttle) at 3000rpm will use the same fuel as going uphill in the same gear at 3000rpm. >>

There's another small point to consider when comparing petrol with diesel. Especially in a lower gear, a diesel engine can propel the car on a closed throttle without stalling, while the driver of a petrol engine will need to prevent that happening, so may use more throttle ?