Fuel Economy and published mpg figures

Ta me anseo

Hello all!

Quick question regarding the published mpg figures you see for new and used cars. They normally give Urban, Extra-Urban and combined cycle figures.

So, can anyone give me a definition of each cycle? Of course Urban is pottering around town, but what is meant by Extra-Urban? It is obviously the most economical cycle. In other words it is the cycle that maximises fuel economy. I know it's not motorway driving as that is too fuel inefficient. So what is it?

I remember years ago and old Toyota Camry we had, had a speed figure published in the manual for the best fuel economy. It was something like 55mph. Would that be similar to Extra-Urban?

The whole question rises because: I had a 1.4 petrol saloon that had a published combined figure of about 40mpg. I always got about 41.5 from it. Now I'm in a 2 litre diesel with figures of 40 Urban, 60 Extra-Urban and 50 combined. However I am struggling to get better than 42mpg.

I know fuel economy has lots to do with driving style and environment, but I drive gently and carefully. About 35% motorway driving, 45% at 30 to 50 mph and the rest is pottering. I never accelerate hard and generally try to use my brakes as little as possible. The theory being that brake use is simply fuel out the window. So I take my foot off the accelerator good and early and let the car naturally slow down for an upcoming junction, etc, etc. So why are my figures way off? Hmmmmm.....

Ta me anseo

Just found this online:

Official fuel economy tests (one of which covers the whole of Europe) do not actually measure the amount of fuel used but instead compare the carbon content of the exhaust with the carbon content of the air that entered the engine - exactly the same procedure which also determines a car's CO2 rating. European test results come in three parts - one for a notional urban cycle representing city driving, another for an extra-urban cycle representing open-road driving, and a third which is a combination of the first two.

The tests are so stringent that the results achieved by two different cars give a reasonable idea of their comparitive fuel economy, but it can't be assumed that any of the figures will be exactly matched in the real world.

May be the answer to my question! If the figures are only notional then I guess that's why I'm way off. Using CO2 figures to determine diesel fuel economy is pointless as CO2 is only a fraction of the emissions when compared to petrol.

So, how does anyone elses diesel fuel economy compare to the published values?

Anan1

Ta me anseo wrote: »

Using CO2 figures to determine diesel fuel economy is pointless as CO2 is only a fraction of the emissions when compared to petrol.

I would have thought that CO2 figures would be directly related to the quantity of fuel burnt? Aren't diesel CO2 emmissions lower because diesels tend to burn less fuel?

Ta me anseo

Reading a little more, it would appear that you are right. The amount of CO2 generated seems to pretty constant for each litre of petrol or diesel. So the CO2 figures should be fairly accurate when used to calculate the fuel economy of an engine. So, why can't I get anything near 50mpg!??

Anan1

Is your car brand-new? Or automatic?

mike65

Are these fuel figures not based on bench-test environment?

Mike.

E92

Diesels produce 13% more CO2 than petrols for the same quantity of fuel burned.

So a petrol doing 50 mpg pollutes as much CO2 as a diesel doing 56.5 mpg.

Ta me anseo

Car is 3 years old and manual.

I suppose if I am spending a fair bit of time on the motorway, I will lose out. I remember something like for every 5mph over 60mph, fuel consumption goes up by 5% or something like that. Might try the back roads for a tank full for the fun and see what happens! I know I can keep a steady 50ish there. Although the distance to work is further via the back roads so it kind of defeats the purpose!

Incidentally, anyone know how much A/C would normally reduce your fuel economy by?

mike65

They say 5% typically, having your windows down is about the same.

Mike.

JohnBoy

the car is basically put on a rolling road which has a standardised set of loads on it to simulate driving up hills and the like, there's stops, starts, slowing down, speeding up etc. its not nessecarily a true reflection of a cars economy but its a standardised test to compare cars fuel economy relative to each other

Ta me anseo

All the same, I can't figure out why my previous car gave me just about bang on combined cycle fuel economy. Within 1 or 2 mpg of the published figure. Now, with a different car, doing the same driving, I am nowhere near it??? How come?