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8.1 / 4L85 highway setup

Jon81K5

Jon81K5

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I’ll start by saying I’ve spent the past few days researching this. I’m aware of the facts, so at this point I’m looking more for opinions considering this is in a K5 not the stock application.
Have a 91’ K5 with 8.1, 4L85, 5.13s and 37” tires. My end goal for the truck is to maintain the Offroad ability, but it eventually endeavor on cross country trips, so good highway manners are a must.

Going off the .75 OD found in the 4L85 as well as the tach, here are some numbers
65MPH - 2270
70MPH - 2450
75MPH - 2620

Not too far off from the same setup in Avalanche/Suburban with 8.1 w/4.10 option
65MPH - 2240
70MPH - 2410
75MPH - 2580
Pretty similar especially when you take unit account the larger rolling mass of the 37s
However 4.10s was for towing in the 8-10k range, something I don’t plan on doing in the K5.

Fuel consumption aside, am I chasing my tail considering swapping in lower gears for a few hundred RPM? The intent is to be able to cruise at 70-75 for 6-10 hours without issues.
I don’t want to swap any larger tire then a 37” since it seems to have the most tread options verses the mainly MT tread when you go any > 37” diameter.

Leave it be, nice quiet exhaust, set cruise for 70 and go? Or tear apart some axles ?

Thanks in advance
 
have you played with this site ? http://www.grimmjeeper.com/gears.html

your 5.13 / 37" @ 70mph = 2446 rpm
swap to 4.56 / 37" @ 70 mph = 2174 rpm

not a whole lot to gain there in rpm savings . the big block 8.1 tourqe curve will prob over come the loss in gear fairly easy . but as you said is the price point worth it ?

might be worth more in the end to do a 6l80 / 6l90 swap and get deeper first gear and double o/d . that would bring your 5.13 / 37" combo @ 70 down to the 2170 rpm range and i think a bigger bang for the buck . but this to will require some rework for sure .
 
Thanks k30, that site was exactly what I was looking for
 
Leave it be. That should be a workable combo for that size of tire.
 
I’ll pick up a vacuum gage and see what’s up, but based of the feedback I’m feeling better about leaving it be
 
8.1s can be made more respectable with a decent cam and tune. Easily getting above 13 on the highway. I mean not earth shattering but 15 is possible out of an empty one driven like an adult.
 
My 6.2 diesel is lucky to get 15 mpg...I'd glady swap it for an 8.1 and live with 2 mpg less!..then I could burn gas and rubber instead of more expensive diesel fuel !..:burnout:
 
Never calculated highway economy, so drove @ 65-70 for 35 miles today with a fairly aggressive foot and consumed just over 3 gallons... ~11MPG. Sweet! I can live with that considering the running gear under it. Don’t see why I can’t bump those numbers up with A/Ts and some engine tuning considering the out of balance M/Ts it’s on now.
 
This is part of the reason transmissions are getting more gears lately. I was having a similar dilemma of towing vs. cruising unloaded with 6.0/4L80E. I had been thinking of an add-on OD unit, like Ranger, but the 27% is clearly intended for vehicles without factory overdrive (double OD is 0.548). The Gear Vendors unit is better at 22%, but that's still too similar to the built-in gear splits. It seems like what a 4L80 needs to truly split gears is probably more like 15% (makes 1.258, 0.85 and 0.638 available). Then you look at the 6L80 and it can do all that plus the granny low...

4L80/85 (with GV)
1 2.48
1.5 1.934
2 1.48
2.5 1.154
3 1.00
3.5 .78
4 0.75
4.5 0.585
R 2.07

6L80/90
1 4.027
2 2.364
3 1.532
4 1.152
5 0.852
6 0.667
R 3.064
 
The ‘06 and up Allison is also appealing

1) 3.10
2) 1.81
3) 1.41
4) 1.00
5) 0.71
6) 0.61
R) 4.49
 
Yeah, leave the setup the way it is. I think you are setup good for lots of different scenarios. It's not going to be perfect in any one scenario, but get the job done pretty good on all fronts.

When I first swapped my 8.1/NV4500 into my V3500 (lot larger/heavier truck than yours I know, but still) I took it on a road trip with the truck not loaded and I got 11.5 mpg's. I got a custom tune on the ECM from Team 208 Motorsports and my goodness it woke up this stock 8.1 a bunch. I have no doubt I'm over 400hp and probably right about 500 ft lbs. I have not calculated the mpg's in a similar scenario as before since the tune, but my anticipation is that it's a tad bit better. I would guess I'm getting 12-13 highway with it now. All I know is loaded to the gills (had my camper in the bed filled with gear/tools/parts and my trailer hooked to the back towing my K5 with lots of tools/spares/parts...we were guessing 17-18k GVW) and it was getting about 8 mpg's on the trip. Considering how heavy that was...I was completely fine with those numbers.
 
Jon81K5 said:
The ‘06 and up Allison is also appealing

1) 3.10
2) 1.81
3) 1.41
4) 1.00
5) 0.71
6) 0.61
R) 4.49
Click to expand...
yaa but there HUGE units . @SpeedlabDan just did 8.1/allison swap in a crew cab and he cant even use the ord shifter kit had to go cable shifters . but he also did a magnum/205 combo . and adapters for allison to older stuff = $$$
 
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