Hi,

I suspected my trip computer wasn't giving me completely accurate results with the average speed. I tested this today on a motorway journey and found this to be the case. I reset the counter and travelled around 20 miles with the cruise control set at 70mph. But according to the trip counter I'd averaged 58mph during this time. I appreciate the speedo is always a few mph out... but 12mph??? When I collected the car the dealer did say it takes a few hundred miles or so for the trip counters to settle in, but I've done over 1,500 miles now. I'm a bit concerned that if this is out by so much, are the other readings out too?

I'm due to take the car in soon for a courtesy service and will mention this, but just wondered if anyone else had noticed this or if mine's a one off?

test it with a GPS a 20 mile stretch of FLAT motorway driven at a constant speed of 60 mph will take twenty minutes.

i challenge you to find a stretch of motorway that is flat for 20 straight miles! dont forget that even with cruise control set a hill will slow the car or speed it up depending on which way it goes.

you should be pretty much bang on using this method. then you will be able to check the average speed computer.

test it with a GPS a 20 mile stretch of FLAT motorway driven at a constant speed of 60 mph will take twenty minutes.

i challenge you to find a stretch of motorway that is flat for 20 straight miles! dont forget that even with cruise control set a hill will slow the car or speed it up depending on which way it goes.

you should be pretty much bang on using this method. then you will be able to check the average speed computer.

I appreciate what you're saying about a flat motorway, but there aren't that many slopes on the M4 and although the car may slow very slightly before the acceleration kicks in, it barely drops below 67-68mph. If the average was showing around 67-68mph I'd accept that to be about right. But over 20 miles when the speed doesn't drop below 65mph a single time, how can it possibly average 58mph? The MPG seems to be a bit out too. That's showing an average of 47mpg, but my calculations work it out to be closer to 54mpg.

Today while I was out I checked the trip counter was working accurately by checking the mileage my car thought I'd done against what my sat nav thought. That seems ok.

I guess it's only a computer! But I really was just curious at the accuracy of others in case it's just mine.

I appreciate what you're saying about a flat motorway, but there aren't that many slopes on the M4 and although the car may slow very slightly before the acceleration kicks in, it barely drops below 67-68mph. If the average was showing around 67-68mph I'd accept that to be about right. But over 20 miles when the speed doesn't drop below 65mph a single time, how can it possibly average 58mph? The MPG seems to be a bit out too. That's showing an average of 47mpg, but my calculations work it out to be closer to 54mpg.

Today while I was out I checked the trip counter was working accurately by checking the mileage my car thought I'd done against what my sat nav thought. That seems ok.

I guess it's only a computer! But I really was just curious at the accuracy of others in case it's just mine.

I think my speedo is reasonably accurate--but I would agree that the trip computer is a little erratic.

It's certainly not as good as the one on my previous car (Astra) as that was crack on with mpg etc.

However I monitor my mpg the old fashioned way--mileage divided by litres multiplied by 4.554.

All this means that I just don't trust the computer when it tells me how many miles I've left in the tank though..........