My 2.8 CRD Auto does 46 mpg on the open road at 55 mph. At 65 mph it does 43 mpg. Over 65 the economy drops off, but I have not tested that. In the town the economy is terrible...I have seen 11 mpg on the trip computer in a really bad traffic jam.

My driving is probably 30% city mileage and 70% A roads/motorways, and I get 38 mpg on average. This does drop to 34 mpg if i do more short local trips.

Needless to say I do drive carefully with a view to getting good economy. Your 26 mpg could be down to the kind of trips you do....do you do alot of town driving. I once reset the trip computer in a busy town centre. In stop go traffic I was getting only 11 mpg! Also if you do alot of short trips where the engine does not get up to temperature, this will hurt economy badly.

Although Autos traditionally hurt fuel economy, the effect becomes less pronounced on bigger engines. There is only a couple of mpg difference in the combined figure of the 2.8 auto and 2.5 manual. Alot of that difference will be down to the fact the Auto version is 0.3L bigger in displacement.

The tuning module on my Rover 75 was a box that plugged into the fuel rail pressure sensor. It had settings to alter the power and throttle sensitivity settings. It made what was a dog of a car really lively and powerful.

I had an early 75 with a 113 bhp diesel. Rover later uprated the engine to produce 135 bhp, so I was confident the tuning module would not break anything in the drivetrain. It boosted 113 bhp up to 145 bhp. But it definately did not improve fuel economy....if anything made it slightly worse.

 

Obviously. The only way of giving a standard car more power is by burning more fuel on each combustion cycle. Doesn't take a brain surgeon to work that one out.

I spent over 15 years blueprinting race car engines and anyone who thinks you can get real gains without doing proper engineering is a fool.

As I stated in my earlier post these addons work by fooling the ECU to obtain a longer injector pulse duration per cycle.

 

I'm sorry to tell you, but I disagree

IT IS possible to gain more power without burning more fuel, and that is the way some of this gismos work.

for starters, every factory engine is tuned to meet balance between three things: economy, ecology and fuel consumption. if you mess with one of them, you have to mess at least one other of the group.
in theory, if you tune your engine to keep the economy, but to be worse in ecology, you gain something in fuel consumption.
in practice, it comes down to fooling the sensors and the ECU, but not only the ones you said.
increasing the fuel pressure will also squirt more fuel into the cylinder in the same time, so that one is not good for us.

what you can do, is to alter the intake air temperature (as the air is colder). that will be interpreted by the ECU as "I can advance the ignition (or injection timing on diesel)".
do I need to explain?
more advance will give you the same force on the piston (as the same amount of fuel is injected) on a differenet crank angle, and therefore more torque. which then comes to more power.

needless to say, I am strongly against that.
on petrol engines that brings you closer to detonations (and possibly less power) and on diesel engines you push up the temperatures inside the engine and more pollution on the exhaust.

this is possible on petrol and diesel engines with ECU controled timing and electronic fuel injection.

 

2 doctors

the best mpg you can get by rather quickly accelerating to desired speed and then to maintain it with cruise control.
there is no saving of fuel in "softly, softly" touching the accelerator.

explanation on demand

 

Not sure I agree with this and it would be easy to test - find a stretch of clear road, ideally half a mile long. At the start of it, reset your average MPG and accelerate hard to 70 mph and then hold steady until a set point. The go back to the start and do the same again, but accelerating more steadily, but making sure you are up to 70mph by the same point - I would be fairly certain that the average fuel consumption would be worse on the first run over the second.

 

there are 2 thing wrong in your line of thinking.

1. the distance is too short. we do not travel 1/2 mile to work, but tens of miles on each run. measuring fuel consumption on such a short distance will surely make it your way.
2. you can not make 70 mph at the same point if you accelerate harder then the first time. and again, if it's such a short distance, you have to consume more fuel by accelerating faster. the faster you accelerate, more fuel will engine consume.
but when it comes to long distances, the story changes.

 

1. Why is the distance too short - once you are at 70mph, the fuel consumption is the same from the set point on, it's the consumption getting to 70mph over a set distance that is being tested.
2. If you feel that half a mile is too short, then make it a mile, doesnn't make any difference to the point of the test

 

you don't see the big picture.

again, if you make 10 runs with the same car on a 1 mile track, it's obvious that you will burn most fuel on the run which was the quickest.
but when it comes to long runs, it's not that simple.

example: lets take a 200 mile distance between two cities.
what is the most economic way to make that? driving in highest gear with constant speed (say, 60 mph) becouse that's when you can get 40 mpg.
but, in real life, we all know that's not possible. the car has to be accelerated from stand still to that constant speed of about 60 mph. and that is the time when your engine sucks fuel and you have bad fuel economy.

so, it is imperative to shorten the time of acceleration as much as possible, becouse that will make longer that time when your engine is running on 40 mpg. pure mathematics.
in time of acceleration, it is irrelevant wheather your car makes 2mpg, 5 mpg or 10 mpg. the rest of the 200 mile run makes it irrelevant, becouse you will be on a 40 mpg sooner and that large fuel consumption will not effect total fuel consumption that much.

on the other hand, pressinig the accelerator softly will extend the time when your car is making bad mpg and increasing total fuel consumption, as we can see from the doc's expirience.
driving like that, I can make 100 km strictly city driving with 9,5 liters of gasoline. the car beeing small SUV with 4x4 and a 2 liter 140 HP engine.

now, the problem is, how do you determin how fast do you accelerate.
that's different for every car. depends on the size of the engine, weight, type of the fuel, gearbox......
I can explain if you are intersted. important thing is that you undestand maths in fuel consumption. you can even make your own calculation.

you can calculate how much fuel you will burn if:
-you accelerate 2 minuites on 10 mpg, and then 2 hours of 40 mpg.
-you accelerate only 30 secs on 3 mpg and then 2 hours and 30 secs of 40 mpg.
on that one run, the difference will not be that great, but in a city driving, accelerating 5 times every 10 minuites......

 

OK, I'll give you 2 examples. We'll assume a final speed of 32 metres per sec (just over 70mph) at which we get 40mpg, and a steady acceleration for the sake of mathematical simplicity.

First, accelerating at 2 m/s2 (that's metres per second squared) - this gives time to final speed of of 16 seconds (using v=u+at) which represents reasonably brisk acceleration. Using a fuel consumption of 12mpg during the acceleration, (about right for my CRD), and using s=ut+0.5at*t (t*t is t squared), since u is zero, this becomes s=0.5at*t, the distance covered is 256 metres, and the fuel during acceleration is 0.060 litres.
The remaining distance to cover 1km is 744 metres, and will used a further 0.053 litres, giving a total fuel usage for the kilometre of 0.113 litres.

Now, accelerating at 1 m/s2 and assuming a fuel consumption of 25mpg (again, about right for my CRD with gentle acceleration) for the acceleration, time for acceleration is 32 seconds, the distance covered is 512 metres and fuel used over the acceleration is 0.058 litres. The remaining distance to 1km is 0.488 km, which will require an additional 0.34 litres, giving a total fuel burn of 0.092 litres, which is a 0.021 litres less, or 22% improvement over the hard acceleration.

Using cruise control over a long distance will reduce the percentage, but the 0.021 litres doesn't change - and if you forget the cruise and accelerate 5 times over 10 minutes, as per your last statement, then the fuel saving for the journey with gentle acceleration becomes 0.105 litres.

Whichever type of journey you have, the gentle acceleration gives the better fuel consumption.

 

and what have I said up there?
total fuel consupmtion depends on many factors, the biggest one is type of engine and type of fuel.

first of all, your car is diesel.
which means that you can accelerate in two ways. by injecting more fuel into the chamber and by changing the time of pre-injection.
on gas engines, there are three ways to do that. by increasing the amount of air-fuel charge that engine inhales, by changing the ignition point and by inrichening the air-fuel charge.

you have a turbo engine and a lot of torque in low rpm's.
gas engine is weak on low rpm and accelerating from there will increase fuel consumption greatly.
by driving like that (mild acc.), you allways keep your engine in rpm with max torque, and that's also the rpm with minimal specific fuel consumption. in total - you get good mpg.
with gas engine, max torque is far ahead in rpm's and specific fuel consuption is very high on low rpm.

I guess we both agree that consumption is the highest during acceleration. logicaly, less acceleration - the lower consumption.
and with diesel, you get only 2 times larger consumption between mild and hard acceleration.
with gas engine, it raises 5, 8 or even 10 times more. that's what makes it uneconomical.