4.56 or 4.88...that is my question.

[guruerror]

Here's my mods:

K&N cold air intake
Gibson shorty headers
Flowmaster exhaust
Cognito upper control arm leveling kit
Cognito HD OEM-style tie rods
Cognito pitman/idler kit/support
Bilstein front shocks
37x12.50x17 BFG Mud Terrains

Since the mods the truck feels like it can't get out of its own way...like its working its ass off just to move down the road.

I'd like to get the truck back to the stock feel regarding acceleration and shifting...and figured gears would be the next step to help the engine move the wheels more effectively (and maybe more efficiently...while mileage is not my biggest concern, it is on my mind. Before the above mods I was averaging 12.1mpg, now I'm at about 10.4mpg.)

I've orders a Superchips Cortex 2950 to reprogram for the tire size change but am having difficulty deciding between 4.56 or 4.88.

The H2 is pretty much my daily driver. I have a convertible, but it only comes out in the summer. The truck's driving breaks down as follows:

Approximately 18,000 miles per year
City (50%)
Highway (35%)
Off-Road (15%)

So...GEAR EXPERTS...what changes to acceleration, shifting, MPG, etc. should I expect to occur by:

1. Reprogramming with the Cortex for the change in tire size WITHOUT changing gears?

2. Reprogramming with the Cortex for the change in tire size AND changing my stock gears to 4.56?

3. Reprogramming with the Cortex for the change in tire size AND changing my stock gears to 4.88?

Thanks in advance!

[timgco]

I used the Hypertech and the Superchips programmers on my past H2's. Sin ceyou can change the shift patterns and shift firmness, you can get better than the stock feel back.

If you do gears, TomP and RuggedH2 have both done 4:56's. Tom P is running 37's and Rugged run 37's for his daily driver and than switches out to 38's for off road trips.

[tomp]

Tim, I thought Rugged had 4.88s?

If you are only going with 37s - get the 4.56s. If you do the 4.88s with 37s on a daily driver, you will virtually have no first gear (it will shift to 2nd too fast) and shifting will be too frequent

[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.'

[OrangeCrush]

Well I hope you do get 12 mpg... just after having 2 H2's and about every possible combination, I have yet to average 12mpg even when it was stock.

Mark

[pteam]

Simple things to improve gas milage:

remove roof rack bars and offroad lights on the roof (there was a study that this alone can drop your milage by 10%)
remove spare tire
remove extra weight not needed
install k&n air filter and keep it clean
inflate tires to 5%-10% under max air pressure and keep an eye on them
use mobil 1 gas saving fully synthetic oil
put the car in neutral when coming to a stop or going down a hill

With my 08 h2 I can average 13.5-14mpg in the winter and higher in the summer mostly city driving. I can hit 16s on the highway in the summer time. This is different than the older h2s as it has 25% more horsepower and a 6 speed transmission but all these things add up.

[OrangeCrush]

Well, I ain't worried about it.... I ordered my Duramax 6.6, allison transmission and transfer case yesterday.

I won't be installing them probably till summer but the quest begins.


Mark

[pteam]

Orange, Best of luck!

[OrangeCrush]

Originally Posted by pteam

Orange, Best of luck!

Yeah, I'm gonna need it... I seem to be problematic when it comes to conversions and the such.



Mark

[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.

[OrangeCrush]

Originally Posted by guruerror

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.

That's cool.... there are many people on the forum getting higher mileage but once you put on larger tires, all hell breaks loose.

My gears should be here this week... hopefully they'll be here soon enough I can get them installed by the weekend.

I'll keep ya'll informed as I make progress (which won't be till summer probably)

Mark