4.56 or 4.88...that is my question.

[charles06]

i have 4.88's with 38's, and it's perfect. I also have the Superchips programmer, but the gears made the bigger difference.

Charles

[OrangeCrush]

4.56 gears with 38's (which actually measure about 37) is the exact equivalent of the stock 33" tall tires (though they are listed as 35's, they are really only 33" tall) with 4.10 gears.

When I did my first H2, I did the 38's and the truck was gutless. I lost 2-3 mpg on top of it.

I installed the 4.56's and it brought the gear ratio (and speedometer) right back to where it was from the factory.

The truck gained my lost mileage.

Now, having said that... I would have gone with 4.88's.

In fact, I just made a post because I JUST got off the phone with Summit and ordered some 4.88's.

Even with a supercharger, the stock 4.10's with 38's aren't enough.

Mark

[charles06]

When I was tire shopping, I did a little research before ordering the tires. On some of the tire manufacturers' websites, they list the "actual" tire diameter. The 38's I have are very close to a 38. They're like 37.8 inches, or something. But beware, as stated above, many tires are not the advertised size.

The service manager at my Hummer dealership stated that he's had a few lifted H2's come through his shop, but none of them drive as well as mine, unless they have a supercharger.

[guruerror]

Thanks to everyone for their feedback.

After reading through this and other posts, educating myself on exactly "how" gears work, and playing around with figures in 4lo.com's gear/tire calculators, I've elected to go with the 4.56s from trailduty.com.

I think the 4.56s together with my other mods will give me exactly what I'm looking for...a little better then stock performance but close to stock mpg. I'll post back with some of my own feedback once I get the job done. I've been tracking my mpg pretty closely, so I should be able to post some fairly solid numbers for those of you who are concerned with that.

I'm scheduled to drop the truck off at the dealer Monday...they quoted me $558 for the front, $558 for the rear, plus fluid and miscellaneous expenses; with the total not to exceed $1,400. That may be a little high, but I trust their mechanics as they've done all my other work (warranty and aftermarket) for years and it's always been top notch.

I'll keep you posted...thanks again.

[guruerror]

Still waiting on the 4.56s to arrive, but I moved forward with the reprogram using the Superchip Cortex 2950.

I selected a performance tune and programmed in new tire size. NICE results! Based upon my typical city/highway driving to and from work yesterday and today I'm back up to 12.0 mpg and the truck's acutally got some pep in its step.

I haven't felt an automatic shift like that since I put a shift kit in my '70 Fastback years ago. The new shift points are awesome...really starting to get the power back to the wheels. I think the gears will be the icing on the cake.

I guess I'm just impressed that a 'tune' could make such an improvement.

[OrangeCrush]

Originally Posted by guruerror

Still waiting on the 4.56s to arrive, but I moved forward with the reprogram using the Superchip Cortex 2950.

I selected a performance tune and programmed in new tire size. NICE results! Based upon my typical city/highway driving to and from work yesterday and today I'm back up to 12.0 mpg and the truck's acutally got some pep in its step.

I haven't felt an automatic shift like that since I put a shift kit in my '70 Fastback years ago. The new shift points are awesome...really starting to get the power back to the wheels. I think the gears will be the icing on the cake.

I guess I'm just impressed that a 'tune' could make such an improvement.

Drive the truck for a week like normal and tell us that you averaged 12 mpg.

I'm betting not.

A performance tune will usually drop your mileage roughly 2 mpg. If you're averaging 12 mpg, you either a) are babying it or b) don't have enough driving to collect good data.

Typically bigger tires will give you 9 mpg. Installing gears will usually bring it up 2 mpg. A performance tune won't do that.

Drive a few weeks and let us know.

Mark

[guruerror]

Well I'll drive it 'til Sunday and report back. Sunday night it gets dropped off for the gear work; but I've got a long haul this weekend into western Maryland so that should give me some good data. Lot's of hills.

My feeling is the 12 is going to be pretty spot on...it immediately dropped to 10 and some change after the 37s and stayed there. The fact that I'm two days in and it's still reading 12 is encouraging.

...and there was no babying it, once I noticed the added 'umph' I stayed in it harder then usual. It was a little 'refreshing.'

[guruerror]

Originally Posted by OrangeCrush

Drive the truck for a week like normal and tell us that you averaged 12 mpg.

I'm betting not.

A performance tune will usually drop your mileage roughly 2 mpg. If you're averaging 12 mpg, you either a) are babying it or b) don't have enough driving to collect good data.

Typically bigger tires will give you 9 mpg. Installing gears will usually bring it up 2 mpg. A performance tune won't do that.

Drive a few weeks and let us know.

Mark

Orange, okay, you called it...10.9! Up from my 10.4...but not where I thought it was going to land.

Oh well, looking forward to driving it with the new gears...anything to give it more umph that keeps me out of the single digits.

Keep us posted on the conversion and the associated costs. Good luck.