C180 petrol courtesy car just 27mpg?

[st4]

I owned a "59" C180K sport estate auto and averaged 29mpg with a reasonable cross section of driving.

It had very little mid range shove, our C220 cdi is a vast improvement in economy and performance. *** will confirm it is the finest car money can buy

Because it is, but yours is not as fine as mine ad it's a 7g not a 5g and it's in an incorrect paint scheme of red

Back on topic my obc over reads as I'm averaging 38mpg on a tank, not 40mpg. Nothing serious but something to be aware of.

 

[Valkirk]

Bought a C250 petrol and wasn't impressed with the economy, comparing it to previous car (5.0L V8)

C250
Cold start 5 mile run: 20-21 mpg (Just)
Motoroway run: 33-35

ISF
Cold start 5 mile run: 18-20 mpg
Motorway run: 28-30

It does seem to be getting better as time goes on but I really was hoping for more. Think the next car may well move over to the dark side

 

[gIzzE]

My last tankful was 43mpg but calculations show that I only got 40.66, 5% out is fine.

My speedo however is very close versus gps, 73mph on the speedo is 70mph on the gps, many cars I have had have been showing over 76mph on the speedo and 70mph on gps.

So, does that mean that the odometer is 10% over?
A car I sell with 100k on it has only really done 90k miles?

Or is the odometer correct it is just the speedo that is set over?


How does this effect MPG?
What is the relation to calculated mpg vs speedo over read?

Do those that are getting optimistic mpg readings by 10% also have a speedo that over reads by 10%?

 

[gIzzE]

Every petrol I have owned as averaged between 26-30mpg.

Whether it was a 1.8na, or a 3.0 twin turbo, or the tuned 3.2 S54 in the M3's.

The exception to this was the V8 Audi S4 avant, that averaged 18mpg.

 

[John]

Bought a C250 petrol and wasn't impressed with the economy, comparing it to previous car (5.0L V8)

C250
Cold start 5 mile run: 20-21 mpg (Just)
Motoroway run: 33-35

ISF
Cold start 5 mile run: 18-20 mpg
Motorway run: 28-30

It does seem to be getting better as time goes on but I really was hoping for more. Think the next car may well move over to the dark side

That is nuts for the C250 - you would be using bugger all of the power of the ISF at normal driving speeds as well.

 

[Stratman]

The mileage used in the MPG calculation will be the same for either method (abacus or OBC), so the amount of fuel used must be the figure in doubt.

I'd be more inclined to believe the car's ECU than the very variable garage pumps. They are subject to defined accuracy (-0.5% +1%), so what you read on the display is pretty close to the amount you've put in the car, but the cutout on pumps varies wildly. I filled up yesterday and spent a couple of minutes slowly feeding in the last few dregs and was sure I should have been able to get in more, but it still kept cutting out. Sure enough, the fuel gauge only showed a 7/8 full tank. The previous fillup spilled some as the pump didn't cut out until there was a gusher back up the filler tube.

If you think all fuel pumps are calibrated to cut out at exactly the same point, and are more accurate than the ECU in your car then I'm happy for you but I think you're wrong.

 

[tali]

Ken Gibson of The Sun (yes i know...) tested a shooting brake cls350 petrol........46 mpg he claims he achieved.

 

[V12]

Stratman, is that between different fuel stations or just between different pumps in one station?

I've never investigated that aspect before. Very interesting in a geeky way.

Cdi engine ecus have no idea how much fuel is actually being burnt and how much is being returned to the tank. I'm no expert but know for sure that the cdi ecu uses a sketchy estimation as to the mpg figure.

Sent from my GT-I9300 using MBClub UK

 

[Dieselman]

You can't. Every car over reads. Just not by as much as the obd does for mpg.

Sent from my GT-I9300 using MBClub UK

Not so. The speedometer is set to over-read, the mileometer is accurate...or thereabouts.


sent from my 1982 Casio LED watch.

 

[Dieselman]

I'd be more inclined to believe the car's ECU than the very variable garage pumps. They are subject to defined accuracy (-0.5% +1%), so what you read on the display is pretty close to the amount you've put in the car, but the cutout on pumps varies wildly.

Over a few fills the average will be very accurate...it can't be any other way.

It's not the nozzle cut-out that is incorrect as that relies on nothing more than a pitot tube down the nozzle, it's the way the fuel settles in the tank makes the variance.

Sorry, If you think I'm arguing because I'm stating facts.

 

[ming]

Why is it that you say speedometer set to over read? Mine seems to be the same as the sat nav.

Sent indeed by an iPad
But a steam driven one designed by Brunel
Not many about

 

[Stratman]

I'm just intrigued by why people believe the odometer but not the ECU.

 

[Dieselman]

Why is it that you say speedometer set to over read? Mine seems to be the same as the sat nav.

Sent indeed by an iPad
But a steam driven one designed by Brunel
Not many about

^ LOL:.. Likey, likey.

By law, speedometers have a tolerance of 0+ 10%, to give drivers an additional margin of error. Yours appears to be set to 05 over-read.

 

[Dieselman]

I'm just intrigued by why people believe the odometer but not the ECU.

Without wishing to get into a contentious argument where you spit your dummy out and cry.

The odometer is a simple multiplication of the tyre size and final drive ratio, so is pretty fixed.
the OBD is a calculated fuel volume taken from cross refereeing the theoretical value of power produced per unit of fuel injected multiplied by the duration of the injector opening, all cross referenced to the base reference fuel map.

So, if your car fails to produce as much power EXACTLY per unit of fuel injected as the manufacturer says it will, or if he base fuel table has been altered, the OBD won't read accurately.

It is only a calculation, not actual measured volumes. It's theory, not fact. A bit like most posts on this forum.

 

[V12]

So, how much difference could there be between a tyre with 8mm tread and 2mm? Surely that can impact the odometer and speedo?

Sent from my GT-I9300 using MBClub UK

 

[wemorgan]

So, how much difference could there be between a tyre with 8mm tread and 2mm? Surely that can impact the odometer and speedo?

Sent from my GT-I9300 using MBClub UK

That's roughly a 1% change in circumference

 

[V12]

Thanks for doing the leg work on the maths...

Sent from my GT-I9300 using MBClub UK

 

[st4]

That's roughly a 1% change in circumference

So DM is 99% correct over say 10 tankfuls to get a good reading, in a scientific world, thats reasonably ok. OBCs tend to mis read by 5-10%. The OBC is a bit easier to take your reading from, the paper and pen way a bit more honerous. Depends what you want.

I tend to do things a more basic way. I put twenty in it. I let it drop down, I put another twenty in. I work out how far that twenty gets me. No further than it did in my E320.

Not surprisingly, most my cars better the claimed figures as I drive in quite a sympathetic way most of the time over longish distances. From the engine, I make the same amount of power as I tend to cruise at 60mph along rural routes and because Scotland is not that big, I tend to go on the same ones.

One drive up Glencoe revealed my car did about 40mpg on a tour, just like the last one, and the one before that. One could go quicker round the bends, the other accelerate a bit more quickly.

People on here, and this is not aimed at your or DM, really need to get out there, drive their cars, rather than talk about driving them. I'm planning a tour down to York in a few weeks. I expect the car will do over 40mpg but under 50mpg, so about £60. Thats ok.

 

[Dieselman]

People on here, and this is not aimed at your or DM, really need to get out there, drive their cars, rather than talk about driving them.

Assuming one can afford the fuel...

 

[st4]

Assuming one can afford the fuel...

I spent about £20 to drive around Perthshire. Or was it £25, I did buy some food at the Tesco petrol station.

Ucht who cares, a day out for £20-£25. I ain't complaining.