Update:

I kinda wish I had kept my old tires. They just made the whole rig feel more 'trucklike'. I have seen no change in fuel economy. Not noticeable anyways.

Don't get me wrong, the tires are fine. Quiet, capable in snow, etc etc. But when I had the LT ones, the truck felt 'bulletproof'. Not anymore.

The whole reason for changing back to the original profile with lighter tires was to try and resolve my fuel economy issue. Since the problem is still there, I am still looking for a reason. Just to give you an idea, I get about 75km for ever 1/4 tank. Used to get about 120-140km for the same amount of fuel. I also measure fuel economy based on actual fuel filled. It matches the number I mentioned here.

I was tinkering under the hood the other day, and heard a wicked vacuum leak under throttle application. Anyone know if that would cause a fuel economy issue? I pulled the engine cover off, and didn't find any disconnect hoses, so I'm wondering if my intake gasket is shot.

Thoughts? I think I might start a new thread.

t

Wow, you're getting worse mileage than me as I'm only getting about 380+km before the warning light comes on.

My mileage has improved slightly since working graveyards, due to less traffic and less stop and go.

With the added weight from the LT tires, I filled up to about half tank to see if it'd make any difference for a couple week. No, it made no difference in mileage.