What affects MPG

[NigelDay]

Brim-to-brim, my 'trip computer' is about 5% optimistic. The range is about 1% to 9%, but I put this down to some journeys being long and steady speed, and others being the opposite.

[john.wigley]

Originally Posted by stephend

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.

Thank you, 'stephend'. Until recently, I didn't have a trip computer on my Volvo. The 745 wasn't so fitted, yet I can confidently say that it returned 27.3148 MPG over 62,206 miles. The computer on my V70 indicates an average of 29.6 MPG, compared with my actual figure of 29.6935 over 5182 miles, which I feel is accurate enough.

I find that a real benefit of tracking fuel consumption over an extended period is that one can spot variances which may give an early indication of a potential problem - I once identified a leak on an incorrectly fitted (not by me) fuel filter on my 944 that way!

Kind regards, John.

[ITSv40]

[QUOTEI 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.[/QUOTE]

That has got to be the most un-scientific way imaginable of coming to the conclusion that: my car does 37mpg.

[rippedoffagain]

Originally Posted by ITSv40

That has got to be the most un-scientific way imaginable of coming to the conclusion that: my car does 37mpg.

That's true.

But I didn't say I'd reached the conclusion that my car does 37mpg.

I said I'd calculated that over a few days, it had averaged 37mpg. I believe I also acknowledged the substantial imperfections in my test.

But yes I agree.

But moving on, still early days, but I reset the computer thingy today after filling up completely. I also recorded how many litres of fuel I've put in.

This is totally not scientific, but driving around town a fair bit, the computer thingy reports and average of just under 35mpg. The official 'urban' figure for this car is only 33mpg, so no cause for alarm there yet. Most of my miles over a week are on fast roads, so I'd expect to get somewhere near the extra urban figure there. We'll see next week.

I have however concluded that something is not right. There is an almost imperceptible 'lumpiness' on idle and on slight throttle, and an occasional, slight brief hesitation, only sometimes, if I give the throttle some welly. Apart from that performance so far has been very good for the spec. The car has a full service history otherwise I'd suspect spark plugs are about done, but they should have been changed I'd assume. I'm not too worried yet though. I'll chuck a bottle of forte fuel treatment through next week, and if that doesn't sort it then I'll maybe buy it some new plugs.

[Dibble]

Three things I would add to what John Wigley has to say is gear changes, braking and cruise control. Every gear change you make unnecessarily uses excess fuel. Think about what gear you require and block change to get to it. Try not to brake if you can help it by using your acceleration sense. Don't dab on the brakes.
Some on here might disagree but use your cruise control on every suitable occasion. You don't have to wait until you are on a motorway to use it. You may already use it but if not, you will quickly get used to it.
And no rapid acceleration!

[big square car]

You will only get an accurate result if you fill it brim to brim, ideally at the same pump each time (due to slope of ground)

[Dippydog]

As john.wigley has said I too have the trip computer set to show average mpg not because it worries me how far I'm getting to the gallon or how accurate the readout actually is but if it starts showing "abnormal"variances then it's an indicator of problems.
As an aside I had the battery off my V70[2.4 petrol manual]for a charge up a couple of weeks ago and the computer now shows 20.5mpg on my commute which is 2.7mls each way.

[rippedoffagain]

I've just trawled through the service record. I'm now part reassured and part annoyed. The last time I can see any evidence of a spark plugs change was in 2010 when the car was 3 years old. The 'full service history' looks like it's been little more than an annual oil change.

The annoying bit is that most of the stamps are main dealer stamps, stamped in the very same book that lists the things they're supposed to check/replace. I reckon the previous owner has paid several hundred quid per year for a fivers worth of oil.

[Simmy]

Originally Posted by rippedoffagain

I've just trawled through the service record. I'm now part reassured and part annoyed. The last time I can see any evidence of a spark plugs change was in 2010 when the car was 3 years old. The 'full service history' looks like it's been little more than an annual oil change.

The annoying bit is that most of the stamps are main dealer stamps, stamped in the very same book that lists the things they're supposed to check/replace. I reckon the previous owner has paid several hundred quid per year for a fivers worth of oil.

its a common thing with main dealers . all makes . maximum charge for minimum work . modern spark plugs with precious metal tips can be good for 40k plus though . find a good independent garage but still check work has been done always ask to see any replaced parts.

[canis]

Originally Posted by NigelDay

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

Actually, yes, I've noticed this too. Where the tyres wear their tracks into the roads, the surface is often very smooth. Great in the dry, less resistance. But in the wet, providing there's room and nobody else around, I try to stay out of the worn tracks and on the rougher stuff because it keeps me up out of the water. It stands to reason that displacing water must sap kinetic energy. And better grip too.