4.56 gears?

[Mike97ZJ]

Anyone ever think of upgrading thier gear ratio to 4.56?

Seems to me that with 35" tires, 4.56 would be the optimum gear ratio, not the stock 4.10.

They'd give better performance around town with better acceleration, and with the overdrive gear in the tranny, the RPM's would still be reasonable on the highway. Also, the lower gears would be an asset offroad, allowing for a better crawl ratio and better compression braking while going down hills.

Any thoughts on this?

Oh, and can anyone tell me what thier RPM is while cruising at 70mph on the highway?

Thanks.

[Mike97ZJ]

Anyone ever think of upgrading thier gear ratio to 4.56?

Seems to me that with 35" tires, 4.56 would be the optimum gear ratio, not the stock 4.10.

They'd give better performance around town with better acceleration, and with the overdrive gear in the tranny, the RPM's would still be reasonable on the highway. Also, the lower gears would be an asset offroad, allowing for a better crawl ratio and better compression braking while going down hills.

Any thoughts on this?

Oh, and can anyone tell me what thier RPM is while cruising at 70mph on the highway?

Thanks.

[Steve R]

Hey Mike,

Good to see your sticking around! Next time I'm cruising the freeway I'll check what I'm turning at 70mph...my guess is about 1,800 - 2,200 rpm...but I will have to check for accuracy of course.

As for the gearing...someone was saying that the H2 comes stock set-up to tow and pull heavy items. They felt the gearing was already too low and that a higher gearing might support better gas mileage. If anything, he wanted to go higher gearing, not lower.

As for me, I think it's all good just the way it is, plus I'm lazy and like to leave my faith in the design engineers who built it. I'm sure they tried variations before landing on those gear ratios.

[Mike97ZJ]

Well I think the engineers have to compromise with the bean counters.

My theory is this:

The lowest gearing GM offers in any of it's trucks is 4.10. So going to 4.56 would be an added expense for them, as its not an off the shelf part.

The trucks with the 4.10's are 3/4 and 1 tons, and those come with 245/75R16's stock, which are only about 30" tall.

And I actually believe that in some cases, lower (numerically higher) gears can IMPROVE gas mileage, depending on tire size. With the lower gears, the engine has to work less to accelerate. Since it will accelerate faster, you don't have to be on the gas as much to get up to speed.

Now maybe higher gears CAN help increase gas mileage, but only on flat or downhill highway with the cruise set at 65-70mph. But who drives 100% of the time on perfectly flat highway?

Anyway, just a theory and something to think over.

I've already done the 4.56 gear swap on my imaginary Hummer.

1997 Grand Cherokee TSi
It's got some lift, skids, and rock rails, among other things.

[Hummie2]

Mike...

On mine 70mph=2000rpm. This was on flat level freeway. My Inst Econ was reading 11mpg at this speed probably due to h2's boxlike aerodynamics and the added rolling resistance of that big of a tire. I do know one thing though, fuel economy drops noticebly after about 62 or 63mph on my H2. I guess one might try a trip without using overdrive to kind of get a feel if higher numerically gears might be a step in the right direction at freeway speeds.

Don

[Mike97ZJ]

Thanks for the info.

2K rpm at 70mph, that's pretty low. That's pretty good highway gearing, though. Any lower than that, and the engine would start to bog and the tranny would be kicking down all the time.

That's a good idea, about running with the OD off to see if higher gears will be tollerable. I think they have a 30% OD, so if the gears you were looking at were 30% lower, it would give an exact idea of how it would be. I'm lowsy at math, so I can't figure out what gear ratio running with the OD off would be equal to.

And I hear you about gas mileage dropping as speed increases. I have an instant economy readout in my Jeep, and I notice the same thing.

At 60-65mph, I can get about 18-20mpg, depending on how flat the road is. If I step it up to 70mph, I get about 15-16. Needless to say, I drive alot slower now, and usually set the cruise to just under 65.

BTW, I turn around 2300 RPM at 70mph.

My old 98 Firebird with the six speed turned 2k rpm at 80mph in sixth gear. That car would get about 28mpg on the highway. Of course at much under 80, that sixth gear was useless, because the motor would just bog and the car would not accelerate at all.

[JCJ]

Do we have an OD on-off switch? Where is it?

[Mike97ZJ]

Ford and Chrysler have actual switches to turn the OD off.

In GM vehicles, on the shifter you have the positions "1", "2", "D", and "D" surrounded by a square.

"D" is 3rd gear. D in the square is 4th gear, which is OD.

So to lock it out of overdrive, leave it in "D".

[Hummie2]

OK Mike...

Just got back from running same road same direction same day,weather etc. this time in direct(3 on shifter JCJ). 70mph=2850rpm and Inst Econ bounced between 9-10mpg. 3.73 ratio might help, but could cause loss of milage also as eng would be pretty well off it's torque curve @ 70. Definitely would be harder to get up to cruise speed and would consume more fuel getting there. My "Seat of the Pants Dyno" tells me the H2 is working alot harder at 70mph than at about 63 or so.

don

[Hummie2]

BTW Mike

Running in direct would be about the same as a set of 5.86ratio gears and trans w/30% overdrive. 5.86 minus 30%= 4.10

Don

[Mike97ZJ]

Thanks Hummie.

Wow, 5.86 gears. WAY too low, unless there's a way to cram 44" tires on a H2, lol.

You're right about the 3.73's, those are too high for tires that size. I think it WOULD have the engine running too far off its torque curve with that ratio.

That's actually the same reason I had for the 4.56's, it might better ut the engine in it's optimum torque curve.

But I gather after reading about this same topic on the general discussion board, the 6.0 has enough grunt to pull even large grades at 70+ mph without shifting out of overdrive, so I guess there is plenty of torque available.

I AM still curious as to what the rpm would be at 70mph in OD with the 4.56's though.

[Hummie2]

Mike...

Threw the numbers into calculator for 4.56ratio and came up 2225RPM @ 70mph.

Also worked out for 4.88 ratio and it would be 2381RPM @ 70mph.

Don

[Mike97ZJ]

Don,

Thanks for satisfying my curiosity.

That doesn't sound to bad, 2250rpm @70. SHouldn't have too much of an effect on mileage, and might even be better around town.

And they'd help out if larger tires were ever added.

I think 4.56 might be the lowest gears available for GM axles, I'm not sure if they make 4.88's for them.

[Hummie2]

One word of caution for anyone thinking about changing ratios "Check Parts Compatability for Your Model First".

I had one Bad Experience with GM products. GM likes to lock-up the parts market.

I order a 88 1ton Chevy Ext Cab 4X4 w/7.4L eng when they first came out. The rear axle for that eng was a Dana70, posi-trac was not a factory option. NO Problem, I'll order a "Detroit Locker" and install myself when truck arrives ...WRONG!

The truck arrived and I pull into shop @ work and pull out rear end gears and open carrier, mount gears and bearings on Det. Locker and go to install back in axle housing - WON'T FIT RIGHT. Nothing to do but put gears and bearings back on open carrier an reinstall in truck.

All Dana70's had been the same forever EXCEPT this run in the NEW Chevys had a different pinion gear offset. GM had a SPECIAL run made for this model so they could have EXCLUSIVE parts market.

BY GOSH I was going to have a Detroit Locker in there come HELL or HIGH WATER and I knew a good machinist that specialized in gear work. He built up and machined the locker to fit and it was a "one-off work of art" when he finished, but it wasn't cheap as he was very proud of his work. That thing worked GREAT when I installed it.

The only down side was that engine could drain a 32 gallon gas tank in 150miles when towing my off-road race car trailer. It would make the H2 look like a gas miser.

Moral: "CHECK FIRST" don't assume anything.

Don

[Mike97ZJ]

Good advice.

[Steve R]

[quote]Originally posted by Mike97ZJ:
I've already done the 4.56 gear swap on my imaginary Hummer.


There ya go....I feel some tugging on the line....not quite time to swing & set the hook, but it's coming!

[Steve R]

Don....I'm considering adding a front-locker to my H2...any thoughts on that? I just can't stand the idea of the Rubiclown having one up on us.

Btw, Mike...anything you wanna know about the H2...you just ask Don: he is all-knowing!

[Hummie2]

Steve...

Don't know if they make a locker for front axle on H2. Do you?

My experience has been with rear lockers(Detroit) and front posi-tracs(Spicer Powr-Loc)only about 80% positive on front. Was always told that you would loose steering on front at very much speed if were 100% positive on both axles. Might not be a problem at slower speed though. We were blasting throuh muddy spots and my old CJ5s with rear lockers and front posis and and I know you couldn't turn until you backed off throttle and let it turn then jump back on fuel again.

I used to desert race a CJ8 Scrambler pick-up that I had put Dana44s front and rear. Rear was a Detroit Locker and front was a Spicer Powr-Loc, but I ran it for a different reason in it. An open carrier only has 2 spider gears and Powr-Loc has 4 (in theory 2X as strong). I know we would blow spiders out through the cover with open carrier if front wheels got spinning and then caught good traction. Never did with Powr-Locs 4 spiders. Also had set of worn clutches in that Powr-Loc and it was only about 50-60% positive. Didn't affect steering at all and we ran over 100mph in 4wheel drive in the long straight streches. We had the transfercase blocked in to 4wheel drive so it couldn't jump out of gear when we got to jumping and we stayed in 4wheel all the time. I finally had to give that up though as the last we raced it we did a complete 360 end over end at about 70+mph and ended back up on the wheels about 100yds off the track. When we got out to survey the damage my co-driver said " if we don't quit this one of us is going to kill the other". I fully agreed and besides Im too damn old to bounce around like that anymore.

I'll stick with my H2's plush ride nowdays.

Don

[This message was edited by Hummie2 on January 12, 2003 at 07:53 PM.]

[Mike97ZJ]

Don,

great story ,lol. Glad nobody was hurt. Sounds
like a pretty bad@ss Jeep too. I've always liked the CJ8's.

Steve, I think ARB makes an air locker for the 3/4 ton GM's. Not sure if the H2 diff is EXACTLY the same as the ones in the pickups though. I don't see any reason why it wouldn't be, but you never know.

Could be something to look into.