Trip computer fuel consumtion wrong

Dear all,

my XJ12 1992 Serie 3 has a Trip Computer installed. While working fine, the reading for the fuel consumption is about 10% too low. Wiht this as well average mpg and instan mpg is reading too low by the same amount. I learend that the interface unit in the trunk will convert injection pulse to the consumtion readint (see duncan brillant articel on that). Is there a way to adjust or calibrate this interface unit to get a more true reading?


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Welcome to the list.
Over the past 20 years we have owned and regularly driven four trip computer equipped Jaguars (two XJ6s, one XJ12, and one XJ-S). During that time I occassionally compared trip computer fuel consumption calculations to actual fuel consumption when I filled up the gas tanks. From memory I’d say that most of the time the numbers were a bit better than 10% difference, but not much better than that. I know of no way to calibrate them.


Welcome to the forum Peter, as Paul has mentioned there will be differences. On any of the cars that I have owned (‘89 and ‘92 XJ40 and ‘99 XJR)with trip computers the readings have always been approximate, the only true way to measure is top up to top up.
The same goes for speedo readings generally they can be up to 9% fast, I have upped the tyre ratio on both the wifes Mazda 6 and my ‘99 XJR to get them closer to actual road speed.

Mine is also about 10% more optimistic than reality…

Did you know that they are required by law to show a higher speed than real?
Avoids a great number of legal issues.

Yes, but 9% is ridiculous, I’m now about 3% difference, still over reading.

I think that the difference is not linear, but starts small and increases as you go faster.
My Mercedes also reads 100km/h when it’s actually 92km/h and 40km/h when it’s 37km/h.

My trip computer is remarkably precise, Peter - within the precision of fill and distance measuring…

How did you measure - using satellite navigation and petrol station pump data?

The ‘Fuel’ reading is straight from the injector opening times - converted by the interface unit; readings should be according to petrol filled.

‘Dist’ is measured by the speed transducer, which also runs the speedo - and should read the same as the trip counter on the speedo, independent of the interface unit. Obviously, if the transducer gives the wrong gen both ‘Inst’ and ‘Ave’ will also be wrong.

While the speedo is independent of the trip computer, it could be checked, in a speed trap or via satellite tracking data. As Robin says, speedo may be deliberately misleading - but not necessarily ‘contaminate’ the trip computer data. This just means that the speedo data cannot be used to verify trip computer readings.

Theoretically, ‘Avspd’ can be checked various ways, with some computation - which may be a separate check of the trip computer readings…

My point is that the trip computer and the speedo data must be checked independently of each other. An obvious source of misreading of either or both is the relation between the rotation of the propeller shaft (transducer speed) is tyre size and diff ratio. Nominally, the latter should be countered by use of different transducer gearing or indeed the interface unit - though I don’t know whether alternatives exist or are available. Misfitting will cause misreadings - but neither the transducer nor the interface units are ‘adjustable’…

As an aside; with lots of idling in traffic, the ‘Ave’ will read too high - and ‘Inst’ is useless both in ‘mls’ and ‘km’. Incidentally; I assume you have the same error in both settings…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

An extra thought, Peter…:slight_smile:

There are two variants of the trip computers, one computing in Imperial and one in US gallons. In the former; filling in US gallons and computer computing in Imperial - the consumption data will be too low…so…?

A very long time a lister described conversion from Imp to US by snipping a wire inside - if I remember correctly. Cars delivered to US would of course be set up for US - Canada would likely be Imp…?

One way of testing this is to convert your filled US gallons to litres and use the trip computer ‘km’ setting. A reading of, say, 1,3 L/10km would compare to 17,5 mpg US. Or 21,5 mpg Imp. Which doesn’t quite gel either …:slight_smile:

xj6 85 Sov Europe (UK/NZ)

In my car the “Dist” and the odometer reading on the speedometer have the same 10% difference as with the fuel figures.
It makes me wonder what could cause this inconsistency, and if maybe, as there is no calibration possible (?) at the trip computer, there maybe a calibration possible at the speedometer ?
Never the less when I put gas, what goes in is 10% more than what is stated by the computer that has been consumed… unfortunately…

The trip computer just counts the pulses from the transducer, and the transducer gets his signal from the transmission… so it does count, not produce a voltage. If your speedo reads the same difference it only means that it is accurate, and you‘d need other tyres.


I think that may be an EU requirement

I think USA regs require that speedo error be no more than +/- 5%


The trip computer in my :lumped" is nuts. reasonable as it gets a mixed message… As is the speedo, same reason!!
I live with it easily…

MPG interest paled early on. I’ve not the slightest idea as to either of my critters. Only that the lump does better than the Jeep!!!

The Jeep has a “fun” trip computer. Selections available… MPG in real time is fun and interesting. Easily seen digits add to it. Throttle change and it can go from abysymal to wildly impossibly good. AVG easy on the gas pedal and in OD at freeway speed seems about 17… 4.0 six driving a box through the air!!!


Frank has a valid point. Remove the trip computer from the dash, find the part number on the tag at the rear, and reply with the part number.
Jaguar made the trip computers for a variety of markets and some used Imperial gallons instead of U.S. gallons. I own such a car, a Canadian market 1990 V12 Vanden Plas. My trip computer was built for Imperial gallins and reading incorrectly for my U.S. gallons when I got it. I made the internal wiring modification to convert it from Imperial gallons to U.S. gallons shortly after buying it because I found the offset annoying.


Easy solution: drive a known distance. If the trip computer is close, it is either a fuel side thing or the (good idea) wrong gallon conversion - also check L/100km because that makes sense all the time.
I think the EU requirement is to read 0-7 kph over. Not sure if for the needle only or for the odo as well. This would apply to the speedo, not the trip computer, if they were smart. And it would probably mean the same inaccuracy for every car… which they would have compensated for! So, it must be the tyre circumference if it reads 10% over with metric settings.

All my Jags hail from before '72. They are very accurate - per GPS - and as road tests attest (too many tests?). Our modern cars all read about 5kph fast as if a “zero error” is intended. Also have a '98 Holden ute - dead accurate - was a police car though. Paul.

Understandable that that is the case.

My S3 is dead on, bmws (and minis) have or had a real speed and an indicated speed setting, accessible via a secret menu, the cars know.
Feels much better when they read a tad high so you can drive the actual limit without fear :grimacing:

Dear all,
thanykyou for all helpful thoughs you did share.
But basically it did not solve my issue.

Distance reading with my trip coumpter is fine, as it is speedometer reading and all speed related info like avrg. speed.

It is just the fuel used that is wrong. Living in Germany I read litres, so the confusion with gallons (us vs. imperial) is not the issue.

The error I see is:
I fill both tank to the brim, drive a distance. When I refill to the same level the reading of fuel consumed and actually filled in differs about 10 % (eg: reading 60 liters, fill in 66 liters)

The tripcomputer is the one originally deliverd with the car, but being a XJ V12 of the last built (May 1992) there may be all remaining parts used that had been present at the factory still :frowning:

Anyway: the different version relate to my knowledge to US or imperial gallons, so it shall have no impact on litre reading. As pointed out already, the metering is based on the injection impluse count and its timing.
These pulses are taken from ECU and are modified by a interface in the trunk to feed the trip computer.
To adjust this signal (which is pusles still, no constant voltage)it need to be modified to compensate for the 10% misreading.

Any Idea if and how this can be achieved? To my knowledge there are no adjustable elements (potentiometer or the like) in the interface. So a direct adjust seem not to be possible. Does anybody have an diagram of the interface or had done some re-engenering and can advise which element is to change to get a different conversion.

Thank you all for your support.


Just a remark to the speed reading of the speedometer according to European law:
the reading is allowed to be 3% too high (reading 100 when actually driving 97) but there is no tolerance for reading too low. So drivers have no excuse when exceeding speed limits, as their Speedo cannot have shown a lower speed than actually driven. This law had been in place since decades.

Because of that all car manufacturer adjust their system to indicate a slightly higer speed than actualy driven. I think that as well applies for the US market. So in fact you will alway see a deviation from the actualdriven speed of 3-5%, which may be linear or progressive depending on the metering system used,

I was confused on the ‘gallon’ question, Peter - since you used ‘average mpg and instant mpg reading too low’ in your original post…:slight_smile:

I would like some more ‘metric’ numbers. Does the ‘Ave’ reading (in litres) correspond to ‘Fuel’ reading (say 60 litres) or to the ‘actual fill’ (66) litres using ‘Dist’ displayed? And what are typical numbers? At steady cruising, some 50/60 km/h one would expect ‘Inst’ numbers at around 1,3(?) for the V12

You say also that ‘Dist’ reads the same as the odometer? And do both actually show the true distance traveled, measured by map or satellite data? And is the speedometer showing ‘true’ speed? If so, it eliminates the transducer, diff ratio and tyre questions as a source of fuel misreading.

The trip computer actually measures the sum of injector opening duration - with constant fuel pressure it reflects the actual amount of fuel injected. Converted by the interface unit to a language the computer understands. It does not count the number of pulses - pulse width (injection duration) vary with engine load.

This is a possible source of fuel misreads; with incorrect fuel pressure, the actual fuel consumption is different from the one computed by the trip computer. But since this does not depend on distance data given by the (correct) transducer - computations done using ‘actual’ fill will be correct, and different.

The interface unit may indeed be a source of fuel data faults. While it may still give correct distance/speed information; it only converts inputs to trip computer language - and the conversion has different pathways. The only way to assess interface function is by replacement…

There may be more obscure reasons for your misreads; any leaks, or tank crossfeeds, will not be registered by the trip computer - and, depending on fill procedures may lead to anomalies…

My trip computer is ‘spot on’, and indeed, David’s experience confirms that if everything is as intended the trip computer is meant to be precise…:slight_smile:

xj6 85 Sov Europe (UK/NZ)