MPG half of that advertised! How to get out of car agreement?

almillar

OP - We've established that the seller was quoting the correct combined MPG from the manufacturer for the right engine in the right car. He's kindof done his job.
Alfa Romeo do the same tests as all the other manufacturers, as you've been advised above, under strict conditions indoors, involving 'driving' 2 routes, one emulating city driving, the other country driving, and the third one is these two combined.
All cars do the same test, and you can compare car vs car this way.
Problem is, it doesn't necessarily reflect people's driving, and over the years, manufacturers have been able to adapt their cars to make those figures better.
What you need to do when buying a new car is work out your CURRENT MPG in your CURRENT car, and see how it compares to what the manufacturer says. You should be in around the same place in your new car.
So, what was your old car, year, engine?

mb101_2

DominicH wrote: »

So are you saying that if you take a small car on a long motorway drive, it would be more efficient if it were carrying a greater load? Say, some bricks in the boot, or extra passengers?

Haha, I noticed that was nonsense, but it is somewhat O/T...

Technically you might see an infinitesimal benefit in aerodynamic efficiency from the car riding lower on its suspension (!), which would obviously be more than offset by the increased fuel required to repeatedly accelerate the greater mass throughout the journey.

It is of course true that a smaller engined car won't use any more fuel carrying a greater mass at a constant velocity.

The difference actually comes about because a smaller engined car may use more fuel than a larger engined car to accelerate the same mass, or to overcome the same constant air and rolling resistance at constant velocity.

kaya

It's the manufacturers who claim that MPG , car supermarket are just passing the info on. Look at the new Mitsubishi Outlander ad on TV and read the small print next to the claimed MPG , it's something like "advertised mpg is for comparison to other manufacturers vehicles and does not nessecarily represent real world driving" then consider the new BMW i8 that has a claimed 100 and something MPG consumption yet in real world tests on top gear and with other drivers who piped up on Twitter after the show was aired managed a measley 31mpg, it's all about marketing and selling a product to you and the manufacturers have stupidly highly paid legal departments that ensure they can say these things without fear of retribution. Even the yoghurt crunching sandal weavers favourite the Prius gave really crappy MPG as soon as they changed the test criteria a few years after launch, aside from testing the car is not faulty you haven't got a leg to stand on I'm afraid, caveat emptor

Strider590

DominicH wrote: »

So are you saying that if you take a small car on a long motorway drive, it would be more efficient if it were carrying a greater load? Say, some bricks in the boot, or extra passengers?

No, because it's got a smaller engine................. The point being it has to work much harder to keep up, any gains in momentum from the additional weight, would most likely be cancelled out by the fact the vehicle would struggle to reach normal speeds.

poppasmurf_bewdley

I was always taught that the way to achieve the best MPG possible was to drive as though you had a glass of water stuck on your bonnet and you don't want to spill a single drop.

Try it - it does work!

Tobster86

poppasmurf_bewdley wrote: »

I was always taught that the way to achieve the best MPG possible was to drive as though you had a glass of water stuck on your bonnet and you don't want to spill a single drop.

Try it - it does work!

That is highly subjective to the type of car, and I know for a fact it's not true for a V8 with a torque converter auto box which is much happier to sprint up to speed in a short time and then cruise, and will drink like !!!!!!y otherwise.

The worst thing for fuel economy is having to brake. You had to burn fuel to gain momentum that you're now having to turn into sound and heat.

maninthestreet

Strider590 wrote: »

Well it's quite simple really, a town runabout has the aerodynamics of a house brick and a small engine. If you put that next to a big heavy motorway mile muncher and then add a headwind, the small car is going to have to work much harder to keep up.

The drag coefficient of the Mito is 0.29 - hardly a house brick...

maninthestreet

Strider590 wrote: »

No, because it's got a smaller engine................. The point being it has to work much harder to keep up, any gains in momentum from the additional weight, would most likely be cancelled out by the fact the vehicle would struggle to reach normal speeds.

Strange - VAG have cylinder-on-demand engines that switch from 4 cylinder to 2 cylinder operation at low and medium loads, resulting in significantly improved fuel consumption. When travelling at constant velocity, the engine of a car only has to produce enough force to overcome friction and air resistance.

Johnmcl7

Tobster86 wrote: »

That is highly subjective to the type of car, and I know for a fact it's not true for a V8 with a torque converter auto box which is much happier to sprint up to speed in a short time and then cruise, and will drink like !!!!!!y otherwise.

The worst thing for fuel economy is having to brake. You had to burn fuel to gain momentum that you're now having to turn into sound and heat.

I find it more than a little unlikely that sprinting the car up to speed is more fuel efficient than doing so gradually, I've never seen a car that's efficient under heavy acceleration.

John

Tobster86

Johnmcl7 wrote: »

I find it more than a little unlikely that sprinting the car up to speed is more fuel efficient than doing so gradually, I've never seen a car that's efficient under heavy acceleration.

John

I wasn't suggesting spinning-the-wheels boy racerishness; just a good 40% or so of the throttle with gear changes happening at just over 3k RPM. By the vehicle's own mechanisms of measurement this uses significantly less fuel than gentle acceleration in the sub 2k range.