MPG-ometer Errors?

[robbi-wan]

Why is it that the computer on the car gets the average MPG wrong??
I filled up again today and calculated the Real MPG vs what the computer was telling me, and the difference was 6mpg

I think i am correct in assuming that the injection system is a precision piece of engineering and that the odometer is accurate [other contributors have compared it to GPS with only minute differences]: so where is the error coming from?
There must be a sensor of sorts giving out fuel flow information for the average to be calculated so could it be inaccurate / faulty??
rob

 

[malcolm E53 AMG]

Interesting point!
I've never done a physical check against the computer, next time I brim it I will do that and report back. Mind you I'm very happy with the fact that this car passes a lot more filling stations than the last one.

 

[robbi-wan]

It is not that i'm unhappy with the economy: 490 miles @ 37mpg is, i think, pretty good. It's just that it is not to be trusted, which is a bit if a let down.
Is this 'characteristic' representative of other (lesser?!) cars.....

 

[television]

. It's just that it is not to be trusted, which is a bit if a let down.
.....[/QUOTE]

Years ago we never went anywhere without a spare can of petrol, you have been spoilt.

malcolm

 

[malcolm E53 AMG]

I'm averaging about 34, according to the computer and I think its pretty accurate. You usually get a feel for these things, probably why you decided to check yours.

 

[l5foye]

robbi-wan
37mpg from a 220CDI is poor. Any idea what your average speed was? My MB technician friend had a driver call to complain about getting only 37 mpg from his 220. My friend took it on a cross country route -average speed 39 mph (this included round town ) -result 52.5mpg.

 

[tamrsoft]

I have done a comparative check of my trip computer mpg calculations against actual fuel used versus mileage covered (presumably the most accurate method) over about 500 miles and found the trip computer to be about 10% optimistic (42mpg against 38mpg actual). Metering injections cycles against distance covered should be pretty darn accurate with modern electronics but it appears not to be.

 

[jberks]

The computer is likely to be a little optomistic as it will be basing the distance covered on the speedo which itself is a touch optomistic (compare it to your gps readout). Whenever I have done a brim to brim check I've found that the average is around 35-36. The trip computer generally reads around 41 but interestingly the overall computer display based on 9,000 miles is 35.6 so pretty much bang on.

I think we tend to watch the computer on a run when its showing 40+ but ignore it when pottering around town and reading 23. This tends to bring the real average down a fair bit.

 

[robbi-wan]

. It's just that it is not to be trusted, which is a bit if a let down.
.....

Years ago we never went anywhere without a spare can of petrol, you have been spoilt.

malcolm[/QUOTE]

It was not so many years ago that i never went without a spare can....:grin:

 

[robbi-wan]

robbi-wan
37mpg from a 220CDI is poor. Any idea what your average speed was? My MB technician friend had a driver call to complain about getting only 37 mpg from his 220. My friend took it on a cross country route -average speed 39 mph (this included round town ) -result 52.5mpg.

l5foye, average of 37 mpg is for the normal drive to work, taking kids places, shopping etc.....(average speed not much), and usually over about 2 weeks.
When i do get away from the daily grind, longer distances yeild 50++mpg, so i'm fairly sure (hopeful?) nothing is wrong with the car.
rob

 

[television]

These trip computers can be callibrated at all levels, too many too put up here, ask when the car goes in for service.

Malcolm

 

[Blobcat]

The computer is 'optimistic' as it looks good. Car manufacturer’s reply on owners believing the information they are given.
I built a calculator in Excel to work out my fuel economy and my E320cdi has averaged 36.5mpg over 86,000miles. The computer shows >40mpg.
The mileometer is correct in my car (GPS Tested) however I note that the Speedo shows 2-3mph more than actual speed. Manufacturers have a 10% tolerance for Speedo’s but must not show less than actual speed.
When showing speeds >120 the Speedo is even more out. One of my bikes has shown >165mph when in reality it was doing 143mph (Not in this country officer). Manufacturers do this as again it looks good (mine’s faster than yours and all that)
Bottom line if you want to know keep the records yourself over a long period.