on board computer mpg

[pipehose]

when i start my o3 c220 cdi the afterstart mpg reads 43.7mpg. by the time i get to motorway it reads 35 mpg. it will not go over 40mpg on motorway no matter what speed i drive or for how long i drive . but if i press reset button
the mpg will go up to 50 mpg and sometimes more than 50 mpg is this normal or does the computer need reclibrated or something
thanks greg

 

[Brian WH]

when i start my o3 c220 cdi the afterstart mpg reads 43.7mpg. by the time i get to motorway it reads 35 mpg. it will not go over 40mpg on motorway no matter what speed i drive or for how long i drive . but if i press reset button
the mpg will go up to 50 mpg and sometimes more than 50 mpg is this normal or does the computer need reclibrated or something
thanks greg

Does seem strange, worth asking whilst under warranty mate.

 

[Alfie]

I think what you are seeing is normal/explainable.

Typically all cars consume an awful lot of fuel in the first few miles until the engine has reached optimum temperature. This is due to increased frictional forces etc within the engine when its cold. This can be as much as twice the normal consumption and is also the time the engine wear is at its highest. So I would expect the mpg to drop especially if your trip to the motorway involves slow traffic and/or stop starting.

The longer you leave the mpg indicator the more driving you need to do to increase the mpg. This is because the more data that it has between resets the more accurate it can compute the average mpg. If you reset the counter then all it has to go on is the data between the current time and the reset. So if you reset it on the motorway and you are slipstreaming a van or something, then your average will leap up. Come out of the slipstream or accelerate hard and you'll see it drop.

The computer in my ML can be made to read an average of 70mpg if I reset it and take my foot off the pedal. To get an accurate reading for your journey, reset the computer after starting the car/before setting off.

Remember that if you do 35 miles at say 20mpg average, to get that average up to 40mpg you have to a further 35 miles at 60mpg or 17.5 miles at 120mpg!

 

[JohnM]

Remember that if you do 35 miles at say 20mpg average, to get that average up to 40mpg you have to a further 35 miles at 60mpg or 17.5 miles at 120mpg!

Much worse than that actually as the overall mpg is not the average of the individual mpg figures but rather total distance divided by total fuel - if you did 35 miles at 20mpg you would have to cover another 35 miles on no fuel at all to achieve 40mpg average, that's what makes it so difficult to get the average mpg up after any significant distance at poor mpg. Given pipehose's 35mpg figure when he hits the motorway, he'd have to average 45mpg for 1.3 times as far again as he has already travelled to break 40mpg overall. That probably means travelling at not more than 65mph or so for the whole motorway distance, assuming it is far enough.

 

[Alfie]

Much worse than that actually as the overall mpg is not the average of the individual mpg figures but rather total distance divided by total fuel - if you did 35 miles at 20mpg you would have to cover another 35 miles on no fuel at all to achieve 40mpg average, that's what makes it so difficult to get the average mpg up after any significant distance at poor mpg. Given pipehose's 35mpg figure when he hits the motorway, he'd have to average 45mpg for 1.3 times as far again as he has already travelled to break 40mpg overall. That probably means travelling at not more than 65mph or so for the whole motorway distance, assuming it is far enough.

Yep, sorry you are quite correct.

Given what JohnM has said, I think what you are seeing on the computer is to be expected.