OBC MPG Accuracy

Irish_Tom

Irish_Tom

Irish_Tom

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Joined
18 Oct 2002
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I've been quite anal recently when it comes to recording the MPG of my Mk3 Mondeo 2.0 TDCi.

I've had the dreaded 'overcooling' issue and noticed a distinct difference in the indicated and actual MPG. I thought this was due to the same cause as the overcooling (faulty cylinder head temperature sensor and thermostats) but those have now been replaced.

Although I'm seeing a much improved actual MPG and the difference between indicated and actual has improved, it is still quite a way off.

When I was overcooling, the worst discrepancy was 12.6mpg (54.6 indicated, 42 actual) now I've just done my first fill-up with a 'working' car and I'm still seeing a seven mpg discrepancy (57.7 indicated, 50.7 actual).

Is this standard for the Mk3 Mondeo? How does it compare to other cars?

I'm not expecting it to be 100% accurate but a 12% tolerance seems to be a bit high.
 
Is this measuring by brimming, driving, then brimming (one or two clicks) and calculating?

Mine is never more than about 1mpg out.
 
My Mk3 still has a knackered oil cooler stat that overcools too.

My typical mpg on the display has been 49mpg for about the past 6 months, however the real figure calculated while brimming every time is ~45, so 8-9%.

I know many car makers specifically program in this to over-read (which is a bit cheeky if you ask me).
 
Its varied depending on car im my experience, my MY06 Focus TDCi OBC was always optimistic by about 4mpg, my mates old fiesta ST was optimistic by about the same.

However the one in his current Auto A3 2.0T is pessimistic by about 2mpg, same for our work van a Nissan NV200, thats pessimistic by about 3mpg

These are all worked out over a long period of time brimming to the first click.
 
My indicated tank average mpg is usually between 2-4mpg optimistic vs. calculated.

It's a Fiesta, not a Mondeo Mk3, just to be clear.
 
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