Trip Computer

[mbenz76]

Not sure how accurate it is supposed to be but I am not convinced by the MPG reading right.

Reset at petrol station yesterday morning (reset trip, from reset and from start). MPG on from start was around 22 when I got home. This morning the from start had reset (as it does after about 4 hours I believe) and after getting to work it shows about 25.

The from reset however shows MPG if 19! How can this be when the only 2 journeys I made since resetting have been 22 and 25?

Ok so it might not be 100% accurate (although no reason for it not to be as it can measure fuel being used etc.) but basic maths tells me that 22 and 25 don't average 19!

Has anyone else noticed this?

 

[Frugal]

I've been calibrating mine against actual fill-up data from new. It's showing about 5% optimistic at the moment which I can live with but I'm intrigued to find out whether I'm right to suspect that there is greater inaccuracy over short stop/start journeys. The trip computer is one of my few minor disappointments with the car, having previously driven an Audi that was both more accurate and updated the display data (eg average mpg from reset) much more dynamically.

 

[Shude]

I've been calibrating mine against actual fill-up data from new. It's showing about 5% optimistic at the moment which I can live with but I'm intrigued to find out whether I'm right to suspect that there is greater inaccuracy over short stop/start journeys. The trip computer is one of my few minor disappointments with the car, having previously driven an Audi that was both more accurate and updated the display data (eg average mpg from reset) much more dynamically.

Mine appears accurate, I tend to get exactly what I expect from a full tank vs trip computer but on engine start it gives the wonderfully pessimistic 12mpg average for the trip and then if I don't pull away immediately it's quickly into single figures!

 

[BTB 500]

basic maths tells me that 22 and 25 don't average 19!

Did you have the car parked with the engine running at any point after you'd looked at the MPG figures (or between the two trips)? As Shude pointed out, that can give very low mpg figues.

As an aside, averaging two trip MPG figures won't match the overall MPG anyway.

Suppose you do 100 miles and use 4 gallons = 25 mpg.

Then you do 200 miles and use 6 gallons = 33.3 mpg.

The average of 25 and 33.3 is 29.2.

But overall you have done 300 miles and used 10 gallons = 30 mpg.

 

[steve333]

By doing brim to brim manual calculations in all the mb's i've owned i find the trip computer readings to be roughly 10% optimistic to real life mpg.

 

[mbenz76]

Did you have the car parked with the engine running at any point after you'd looked at the MPG figures (or between the two trips)? As Shude pointed out, that can give very low mpg figues.

Not done any exessive ticking over except in traffic but this surely should have been taken into account by the "since start" figure as it keeps changing when stood still any way. The final figure when i turned the engine off is what I mentioned so would include driving and standing time surely?

 

[st4]

When you reset the OBC it counts down from 40odd. Over short distances its woefully inaccurate. The reset figure will be better, but it will have counted down from 40odd. You need to leave the reset figure alone and can only rely on the after start reading after about 20miles.

I only reset mine after a service.

 

[AndyBITR]

I own a 2008 W204 C220 CDi Sport and the trip computer had a 5200 mile average of 45MPG, but based on tank to tank fill up calculations it was was anything between 38-42.8MPG.

This week I reset the computer and recorded an average of 48.2MPG over 558miles. When I filled the tank it took 60L or 13.2 gallons indicating only 42.2MPG actual. Is there anything my MB dealer can do to reset/re-calibrate my trip computer to be more accurate?

On the actual MPG front and looking to improve my engines economy, is it safe to give it a shot of BG244 in the tank to clean my fuel system and injectors? I have used this on an Audi 2.0Tdi engine, but has anyone had good results with this on their MB engine?