I'm not sure. 3-4 mph maybe.

My H2 is being regeared right now I'll report back when it's finished and broken in some.

 

So I have been running with my new 4.56 gears since Friday now.

This is the way it should have driven from the factory. The only time I come out of 6th gear on the highway now is when there is a pretty good size hill to climb. No more downshift to 5th for every little incline in the road. My rpms went from about 1750 in 6th at 75mph to 2150 at 75mph. In the area where I drive manly from fuel economy has gone from 12 mpg to around 13. Maybe getting closer to 14. More testing will tell. I would assume this is because it is more efficient to stay at 2100 rpms rather than constantly going from 1700-3500 which is what used to happen with all the 6,5,4 shifting.

 

Thanks a ton for posting this info!
I'm still scratching my head about why I don't sit around 1800 @ 75. Regearing would lower my RPMs 50 to 150.
I'm curious what your DIC says about the instantaneous MPG at say, 40, 55, and 70 (I get 1800 rpms @ 70, makes me wonder if my transmission has a problem).
How's your 0-60?

 

My 0-60 is roughly 7 seconds. At 60 the dic reads 16mpg on flat surface. Not sure what it says on the my stretch of the interstate which is where I actually do my mpg test. But in the past I have seen that the dic is actually really close to hand calculated on my 08.

 

That's fast! I think stock gears puts 0-60 at 8.9 seconds. Drop in 5.88 gears & get a 5 second H2.
Do you feel the MPG at 60 is better or worse that before? I'm just curious. I know everyone will scream, "it's a hummer!", but nothing wrong with gaining MPG.
The DIC is very accurate. I'm always surprised with that. I've found mine is usually within 0.1 to 0.2 of a mpg. Even on diesel conversions (not just H2s, but Yukons, Suburbans) & stock 2500s/3500s, they seem to be incredibly accurate. I'm always blown away by that. DICs used to be entirely inaccurate, but that's changed.

I take it that you're overall happy with the upgrade?

 

I'm really not sure about the mpg at 60.

And I am extremely happy with the gears. It's the best thing I have done to this monster.

 

You're making me want to regear!
The fact that your overall MPG has gone up is just awesome!

 

Well that's hwy mpg. I'd imagine city has dropped but idk. I never measure that. Too many variables. And I live an hour from a small city so anytime I need something that's how far I have to go. So most of my driving is hwy.

 

Synthetic here is your pic of DIC mpg at 60 mph. I had reset the the dic about 5 miles before I took this pic. Very flat road basically no hills or dips.

 

I looked at my tac at 60mph and it's at 1500rpm's. Mine is an 08.