Car mpg question

You've probably explained why. Petrol cars need a much richer mixture when starting from cold and during warm up. Which makes them very much worse than diesel for short trips. The difference when both are fully hot becomes much less.

for most drivers fuel costs are one of the lower expenses over the course of ownership. The only way to compare is on paper, write down every known expense each vehicle will incur over its life, include finance costs or loss of interest on savings, be realistic about servicing and depreciation. tax and fuel are minor compared with the rest.

Do the calculations and you will truly realise how much your choices will cost.

What I didn't say was that the Corsa, even when used for several consecutive trips of over 200 miles, in summer weather, never showed more than a couple of mpg improvement.

It's also true that even in winter the Corsa warms up in about 5 miles, whereas the diesels take nearer 20 miles, so I would not count trips of

I think you will find that diesel in the UK, with 5% "bio" products, contains less energy per unit volume than petrol.

I think you'll find that it contains more, bio additive or not. It may be close per unit weight but by volume diesel still wins.

Tim

Think you missed the point. A diesel doesn't need the same amount of extra fuel when running below normal temperature than a petrol.

But I've no idea whether your Corsa is typical of all small petrol engines.

Changing a 3 year old low miles car means that overall runnings costs don't matter much.

I agree, but the difference in total costs of the 13 year old corsa to anything new will be enormous. Unless there is a good reason to change stick with what you have is generally a good idea.

Ah - sorry. Was 10 years out with the age of the Corsa. Blame it on mine. ;-)

I've seen estimates that for engines of similar power the diesel is about 15% more efficient than the petrol - this is over the whole driving cycle so presumably includes the extra inefficiency of warming up the petrol engine. But this ought to mean that by comparison with the 40mpg Corsa my diesels might have been expected to achieve 46mpg - not the 55 mpg I saw.

However, the Corsa is a very old design of engine. It has a carburettor!

By contrast the diesels have fuel injection with computer controlled metering - which must add to the initial purchase price. But does the intelligent metering make that much difference?

And would a more modern design of petrol engine with fuel injection be a significant improvement by contrast with a carburettor engine?

Are there any other design improvements in modern petrol engines that make them more competitive with diesels (for fuel efficiency)?

We've done that already - for several years. But repaired items are going wrong again for the second time!

For example the bulkhead between the cabin and the engine - this leaked rainwater so the carpets were always wet. Some years ago the local dealer dismantled the whole area, dried the carpets and re-sealed the bulkhead - for a cost at the time that was probably more than the car's secondhand value. It's now leaking again. I think it's actually a design fault - in the past I've had much older cars which didn't leak there.

Plus - driver's door window mechanism - it's loose and rattles horribly when the door is shut. But it does still work, so the cost and aggro of dismantling it isn't justified.

So given that the Mrs has some money available, a new car seems like a nice idea.

Is it likely that she can get a similar sized petrol-engined model that for her driving will achieve nearer 55 mpg?

Any car built this century with a carb was done so down to a price. After all, EFI had been around for about 20 years by then. You could certainly expect a very worthwhile improvement in efficiency simply by changing the same basic engine to EFI. A modern engine will also have the ignition controlled by the ECU which gives even more advantages.

A 2004 car with a carb? Really? I thought they'd all gone in the mid 90s.

I stand corrected, I am surprised that the density difference is so great

"As of 2010, the density of petroleum diesel is about 0.832 kg/L (6.943 lb/US gal), about 11.6% more than ethanol-free petrol (gasoline), which has a density of about 0.745 kg/L (6.217 lb/US gal)."

No, it doesn't.

yaris hybrid would be ideal.

My car is 3 years old next month so I might px it for a new one It is a diesel and although it runs fine and returns 50ish MPG I think I'd rather go back to petrol. I looked at the oil in it the other week and was disgusted how filthy it was after just 5,000 miles. Diesel cars are not as clean as claimed.

Tom G laid this down on his screen :

The oil in a diesel will be black after just 50 miles, that is normal.

Plus points for a diesel...

They are much more fuel efficient, better mpg. They have more slogging power at low revs/ less gear changes needed. More power than a comparable petrol. They last longer than petrol engines.

Negatives.. They take a long time to reach operating temperature, some might never properly reach it in winter without help. So not suited to short journeys. More expensive to buy.