One of the difficulties in assessing change in vehicles over a procession of models is getting past the obfuscation that normally accompanies car publicity. Both in advertising and PR material, every model is always billed as being vastly better than the preceding model.

Of course, in many ways newer models or versions often are better – but in other ways sometimes they are not.

One example of this is the weight of cars: as we all know, cars of today are much heavier than the cars of yesterday. However that’s a process that has been largely unremarked upon as it has been occurring. The upshot is we only now say: “Hell! Look at how heavy new cars have become!”

But this insidious change occurs in other design aspects as well.

When I recently bought a 2-litre Peugeot 307 HDi and started looking at the fuel consumption figures, it occurred to me that over the life of that model, fuel consumption appeared to have gone backwards. Yes, it looks like it actually got worse in subsequent iterations.

In 2001, when the 307 HDi was locally released, Peugeot listed the fuel consumption at 4.3 litres/100 (highway) and 6.9 litres/100km (city). When we at AutoSpeed tested the car, we got a poorest figure of 6.2 litres/100 – and that was in a week of heavy Sydney traffic.

That car used a 2 litre, 8 valve, non-intercooled turbo diesel developing 66kW and 205Nm.

In 2005, the engine in 2 litre form sprouted a 16 valve head and intercooler. Power jumped to 100kW and torque to 320Nm.

But fuel consumption also rose to (on Peugeot’s figures) to 4.5 (highway) and 7.1 (city).

But it gets more complex. Not only did the later 307 have the different motor, in sedan form it also had a 6-speed transmission. However, the wagons stayed with a 5-speed trans. The wagon’s Peugeot-sourced figures are 4.7 and 7.3.

When we tested one of these cars, we got 6.4 litres/100km in a week of gently driven (mostly freeway) travel. That car also showed that over the previous 3000 kilometres, it had achieved an average of 7.5 litres/100km.

The change of engine certainly brought more performance. But fuel consumption worsened – comparing the 5-speed original 2 litre HDi with the later 6-speed 2 litre HDi, official fuel consumption rose by about 3.5 per cent. Comparing the original 5-speed 2 litre HDi hatch with the later 5-speed 2 litre HDi wagon, it rose by about 7 per cent.

There’s yet another variation available on the theme – mid model cycle, Peugeot released a 1.6 litre HDi model, and that car returned better fuel consumption figures (4.3 and 6.1) than the original… and still had improved performance.

It’s easy to be quite positive about all these figures. For example, the official fuel economy penalty of the later 2 litre HDi was only about 3.5 per cent and yet the power increase was over 50 per cent!

Wow – what’s wrong with that?

Nothing, except looking at our real-world road test data, I think it’s likely that the fuel consumption penalty of the later 2 litre diesel is more like 20 percent.

So the volume selling diesel 307 actually got worse in its fuel economy over the model life, with the 1.6 HDi effectively taking the place of the original 2 litre.

The worsening fuel consumption figures are difficult to dig out of the specs, mixed as they are with other changes. (Over the model life of the car, the kerb weight of the 2 litre HDi 307 went up by an amazing 178kg!)

But at the time no-one (including me) stood up and said: “Yes, there is more power. But here is a car that is going backwards in its fuel consumption, in a category where fuel economy is the very reason that people buy these cars.”

Just as no-one comments when new models are 10 or 20kg heavier, or a 3 or 5 centimetres bigger, or – yes – get worse in fuel consumption by a few percent here or there….

Footnote: The newly released 308 HDi 2-litre has Peugeot fuel figures of 4.5 and 7.2 litres/100km – a fraction poorer than the last 6-speed manual trans 307. In 6-speed auto trans form, the city fuel consumption figure of the 2-litre 308 diesel is staggeringly bad 9.4 litres/100km! However, the smaller engine 1.6 litre HDi 308 has excellent figures of 4.1 and 6.2 litres/100km. Less impressive is its CO2 output of 130 grams/kilometre – way back in 2001, the 307 HDi 2-litre was listed at 138….

*see here