Tech question: I am replacing the front diff (case broken) on my '04 H2. The replacement diff has a 3.73 ratio (out of an '08, cases are hard to find!), original is 4.10. I'm considering changing the rear to 3.73 to improve MPG's (don't do much offroading). My concern is with the speedo/computer, does anyone know what I would need to change (sensors, etc.) or just flash the computer? Where is speedo signal taken from? Any other thoughts? I need to order replacement gears ASAP, so I need to know which way to go...

Thanks!
Joe.

 

Holy crap Batman! Google a gear/tire ratio chart to have a look at where you need to be for gears. 3:73's aren't even in the ballpark. I'd really recommend going the other way, to 4:56's or even 4:88's when running a 35 inch tire and a possibility to go to 37. When you pull up the chart it will list the optimal gearing for tire size for performance and mileage.
Walk, don't run from that 3:73!

 

really... dont try the "I want to save on gas" around here.

If you are not going to do any off roading, and want the best performance for your Hummer, stick witht the 4'10s.

that is stock for the H2.
35 inch tires are also stock for the H2 ( 33 inchs if measured while mounted on the vehicle).

dont go to a higher ratio.. there are other things to consider. the 08 H2 also has a 6 speed tranny, which kinda makes a difference with those higher gear ratios. your 04 only has a 4 speed tranny.

i would recommend checking out a junk yard for a 03 to 05 hummer diff, and get that instead of the 08 one.

also... i would recommend the 4.56s if you decide to go with bigger tires. i run 37 inch tires, which call for 4.56s. i would go for 4.88s, but i plan on doing some off roading.
4.56's + 37 tires = 35 inch tires and 4.10 ratio.

you are looking at about $350 just for 1 set of gears ( gears and installer kit) then, you are looking at about $400 to 500 to have it installed. i figured if you could install it yourself, you wouldnt be asking this.
no computer flashing or reprogramming to have to worry about.

If you went with anything higher than 4.10's, im guessing you will need a custom tune $$$$.. and thats assuming you already own a tuner to program with, and unless it is a newer model, you will need to purchase a new one that accepts custom tunes.

so if you bought the 03 to 05 diff, all your paying for is the diff, and maybe some beer foe some buddies to help install it.

otheriwse, if you are going with whatever else, double those prices above to have matching gear ratios.

in the end, its your money, but using a 08 diff compared to a 03, to 05, isnt going to matter.

If you are going to go with the higher ratio, check to see if they have the tranny from an 08 that you can buy after you toast your current one

 

Well if you are going to change to 3.73's you are going to need to change out the rear diff too.

You cannot run 2 different gear ratios in your diffs.

Also you are downgrading if you go to 3.73's. alot of the guys with the 08's are huniting for older h2 4.10 gears.

 

Thanks, guys for the info. Like I said, the front diff lunched itself in some deep snow, had to replace it anyway, and thought that changing the rear diff to match the 3.73 would be viable, that's why I asked. I'm not concerned about the mechanicals, there are plenty of Tahoe's/Suburbans/pickups running 3.73 gears, in fact they were plentiful when I was looking. I opted to replace the 3.73 in the front with a new 4.10 set from Riverside Gear, they had a set for $199.95 (GM 9.25 IFS), and had it shipped overnight for 55 bucks, not bad...

My biggest problem was in finding a housing, seems that the junk yards were all out of them, I ordered the '08 diff after confirming that the housing was the same. The junk yard people were as useful as nipples on a bullfrog in finding parts. I even contacted Quality Auto Parts in Chicago via Automotix.com, and the unit that they had was bad. When I talked to the local Hummer dealer, they said that there were 2 others that broke their front diff locally and were looking for parts, so I guess the junk yards are pretty well cleaned out now..

My biggest concern going forward is that this could happen again, I wasn't even in a serious 4-wheeling operation when this one broke (just some deep snow, something that any other 4x4 would have handled OK), what's going to happen if I decide to do even some trail driving??????

I'll post some pics of the diff when I get to see it...

Once again, thanks for your input!!! Have fun!!

Joe.