Mondeo TDCI question

I have one of these. I got 40 mpg before using an Inforad camera detector, and now get 50 mpg.

Rob Graham

The onboard computer is going to be more accurate than doing a spreadsheet but unless the same driver is driving the both cars over the same journey at the same time, there's going to be a variance.

And then there's the power to weight ratio and higher gearing of larger engines. You get a sweet spot where a larger engine is able to move the car easier and is more frugal at cruising speeds due to the higher gearing. Smaller engine doesn't always equal more economical in the same car. On my Capri, a 2L engine is 10% more economical than a 1.6. I've found on older petrol Mondeos that the 1.8 is more economical than the 1.6 or 2L.

I have to disagree there; onboard computers are usually inaccurate at this calculation. This is because they measure flow rates, not the actual volume of fuel going into the engine. I've had them in a Saab and in an Omega and both were on the optimistic side.

Fuel calculations by brimming the tank are not always accurate in the short term, but long-term they are far more accurate. I used to do it; can't be bothered any more.

The only way to settle the OP's question is for both to brim their tanks, then again after as many miles as possible, and calculate the mpg for each (this is after all the method used in economy runs). I suspect the 2.2 is more economical because it is running well inside its capability.

I'm usually suspicious of the accuracy of measurements, so am wondering about the following:

- Presumably the litres figure displayed on petrol station pumps is pretty accurate (less than 1% error), because if they over-read they might be prosecuted, but any degree of under-reading hits the (reportedly slim) profit margin.

- I don't know how modern cars measure fuel flow. If it's a gizmo inserted in the pipe between the tank and the engine, then I would expect the error to be greater than 2%. If the flow measurement is done by the fancy fuel pump that modern diesels have, I guess that could be more accurate.

- How accurate are car odometers? They probably have an error of at least 2% once things like tyre wear are taken into account.

So is the difference reported between the 2 cars significant?

Don't talk rubbish, of course it isn't.

John

asahartz ( snipped-for-privacy@hotMEATPIEmail.com) gurgled happily, sounding much like they were saying :

The one in my XM was fairly accurate - but certainly nowhere near as good as keeping a full log over thousands of miles would be.

They don't just measure flow rate - because with a modern injected car, so much fuel is recycled back to the tank, it'd be MASSIVELY inaccurate. They calculate based on the opening times of the injectors, so the actual fuel usage.

No explanation as to why I see.

Spot on. SAAB 9000, 10% too good on the computer.

And as the system petrol pumps use is quite basic and has to be very accurate, there's no reason not to use the same methodology in a car that has a variety of sensors to say just how much is being used.

Conor (conor snipped-for-privacy@hotmail.com) gurgled happily, sounding much like they were saying :

Apart from the fact that the speed sensor used to calculate the distance is not exactly accurate.

You'll use the same reading to calculate the mileage yourself, unless you happen to have a GPS...

David Taylor ( snipped-for-privacy@yadt.co.uk) gurgled happily, sounding much like they were saying :

I wanted to give Conor an excuse to rant about tacho calibration again...

You mean the one you take the reading from to put in the spreadsheet to work out the MPG??

:p

There's no need.

OK, a few theories:

You got the wrong conversion value for litres -> gallons, and are working in US gallons (that would give you 50 mpUKg).

You do more motorway miles than he does. (our 1.4TDCi Fester does 60 around town, 50 with mostly motorway. Our 2.0DTi Signum does 45-48 on mostly motorway)

He drives like a grandad, you drive it like you nicked it (although I would expect 46-48 from both).

Bob

I've checked the *internal* speed sensor against GPS. (You turn the ignition off, press the trip button, and then turn the ignition key, and it puts the dashboard into test mode. If you then press the trip button, you get all sorts of diagnostics.

One of them is road speed (resolution of 0.1mph). This matches up, at 50mph, on a straight, flat road (50mph bit of the A55 east of Llandudno) with my sat-nav, so whatever the ECU thinks the speed is, it agrees with the sat nav.

So, as part of the test sequence, the computer controlled speedo needle can accurately point to the "Max" point on the display (which means the computer can accurately control the needle). The computer knows exactly how fast it's going, but it *still* manages to over-read by 8% on the speedo. I recon it's deliberate myself.

Pete.

I have to disagree here. The Mondeo TDCI trip computer is "known" to be accurate to only +/- 10%. Some cars it's spot on. Some it shows 10 over, some it shows 10 under.

Mine shows about 5 over. When I did a shown 63mpg on a trip, I'd done 58. The same trip showing 60mpg, I actually got 55.

They're usually consistent though. One that shows 5 now tends to show 5 always.

Pete.

In message , Pete Smith writes

There is an element of luck then because the EEC can't compensate for tyre pressures or tyre wear, both of which will effect your speedo reading.

..by so little as to be completely insignificant. On a lorry I once took in for a tacho calibration test, it was 0.5MPH out at 56MPH and only because the tyre was half worn. If a 22" tyre with a 20mm or so tread depth only produces an inaccuracy of 1% when half worn, a car tyre will priduce far less.