Indicated vs Actual MPG (depressing, I know!)

[Rich n Em]

Now I understand that instruments like the speedometer under-read by a small percentage - and that indicated values are not always 100% accurate but by what margin for error shoud I expect, roughly, for MPG?

Car in question is a Merc C270cdi - 100k on clock, mechanically sound (AFAIK).

Indicated - over 450 odd miles, was 52mpg
Actual, calculated as accurately as I can from several fill-ups, was 42mpg

Unfortunatly with wife not working currently (maternity) and rising pump prices, cost is king and the difference between 40 and 50 mpg over 15000 miles a year (about £500), is enough to consider a more economical car (I'm saving for a weekend toy!).

So, is a 20% difference between indicated vs actual a cause for concern? Or should I just carry on with my penny-pinching misery?

Pistonheads - sadly, MPG Matters

[Synchromesh]

If I got an average of 42 mpg I'd jump for joy.

[Nash_wrx]

Synchromesh said:

If was got an average of 42 mpg I'd jump for joy.

^^ That

I get low twenties if im lucky

[DanGPR]

How exactly have you been calculating your MPG?

I would be trusting that (depending on your calculations!) over the usually optomistic OBC.

[Rich n Em]

I know, but it's the daily hack, bought cheap only to slog up and down the motorway everyday as cheaply as possible...

[Synchromesh]

Rich n Em said:

I know, but it's the daily hack, bought cheap only to slog up and down the motorway everyday as cheaply as possible...

Changing cars is deceptively expensive and you never know what you're buying. The bes thing to do would be to go 60 instead of 80 on the motorway if you spend a lot of time there.

[Rich n Em]

DanGPR said:

How exactly have you been calculating your MPG?

I would be trusting that (depending on your calculations!) over the usually optomistic OBC.

Well, last fill up did 419 miles on a full tank and I put in 44 litres - which is what, 44 mpg (by my calc) - indicated showed 53.7

[STW2010]

When calculating yourself you have been using UK gallons rather than US gallons haven't you?

In my BMW I get an indicated mpg of 31, and a calculated (when I bother) of about 28. So not massively inaccurate.

Oh, and as above, if I got 40-odd mpg from my car I would be delighted. I have never managed this in any car I've owned.

[Super Slo Mo]

In my experience, each car varies bit a different amount.

For instance, going by the last 3 cars, the Polo claimed an average 52.xx mpg, actual was 51, the Passat claimed 49, actual 47, and the Volvo claims 52, actual 40 (if I'm lucky!).

However, I suspect there's something wrong with the Volvo, as most other people reckon they're about 5 mpg optimistic.

Your car sounds a bit further out than you should expect, it may need a checkup or something.

[Rawwr]

Did you drive until you ran out of fuel?

[LukeBird]

Synchromesh said:

Rich n Em said:

I know, but it's the daily hack, bought cheap only to slog up and down the motorway everyday as cheaply as possible...

Changing cars is deceptively expensive and you never know what you're buying. The bes thing to do would be to go 60 instead of 80 on the motorway if you spend a lot of time there.

This.
Careful driving is king for saving money. Also try with and without cruise (I assume it has it!) as you may find a difference.

[RenesisEvo]

I've found my Focus diesel to be about 1 mpg optimistic with the computer compared to actual, which given the potential errors in my calculations, I figured wasn't too bad at all. Some cars (the Focus for example) have an adjustable 'offset' which you can use to alter the value to get a more accurate readout on the computer.

[Rich n Em]

Synchromesh said:

Changing cars is deceptively expensive and you never know what you're buying. The bes thing to do would be to go 60 instead of 80 on the motorway if you spend a lot of time there.

I've already cut back on the loud pedal at stick to ~65 mph (which makes my petrolhead skin crawl) - and the Merc needs some maintainence that likely won't be cheap, plus the dreaded tin worm is attacking it. Plus tax is up etc.

Essentially it needs some money throwing at it, so I was weighing up a change

[Jayho]

Once you take into the consideration of the depreciation you'll recieve from old car you sell and the new car you buy and all other costs, its gunna probably be more than the "£500" extra you're spending on fuel for the year tbh. + The stress of finding the new car, the worries in the first couple of weeks hoping its not a lemon ect... Really not worth it IMO.

O/T, I get confused with the use of Litres at the pumps and Gallons for "calculations". I'm a bit young so please correct me if I'm wrong, 1 Gallon = 4L?

So my 30L tank = 7.5 Gallons?

280-300 Miles out of my tank = roughly 38MPG?

[STW2010]

Rawwr said:

Did you drive until you ran out of fuel?

You don't need to. Fill the car to the brim and reset the odometer, then when you next fill up (to the brim) read off the miles covered to the volume of fuel.

[Trommel]

Rawwr said:

Did you drive until you ran out of fuel?

I've done that twice in the last week.

Still haven't bettered 14 mpg.

[Fish981]

Rawwr said:

Did you drive until you ran out of fuel?

Why?

[STW2010]

Jayho said:

Once you take into the consideration of the depreciation you'll recieve from old car you sell and the new car you buy and all other costs, its gunna probably be more than the "£500" extra you're spending on fuel for the year tbh. + The stress of finding the new car, the worries in the first couple of weeks hoping its not a lemon ect... Really not worth it IMO.

O/T, I get confused with the use of Litres at the pumps and Gallons for "calculations". I'm a bit young so please correct me if I'm wrong, 1 Gallon = 4L?

So my 30L tank = 7.5 Gallons?

280-300 Miles out of my tank = roughly 38MPG?

Nearly. 1 gallon = 4.45 litres

[Rich n Em]

STW2010 said:

You don't need to. Fill the car to the brim and reset the odometer, then when you next fill up (to the brim) read off the miles covered to the volume of fuel.

This is exactly what I did, over 3-4 fills

[kambites]

Odometers are only guaranteed to be within about 10% accuracy. Most over-read by something like 5%.