4l80e worth it?

[Tdubbyount]

So I've got a 79 k10 shorty, 3/4 ton running gear, mild built 350 and a fresh rebuilt sm465 paired with a 205. I just finished this project and I love every bit of it other than the highway driving! I was told the running gear had 4:10 gears. I'm running 35" tires, and at 70 mph I'm turning 3000 rpm. That gets me about 7 miles to the gallon. Not good. So I'm wondering if it would be worth it it swap out my precious sm465 for an overdrive tranny. I absolutely love having a 4 speed, and I know this transmission will never fail me no matter how hard I beat on it, but I would love to be able to head up to the lakes for the weekend without burning my whole paycheck. This truck gets a lot of off road use, and I tow with it occasionally.
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[Rubble]

Another option might be a NV4500, but either way it's gonna cost something to do the swap.You need to pencil out how many miles your gonna put on to save enough gas to justify the tranny swap.Trying to get decent mileage out of a lifted truck with 35" or bigger tires is asking a lot!! I know,have a couple of them!! I have looked at the same thing for my 86 & it still has the 465 in it. For the price of rebuilding a 4500 or 4L80E,then getting it in your truck,you might be able to buy a lot of gas.Just my thoughts.
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[Rubble]

Another thing to help your gas mileage is slow down,try to keep your speed around 60-65 and see what you mileage does.Unless your running for 7-8 hours at a time,that extra speed won't help you much in a trip under an hour.I used to drive truck & see this lots.
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[Ziegelsteinfaust]

I think the 4L80 or 85 is your only auto option that will stand up to your running gear.

Also I think your gearing is fine, and as the guy above said try to slow down a bit. I have 4.10's, and 32" tall rear tires. If I cruise 60 I have seen as high as 14-15 with bad head gaskets on a dead 350. I just did the head gaskets, and the carb so it should be higher more often. So if you slow down maybe you can get as high as 12mpg, but your tires are your biggest drawback here.

[Rubble]

Just something for you to compare to,my 86 is set up similar to your truck: very mildly built 350/465/208 with 4:10s on 35s with 7" lift. Aerodynamics of a barn wall,if I keep it around 55-60. 2200-2500 rpm,it will get close to 12-13 mpg.That's Imperial gallons..But you have to keep your foot out of it....
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[74 stepside]

It will be around $3000, but a Gear Vender might be your best solution. They make them for stick and automatics. So if you have a 4 speed now, it will split your gears to an 8 speed. I think for the money, it will make the drive more pleasant and pay for itself in fuel costs within the 1st year. Not sure how they work with 4wd, you'd have to ask them..Good luck and Bad A$$ truck!

[Shyguy]

I have a '92 GMC K-3500 and it seems to get the best gas mileage in the 58-60 mph range. It has a 454 and it gets right at 12.5 mpg unloaded and a little over 8 mpg towing our 26' 5th wheel travel trailer. This is with stock tires.

Are you sure your speed-o-meter is reading correctly?

[Tdubbyount]

Thanks for the advise guys! I've been looking into the gear vendor unit a little. Probably not something I can afford at the moment. I've seen a lot of 2wd 4l80e trannys around, how hard is it to convert? Just an output shaft and an adaptor? I'm not very familiar with these transmissions.

And Shyguy, my speedo is almost spot on. Give or take 2 mph
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[Ziegelsteinfaust]

They are easy to convert just pricey vs buy a 4wd already

[Mr. 250r]

Go all in and get the ZF-S650 6spd that was behind late model big blocks and duramaxs lol, strong tranny and you still have a stick and bragging rights

[andrewmp6]

If your good and lucky in 91 the crew cab trucks got the 4l80 with a np205 but was one year only.

[kbar]

I've got a connection for a used heavy duty 4l80 4x4 if you are interested shoot me a pm.