Lacking power and bad fuel economy.

Until recently we owned two Navaras, both 3l diesel STR. One we were selling, the other we had recently bought. The Navara we just sold was a 2003 factory standard with no modifications except bullbar and towbar. It had done about 140,000km.
The other Navara is a 2007 and has done around 70,000. Apart from the usual bullbar, towbar modifications it also has a steel canopy and tray, snorkle and slightly larger tyres.
Now my query is that the 2003 Navara had far more power at both take off/ low speeds and cruising/higher speeds as well as better fuel economy- 10l/100km compared to 13.5/100km for the 2007 Navara. Now I realise the 2007 Navara weighs about 400kg more (I weighed both)it has about 0.322m2 of wind resistance (canopy higher & wider than cab) and the tyres are about 5% bigger in diameter/circumference than original tyres (265/75/R16 compared to 255/70/R16)
Does all of this equate to such a dramatic differnce in power and economy????
I used to own a Hilux with a steel tray and even with it loaded up it made little difference to the fuel economy.
Do bigger tyres mean better fuel economy when cruising but worse when accelerating???...
I'm interested in any views or opinions but if anybody has any real life experience with any of these factors or any hard data that would be great! If you've got this far, thanks for listening!

My 2006 model gets 10L per 100km's all the time.

Ever run injector cleaner through it.

Thought of intercooling it ?

Dave.

You've pretty much identified the vehicular reasons why your 2007 model is less economical. The extra weight means more force required to get it moving. The larger wheels means even more force required to turn the wheel over.

If you just did a highway run - fill the tank, get on the freeway and head in a direction (at the vehicle's "sweet spot"), get off and refuel it - you'll find that it's probably not bad, because it's a diesel.

That "sweet spot" is important. In my D40, the engine reaches maximum torque at about 2000rpm. If I can keep my vehicle travelling at a speed where I've just entered the max torque zone, I'll use surprisingly little fuel (relatively speaking).

Larger tyres will mean that your "sweet spot" has moved UP in speed (not rpm), too. Unfortunately there, the faster you go, the greater the force needed to push the vehicle through the air, so you'll reach a stage (may have reached it) where the air resistance is high enough to negate the advantages in driving at the sweet spot.

I'm not going to mention driving style, because we're not talking about the difference between you and someone else.

If you add those things together, you might easily find a couple of litres per hundred km.

Before you take it on that run, reset the ECU - remove lead from battery negative, press brakes, replace lead, take vehicle for a very gentle drive. That should make the ECU re-learn the fuel mapping.

There may also be things like tyre pressures at play. On a level surface, how hard is it to push the car? You should not have to strain too hard to get it moving. If you do, then tyre pressures might indeed be causing some additional drag.

I can't think of more. Been a long day, I need some sleep!

Tony, does that reset procedure work on the 3L D22's or D40 only?

All of them. Because it's a simple power-down, it will work on any vehicle that remembers its fuel mapping in memory (as long as that memory isn't in a storage device). The Nissan ECUs all seem to hold this information in "volatile" memory - lose power = lose memory.

Don't forget, you need a gentle, unladen stress-free drive after doing it to teach it a nicer fuel mapping.

so what does pressing the brake pedal do?

Consumes the remaining power in the system. Computers sometimes store a small amount of energy for a time, it's part of the circuit that keeps the power supply steady into the computer so there are no spikes or power drops.

You can either press the brake pedal to immediately remove this power, or wait about 5 hours for it to dissipate naturally.

I bet I know what you're going to do!

Tony, thanks for that advice and info. I'll try the resetting the ECU as suggested. I'm also thinking about buying some smaller tyres, although there is still plenty of tread on the ones I have(Goodyear are having a 25% sale at the moment though). I'm also planning on making a wind deflecter in front of the canopy since it does extend some 200mm above the cabin.Can't do too much about the weight part, although I've been told that that's least of the contributing factors?

The weight is only really a big issue for fuel economy when you're accelerating or climbing hills.

I'm not sure I'd change the tyres straight away. You're tossing several hundred $ worth of tyres to save $2-3 per hundred km. It could take thousands of km to recoup that.

Someone here suggested limiting your take-off rpm to 2500. Give that a shot and see how your economy goes?

Another point I forgot about.

Are your brakes dragging ?

Dave.

Dave,"are my brakes dragging? Please explain!

It's possible that the shoes of the rear drum brakes are maintaining contact with the drum itself, which offers resistance to motion.

Easy way to tell - try the roll test. Put the car on level ground, place it in neutral, take the handbrake off. Get out and try to push the vehicle. Can you do it?

If you find it very difficult to push (or impossible) then your brakes might be rubbing and that will mean you are using more power (therefore more fuel) to overcome this additional resistance.

If you can move the vehicle but it feels extremely sluggish and heavy (while you're pushing it), it might be tyre pressure. As the tyre rolls on a surface, the tyre wall has to deform. The lower the pressure, the more the wall of the tyre has to support (because the air pressure is not supporting the weight) and therefore the more the wall will bend. With greater bending comes greater force in making the bend, and that translates into more power used to turn the wheel over = more fuel consumption.

Hope that's clearer than it looks!

Old.Tony said:

Someone here suggested limiting your take-off rpm to 2500. Give that a shot and see how your economy goes?

Click to expand...

jesus reaching 2000 is heaps enuf for me in the d40...

redbook shows d40 weighs 5805 and str 4850. thus d40 almost ton heavier and im getting approx 10l/100km 13.5 doesnt seem right at all.

then again im running cr yd25 motor...so comparison is kinda void.

also how are u getting ur figures? by filling the tank and driving to see how kms ur getting or the dash trip meter (over rated)

The D22 is about 1800kg in stock form dry weight.

The D4o would be about 2000kg.

What Tony said about the dragging brakes, the fronts could be seized aswell.

So it starts easy ?

Dave.