What affects MPG
 

Not volvo specific, just in general, major faults aside, what sort of things significantly affect mpg?

I've just calculated my mpg at 37, when apparently it should be closer to 45. But I've only had the car a short time (since Christmas), and in that time it's been almost constantly sub zero round here. I also discovered that having filled it up when I bought it, I clearly forgot to put the fuel cap back on (previous car had the seal built into the flap, so force of habit and all) so all I have to go on is a few hundred miles in mostly sub zero temp, with no fuel cap on.

Performance seems fine, so I don't currently suspect any major faults.

Based on this, do we think 37 is OK when the official combined figure is 42 and browsing about tinterweb it seems most are getting around 45?

Goodness me, what an open-ended question!

The short answer is that just about anything and everything can have a bearing upon fuel consumption, 'rippedoff again'. Unless you operate in laboratory conditions, every factor must be considered a variable.

As you have observed, ambient temperature is a factor. So to is altitude; cars will burn more fuel in mountainous regions because the air is thinner.

Off the top of my head and in no particular order, the following factors will in my experience affect fuel consumption.

- Mechanical condition - are the brakes binding, for example?
- Tyre pressures - under inflation tends to use more fuel to overcome friction
- Electrical load - the engine has to work harder when more accessories are in use
- Load carried
- Number of passengers carried
- Roof rack disturbs air-flow and carries a fuel penalty
- Towing - especially a large heavy caravan (weight and aero-dynamics)
- Oil viscosity and age
- State of 'tune' of engine
- Journey length and number of cold starts
- Terrain
- Time of day - rush hour driving will use more fuel
- Etcetera, etcetera ...

But the biggest single factor is driving style. Rapid acceleration and heavy braking both use much more fuel than a more sedate style will. Good judgement when approaching traffic lights for example will pay dividends in reduced fuel consumption by minimising braking - which scrubs off speed by converting it into heat and which you have paid for by earlier expenditure of fuel. This is why longer journeys at constant (modest) speeds tend to be more fuel efficient.

As for your specific fuel consumption, only you can say if it is reasonable. Outside the laboratory, the same car driven over the same route under the same conditions by a different driver will almost certainly return a different figure for each.

Apologies for the lengthy answer - but you did ask!

Regards, John.

Yes, but it's a very short and unrepresentative sample period, and I don't really believe people's stated averages anyway, as few actually go to the trouble of calculating mpg, but use the readout instead.

A missing fuel cap will affect performance as the system cannot operate as designed. Replace the cap and then do a proper test over a longer period of time.

Wind direction.

Just one thing to add to john.wigley's excellent answer, and that's not necessarily to trust what the car's mpg meter tells you. My V40 and my 'new' Mondeo actually appear to be accurate to within about 0.5 mpg - but the one on my Avensis was useless: "40 mpg" was mid-30s in reality.

In each case, this was comparing the average value recorded for one tank-full against the actual miles between fill-ups and the amount of fuel to refill the tank.

OP said they calculated the value, but yes I agree with your point, as I posted earlier.

Two others to add.
Wet roads (I'm convinced) increase the rolling resistance of the tyres.
Also, tyres with a poor efficiency rating.

Thanks all for the useful info.

I kind of know I'm panicking prematurely, but I'm just worried in case I've bought another duff motor. I don't think I have. I know my test is far from accurate, because having discovered I'd lost the fuel cap, I figured it best to not fill right up, so I only put 30 quid in. I also can't say with any degree of accuracy how much fuel was in when I put the 30 quid in, or how much is left now. I know I've used APPROXIMATELY my 30 quid, because the fuel gauge is in about the same position now as it was before I put the 30 quid in, but I know realistically there could be quite a difference in actual fuel volume between the two readings.

I have a new fuel cap ready to go on (arrived yesterday). I'll get that sorted, fill it right up, and do a more proper test.

Ahem. That is all, OP!