Help with gears

[TX3GMC]

I am looking at a 85 k10 swb. Has 12 bolt in the rear 10 bolt up front both with 4:56 gears. At 60 mph it's at 3300 rpm. I need to be able to drive this truck at 75 mph, for 60 mile round trips weekly. what will be my best option for replacement gears. Tire size is 35/12.5/17 and is automatic
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[b454rat]

3.42s will keep your Rs lower at that speed, but will be lacking with pulling power and get up n go power. I'd say 3.73 to 4.10s...I had 4.10s with a big block, 4-speed and 35s, was OK. Seemed like it needed OD, and never a problem with power with the 454.

[wilkin250r]

My calculator says 4.56 gears and 35 inch tires comes out to 2700 rpm at 60mph. That's still high for 60mph, but I don't get 3300 unless it's either 29 inch tires and 4.56 gears or it's 35 inch tires and 5.56 gears.

But I can still answer the question. 1800 to 2200 rpm will yield the best gas mileage, but 75mph is pretty fast so we'll let it rev a little higher.

3.42 would be about 2500 rpms @75mph, and would probably be your best bet if 75mph for 60 miles is your primary concern, but if you're ever going to be towing with this vehicle or using it for other uses, 3.73 might be better. It'll rev to about 2800 rpms at 75.

[TX3GMC]

I don't know much with 4x4 would you need to change both gears or just the rear?
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[wilkin250r]

You need to change both.

If you were ever in a financial bind and there's just no possible way to change both, you could change just the rear as long as you NEVER put it into 4-wheel drive. But if you ever want to use 4wd without breaking anything, the front and rear must be the same.

[obijuan]

85 should have a 700r4 and it sounds like the converter isn't locking up.
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[b454rat]

Not always. Seen many trucks that where TH350Cs. Even if it was a 700, it would still go into OD, and wouldn't be turning that many R's.

[wilkin250r]

Agreed. The 700R4 would still shift into overdrive, and although you'd see slightly higher RPMs if the converter wasn't locking up, it would still be lower than a 1:1 drive ratio. No way you're losing 40% of your revs in the converter alone.

[Johnny G]

Originally Posted by obijuan

85 should have a 700r4 and it sounds like the converter isn't locking up.
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I think 700r4 was an option back then. I had an 86 GMC, it had a 350t.