Gas mileage isn?t as good as I thought it was...

[OrangeCrush]

For the last year that I?ve had the duramax in the H2, I?ve been using the DIC to check the gas mileage. Now I know it uses algorithms to calculate the necc. info but I thought it would be fairly accurate.

Last year I drove from NC to FL at about 80 mph and the DIC said I was getting 20 mpg over a 1500 mile trip.

This weekend we drove the same 1500 mile trip averaging 85-90 and the DIC said 17.7 mpg.

A few thoughts...

I?m running the second to highest tune available. (the highest tune allows more mpg but you can?t tow with it)

My odometer and speedometer are right on the money so I?m assuming the DIC is using the right data.


Anyway, calculating the mileage by hand, the gas mileage only ended up being 13 mpg.

That seems more realistic running 85-90 mph but disappointing all the same.

I?m gonna run a long trip at about 60 and see what kind of mileage I really am getting.

Even if the gas mileage is that crappy, it?s still worth the swap for the power.


Mark

[Indy]

Anything over 60 mph and the gas mileage goes up exponentially because of the wind resistance. Even from 60-70 there is a huge jump...way more than from 50-60. So going 80mph will obviously be eating gas/diesel like crazy. And I think that the onboard computer will only work for calculating gas mileage, not diesel

[guruerror]

Originally Posted by Indy

...And I think that the onboard computer will only work for calculating gas mileage, not diesel

Why? Fuel is fluid whether it's gas or diesel. And a gallon is a gallon regardless.

It would stand to reason that the computer is simply calculating how much fuel is pumped thorough the injectors together with how far that makes the vehicle go (based upon the revolutions of the wheel) to work out it's equation on what the MPG is.

If the computer is programmed with accurate information regarding tire size the computer should be accurate...key word is 'should.'

Mark: Did they replace your ECU when they did the conversion? Or just reprogram your existing unit?

[2010_5thgen]

driving any car at 85-90mph isnt going to get you great MPG's. best fuel economy speeds are right around 60-70. anything over that and your pushing higher RPM in the highest gear.

[OrangeCrush]

Originally Posted by guruerror

Why? Fuel is fluid whether it's gas or diesel. And a gallon is a gallon regardless.

It would stand to reason that the computer is simply calculating how much fuel is pumped thorough the injectors together with how far that makes the vehicle go (based upon the revolutions of the wheel) to work out it's equation on what the MPG is.

If the computer is programmed with accurate information regarding tire size the computer should be accurate...key word is 'should.'

Mark: Did they replace your ECU when they did the conversion? Or just reprogram your existing unit?

Yes, the ECU was replaced with that of a diesel. The only thing overwritten would be the performance tune written by a tuner. (Diablo Sport)

Originally Posted by 2010_5thgen

driving any car at 85-90mph isnt going to get you great MPG's. best fuel economy speeds are right around 60-70. anything over that and your pushing higher RPM in the highest gear.

And considering I'm running 37's, I still have 4.56 gears so I imagine that wasn't very helpful...lol.


Mark

[Indy]

It would stand to reason that the computer is simply calculating how much fuel is pumped thorough the injectors together with how far that makes the vehicle go (based upon the revolutions of the wheel) to work out it's equation on what the MPG is.

Don't forget that the injectors of the Diesel engine are different to the gas engine injectors. The readout will be false if this isn't programmed into the system. Anyways, the most accurate way will always be to measure it yourself, although this might require some Math to figure out the distance driven if the tire size and gearing have been changed. So Math does come in useful outside of school

[2010_5thgen]

Originally Posted by OrangeCrush

Yes, the ECU was replaced with that of a diesel. The only thing overwritten would be the performance tune written by a tuner. (Diablo Sport)





And considering I'm running 37's, I still have 4.56 gears so I imagine that wasn't very helpful...lol.


Mark

exactly. im kinda scared to see what my MPG goes to once i lift it and throw on 38's. but i only work 5 miles away from home, and i get a company gas card

[OrangeCrush]

Originally Posted by 2010_5thgen

exactly. im kinda scared to see what my MPG goes to once i lift it and throw on 38's. but i only work 5 miles away from home, and i get a company gas card

Going to bigger tires will lose you 2-3 mpg. I lost that and when I installed steeper gears, I got most of it back. On the bright side, your diesel won't know the difference.


Mark

[2010_5thgen]

i noticed i lost 1mpg just by switching to the Nitto trail grapplers from the stock bfg's. not sure if i want to get into gears on thei truck quite yet.ill do the lift and then go from there. but i still have lots of warranty left.

[MVR 155]

Those speeds are going to eat up alot of fuel regardless..... do you really need the 4.56 gears with the torque of the diesel? I would imagine you could really pick up some mileage swapping back down to 4.10's or even 3.90/3.73's

[guruerror]

Originally Posted by MVR 155

Those speeds are going to eat up alot of fuel regardless..... do you really need the 4.56 gears with the torque of the diesel? I would imagine you could really pick up some mileage swapping back down to 4.10's or even 3.90/3.73's

That's an interesting thought...

pwrhse2k's DMax is on a truck heading back now. He's got the stock gears but is moving to larger tires; we should have him give us some MPG samples after he gets it out an about a bit.

[OrangeCrush]

Originally Posted by MVR 155

Those speeds are going to eat up alot of fuel regardless..... do you really need the 4.56 gears with the torque of the diesel? I would imagine you could really pick up some mileage swapping back down to 4.10's or even 3.90/3.73's

No, I don't but they're left over from when I had a supercharger on the 6.0

I may need to look into that idea...


Thanks,

Mark