Bad Fuel Economy

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Mike4x4

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I have a d40 2007 turbo diesel navara, i usually get about 650-700ks out of the tank on a straight road.

I recently put 33 tyres on it and the egr mod and went for a drive from perth to esperance which was basically a straight road. I only got about 250-300 ks out of the tank.

On the way home i took out the egr mod, i got 350,400 ks. Is this normal?
 
I was going about 120-130, speedo was 10ks out before the tyres but the bigger tyres corrected the speedo.
 
Larger tyres don't correct the speedo. They change where the needle points. The ECU actually knows quite well (on standard tyres) how fast you're going - it's the crappy needle that is about 10% out. If you change your tyres and your needle looks right, then your ECU is now 10% out AND so is the odometer. It's a sad problem. Nissan don't have any legal obligation under ADR to fix it and so they won't. The best you can do to get an accurate speed measurement is to use a GPS.

Ok, the fuel - bigger tyres require more force to turn them over, so with the same engine and gearing, that means more fuel. Our tyres are just 2% larger than standard and we're using more fuel than we ought to be.

Then there's the speed. In the 1800-1900rpm range, the D40 YD25 engine is nearing the top of its torque range and at 2,000rpm it's right at the point where the engine develops the most amount of torque for the fuel going in. If you put more fuel in you get more rpm but not a lot of extra torque - so anything over 2,000rpm at cruise will result in an increase in consumption. On standard tyres that's 95km/h, and that should be just over 100km/h (actual) in your car. Anything beyond that and you're effectively throwing fuel away.

You can change the torque curve with a chip - especially one that adjusts the turbo vanes. Chip It and Unichip come to mind.
 
Thanks fo rthat Tony
I drove the last 1/2 tank trying not to go above 2000 & 2200 rpm and I've got 10% more kms than with my normal driving style before your advise. It's nice to have good blokes like you sharing important informations like this one.
I filled her up again today and I want to see how many more kms I can drive out of the full tank.
Only problem is that 0 to 80 takes about a month!!!LOL Would it better to take the car up to speed (up to 2700 rpm) than drop the RPM?
Cheers
 
I don't mind taking my time getting up to speed, I've done nothing to piss the Comancheros off so I'm in no great hurry. I tend to let my RPM peak at around 2500 while accelerating. I do have a bluetooth OBD adapter and use the Torque application on my smartphone, and I've noticed there's a significant increase in fuel rail pressure (= fuel consumption) between 2500rpm and 3000rpm.

I also tend to work the hills. If I'm going up a hill I'll let the car gradually slow and I'll use the downhill to pick up speed again. I don't mind this - as the other vehicles overtake me they see what they expect - an old fart driving a ute!
 
as the other vehicles overtake me they see what they expect - an old fart driving a ute!

With the van on the back and that bowls hat on your head they just think "oh no not another member of the grey army"
 

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