About ten years ago I conducted an experiment.

After joining the freeway and travelling at the speed limit ie 100 Km/hr (62 mph) I set the cruise control

after zeroing the dash computer and travelled for about half an hour over fairly flat ground.

After that time period the computer read 45 mpg.

I repeated the run recently and the result was 38 mpg.

My fuel consumption over very short runs reads 24.5 mpg and used to be nearer to 29 mpg or thereabouts.

I have recently replaced all the plugs with the specified Iridium ones and the Short and Long FT's

are fine.
The Jag mileage is probably 65 k miles more at 100k miles now.

What else can I do to improve the fuel consumption?

 

I don't have any recommendations but do have a comment on my experience. At one stretch I was switching between three sets of tires/wheels and noted my mileage was measurably impacted by the change, so not sure you can be comparing apples to apples if you have a different brand, model or size tire than during your previous experiment.

 

The weather can make a huge difference, many years ago on a long trip to The South of France on the autoroutes we found, on stopping at service stations enroute that the exhaust tips were a very pale grey colour, the heat of the day and constant speed ( no cruise control in those days on my cars ) was effecting how rich or lean the car was running, whereas normally in ordinary driving locally the exhausts would have a black residue, might be worth thinking about with your results ?

 

Gray color meant your carb was set lean for maximum mileage

 

Hi meirion1,

My first suggestion is to not rely on the car's trip computer to gauge real-world fuel economy. In my own fairly scientific tests, I have found the trip computer to typically be off by about 2 mpg. It's better to start your test at a gas station and fill the tank as full as you can, and end your test at the same station and fill it again as full as you can and measure the fuel used in the distance traveled.

Other things that can affect fuel economy are tire tread rolling resistance and inflation, a malfunctioning coolant temperature sensor or stuck open coolant thermostat, an out-of-spec MAFS, an obstructed air filter, leaking fuel injectors, a full fuel tank vs a 1/4 full fuel tank, incorrect spark plug gap, air leaks causing lean running which prompts the ECM to increase fueling, lazy oxygen sensors, using a higher oil viscosity.... Lots of possibilities.

If you can view Live Data with your scan tool, watch your coolant temperature, MAFS signal and O2S signals and see if they all seem plausible and responding properly. Low coolant temperature is a big red flag since it can cause the ECM to continue to apply cold-start fuel enrichment even after the engine has achieved full operating temperature.

Cheers,

Don

 

Add ... immediate driving history before each test ... a good long run before one but a series of short runs before the other will give different starting conditions which will skew the results;

Add ... different environmental conditions between tests ... air temperature, humidity ...

 

When did you last service your brakes? Dragging brakes use fuel.

 

Many thanks for all replies.

 

I agree with Don B. regarding the tank full to tank refill method of gauging MPGs. Tire pressure should also be to specifications. Use of the sunroof and/or driving with windows down creates turbulence and drag. I've found that with steady speed Interstate driving say, 70MPH and all buttoned up I still get ~28MPG +/- 1 MPG with almost 100K miles on my 2004 4.2 NA XJ.

 

In October 2017 about 4 months after I purchased my 2004 XJ8 I took a trip to Atlanta. The car had ~65K miles on it, new plugs and air filter. Drove the inland route I78 to I81. I did the first 600 miles non-stop on a single tank going between 80 and 85mph. It took almost 20 gallons to refill. Averaged 31 mpg (dash showed 33). Needless to say now at 100K I'm getting less (around 28 mpg on long highway drives and 24 in mixed driving).

 

How old are your catalytic converters?

 

Actually I'm pretty sure there isn't any actual flow metering device in the fuel line from the tank and I can't see a manufacturer fitting such device (not cheap !) just for calculating mpg. As I understand it the 'systems' on the car (most cars these days, I think) calculate fuel usage by inferring it from the opening times of the injectors and assume that the fuel passed is proportional to that time.

The problem with that is that the actual flow through an injector nozzle depends on more than just time; it will also vary with fuel pressure, fuel density (thus temperature) and fuel viscosity ... all of which are variable.

I've always kept a fuel log for my Jag (all my cars back to '72, actually, as my Dad did back to the '30s) and have always filled it brim-to-brim to get mpg for each 'fill'. At the same time I record and then reset the trip so that I can compare the trip mpg with the 'actual'. The variation is quite considerable; the trip mpg is ALWAYS optimistic but more importantly has varied between any thing from 1 mpg to 5 mpg better than 'actual'.

 

It can allow for all the variables.

My car surprised me at how well it calculates MPG.

 

"How old are your catalytic converters?"

Probably as old as the car-it's 2003!

I have bigger things to worry about now, my Jag has had a heart attack.

An ECU fault. The ECU is being repaired by specialists now (I hope)
Bank 1 only affected.