Speed/Gearing/Tire size?!?!

[DriverSideImpact1008]

I was wondering what the best ways to get the RPM's down for highway driving in an M1008. I need to be able to drive fairly frequent 150 mi highway trips comfortably with decent mpg.

I know that currently @ about 65-70 mph the 6.2 is pushing 3100 RPM's or so. I know thats pushing the 6.2's limits.

I've been looking into regearing the diffs, lift and bigger tires, or swapping an 700R4, but i have had a difficult time finding a lot of answers.

Can anyone help me figure this out? Otherwise i may be forced to sell the truck .

 

[BFR]

... http://www.steelsoldiers.com/index.php?name=PNphpBB2&file=viewtopic&t=14467

 

[1956_4x4]

I was wondering what the best ways to get the RPM's down for highway driving in an M1008. I need to be able to drive fairly frequent 150 mi highway trips comfortably with decent mpg.

I know that currently @ about 65-70 mph the 6.2 is pushing 3100 RPM's or so. I know thats pushing the 6.2's limits.

I've been looking into regearing the diffs, lift and bigger tires, or swapping an 700R4, but i have had a difficult time finding a lot of answers.

Can anyone help me figure this out? Otherwise i may be forced to sell the truck .

A lift and bigger tires will not really drop the RPMs enough to make much of a difference. The weight of the tires and the increased wind resistance will work against an increase in gas mileage. I tried 35" tires on my 1031 and ended up dropping back down to 33" tires.

An overdrive transmission is a good start and going to a higher gear in the differentials can help too. The problem with a differential gear change is the need to change both front and rear gears, this can be pretty expensive.

Here is an RPM calculator that might help you decide. http://www.csgnetwork.com/multirpmcalc.html
It's a simple plug and play to let you figure out what the modifications will do for your RPMs.

Hope this helps.

Smitty

 

[DriverSideImpact1008]

RE: Re: Speed/Gearing/Tire size?!?!

Thanks...yeah the gearing i looked into for the 14 Bolt rear was $800 itself. Plus another $300-400 for the front was just as much if not more than what i think a 700r4 would go for.

Anyone have any insight on the 700 trans or perhaps a manual swap?

 

[1956_4x4]

The 700R4 is a good transmission, just stay away from the first few years of production (they hadn't worked out the bugs yet). If you go this route, try to find one from a truck with a 6.2 and grab all components from the donor vehicle.

Another option would be the 4L80E but the swap would cost $$$$.

Smitty

 

[DriverSideImpact1008]

Yeah i had experiance with an early 700 in a caprice shifted pretty funny. Thanks for the info.

 

[CCATLETT1984]

A lift and bigger tires will not really drop the RPMs enough to make much of a difference. The weight of the tires and the increased wind resistance will work against an increase in gas mileage. I tried 35" tires on my 1031 and ended up dropping back down to 33" tires.

I noticed a huge difference, considering running 38" tires is like putting 3.73 gears in the differentials. I can do 70mph all day long and still get ~20mpg driving at 65.

 

[1956_4x4]

A lift and bigger tires will not really drop the RPMs enough to make much of a difference. The weight of the tires and the increased wind resistance will work against an increase in gas mileage. I tried 35" tires on my 1031 and ended up dropping back down to 33" tires.

I noticed a huge difference, considering running 38" tires is like putting 3.73 gears in the differentials. I can do 70mph all day long and still get ~20mpg driving at 65.

That's pretty impressive. The 38" tires will drop the RPMs to just under 2700. That turbo might have a lot to do with it. If the turbo makes that much of a difference I need to look into one. Have you done anything else to help get these numbers? My experience with larger tires and the lift did not net any actual MPG gain.

Smitty

 

[DriverSideImpact1008]

RE: Re: Speed/Gearing/Tire size?!?!

Hmm...how much did those ground hawgs cost you?

 

[CCATLETT1984]

RE: Re: Speed/Gearing/Tire size?!?!

i paid 270 per tire, and that mileage estimate was PRE-turbo. will report on mileage post install when i make the trip to GA later this month.

 

[AJMBLAZER]

RE: Re: Speed/Gearing/Tire size?!?!

With my 36" HMMWV tires I was getting a rough 15mpg. Maybe a little more. Never corrected the speedo but I will after the 38's go on.

Look into a well built 700R4 and you'll find the trans shops of today can build them to withstand anything a 6.2L can throw at them. Make sure the trans is set up for a diesel and not a gas motor.
The hot setup would be to find a 80's GM truck and rob it of it's trans, t-case, crossmembers and mounts, and if you get one with a 14FF the rear driveshaft would even work. Your D60 front is bigger than a front 10 bolt so you'd have to have your front driveshaft modified.

35's +4.56's + 700R4 would be nice, right about a factory performance level. I was contemplating a similar swap but ended up going CC's route and went for the 38's.

If you are serious about regearing go over to www.ColoradoK5.com and see if anyone wants to trade axles with you. There's lots of guys over there with 3.73 and 4.10 geared trucks that would love to get your 4.56 geared axles with the Detroit Locker in the rear.

 

[Ambulancechser]

get a k model...

If you go the 700r route,get a later k model 700...identified by the large letter K on the belhousing.as mentioned,stay away from the early ones.if you intend to tow or use the 700 hard off road,you'll have to spend some bucks to get it properly built.it's not the strongest trans ever made.

i too,am getting about 15 miles per U.S gal',depending on the weather.prairie head winds are killer in an m-1010.imagine driving a pickup that has 37" tires down the hiway at 65' with a 6 foot drag chute out back,and you'll get the idea.but just think...what if i had a 454? maybe half that.

 

[AJMBLAZER]

RE: get a k model...

The 700R4 got a bad reputation because of it's problems when it first came out. Late in it's life GM had it working pretty good and then opened a new bag of worms by going to the electronically shifted version, the 4L60E. That got it's bugs worked out after a while but then GM started to really up engine power and it started to show weaknesses...it's a cycle.

I wouldn't trust any 700R4 you bought out of a junkyard or even a running pickup. Either buy a professionally built 700R4 or find a cheap but serviceable unit and then have it rebuilt to hi-po specs.
I've seen friends buy "good working" auto trans and then soon after discover that "good working" to that particular seller didn't necessarily mean it worked correctly. Personally I'd rather just get a new or rebuilt trans and then go through the hassle of installing it. That way I know it's good to go and up to the tasks at hand.

 

[CCATLETT1984]

RE: get a k model...

FYI, if you have one rebuild, PLEASE ask around the local 4x4 shops for a builder that has a good reputation for things lasting and working right. its not a hard trans to rebuild, but there are certain modifications that must be done for it to handle the diesel and towing. and Don't tow in overdrive, you WILL kill it.

 

[AJMBLAZER]

RE: get a k model...

Heh...ask my dad about towing in OD with our '92 K1500 Blazer...towed a 18' travel trailer up half the Appalachian mountains for most of a day before he realized he was in OD...when the check engine light came on and the thing started shifting in and out of OD...and puking fluid out of the dip stick tube.
I was about to buy it and he tried to play it off like nothing was wrong...$1500 HD trans rebuild I had a Blazer with a HD 700R4.

My trick for finding good trans shops is to snoop around your local cop shop's garage and find out who they use to rebuild their transmissions. Cop cars go through transmissions every couple of years at best so they know who in your area to use.

 

[Croatan_Kid]

Coincidently, I JUST (about 4 hours ago) sourced and purchased all the parts to rebuild and beef up a 86 model 700R4 to go in my CUCV. My buddy is running one with a full manual vavle body behind his 383 stroker mud Jeep and had a spare laying around where I work. He mentioned it to me and I stopped and stared and him in awe. I hadn't even thought about the freebie he had gotten. So we worked out a deal, ordered the parts, and by the time I get the truck registered and insured, the trans will be built and ready to go in. As of the moment, I have $500 in parts and only need a high capacity pan, and a R4 to 208 adapter but we're watching them on E-bay like hawks. So after the trans-plant (hahahahaha) I'm gunna run 35s with no lift for a while until I recover enough fundage to do the 4 inch lift later on in life.

Parts/Price List:

- diesel tq converter (1650 stall) $125
- master rebuild kit w/ kevlar bands and corvette servo etc etc $120
- tv cable $25
- 13 vane pump kit $40
- TransGo 4th gear lock-up kit $45
- TransGo Jr HD shift kit $50
- TransGo Separator plate $65
- +2 qt finned aluminum pan $40
- 700R4/NP208 adapter $40

I'm really excited to do all this. Might have a few driveshaft issues, but we'll figure them out. Should be easy to move the crossmember and we have a few welders at out disposal. All I really want to do after this is lift it and turbo it, but not necessarily in that order. My buddy's 700R4 is holding up to 550hp/620tq so I don't think I'm gunna tear it up, besides we're actually beefing mine more than his It'll be a while, but I'll take plenty of pics and maybe do a small write up on it.

 

[CCATLETT1984]

just be sure to put the diesel governor in the trans, otherwise it will do funky things (well i'm not sure with a full manual valvebody, but i wouldnt want to have to drive that around much, they shift HARD.)

are you going with the upgraded sun shell?
and yeah you will have to change the driveshafts. not by much, but enough to need lengthened/shortened. iirc

 

[Croatan_Kid]

Yeah, I need to get the diesel governor set forgot about that. Oh, and we're not running a full manual on this one, just the Jeep. On the sunshell, I'm lookin me one. Then for the driveshafts, the rear will have enough slip to run it temporarily and I'll just take the front one out. I'm actually thinking about getting a different t-case that I can run a SYE kit on and then get me some new spiffy shafts made up. I think the case on the 700 is only an inch shorter and the adapter is a bit thicker, so it'll be at least driveable. If not, we'll make it longer so I can drive it home which is only a few miles.

Edit: I went and researched it on Novak Adapters knowledge base. The 700R4 is an inch shorter in case length than the TH400.

 

[CCATLETT1984]

ok, you'll have enough in the slip joint of the front axle to get it home with np, just have a shortened shaft made up soon so you dont bottom it out and crack the case.

the rear has way more than enough slip in it. you should be fine there.

 

[Croatan_Kid]

Yeah, I thought the rear would be fine and then for the front I can just take it out until I can get it cut down. Maybe I'll get my buddy to cut it down for me. It won't be balanced but I don't think it necessarily needs to be does it? You don't really run too fast in 4x4 usually. Like I said, it'll at least work until I can get some new fancy ones made