4.10 gears and 31 inch tires?!?!?

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stm3784

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hi guys I have a 78 K20 with a 14 bolt full floating rear, and 4.10 gears. Everything I’ve seen from GM back in the day was 4.10 was a standard gear ratio with 350 and 465/205.

My question is this truck will be used to pull a 7200# camper only if I need it to. So I intend on keep the short gears but living In PA we have hills. I’m not looking to win races, just don’t want to blow the motor. So is it doable?

Electric brakes will be added, and like I said stock 350/465/205 4.10 gears with 31s


As I said it’s only going to be in a pinch. That if I really need to.
Please let me know
 

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It's fine right here for this discussion.

If it's purpose is for "in a pinch", I'd leave it just like it is. Being a manual transmission helps about 300-400 rpm over what an auto trans would lose due to not having a lock up torque converter. Sounds like you'll need that low end grunt the 4.10 offers, not to mention expense to regear 2 axles since it's a 4x4. Also, it's a small block. Small blocks are not near as worried about higher rpms as big blocks are since the rotating assembly of a big block is so much heavier than a small block. With a 31 inch tire and 1:1 transmission ratio with 4.10's you're looking at 2900 rpm at 65mph. As a rule of thumb, you should try to consider 3000 rpm as your max long distance freeway cruising rpm. You're under 3000 at 65mph so if that works for you, then consider it very doable. If you need to get in it a bit a pick up the speed to 70-80 mph for a mile or so to pass some semi's climbing a hill or something of that nature, SEND IT, no big deal. It's a small block, they love rpm. 70mph is going to be 3200 rpm so even if you did that for a distance if in a hurry, you're probably not going to hurt it either. The 3000 rpm is as said, just a rule of thumb. Just make sure you're not running it low on oil and have 25lbs of oil pressure or better and call it good. Just my opinion, other's may vary.
 

stm3784

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Ok that works. I just wasn’t sure about what the smallblock could handle. If it ever does pull it, it’s a mostly 2 lane driving to camp so no highway. But my 1500 with 5.3 and 3.42 rear is the prime mover. Like I said it’s only if needed.
Thanks again
 

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It should handle it but keep an eye on the oil level. It can consume considerably more oil at continuous higher rpm
 

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Wasn't the stock tire size like a 28 or 29" tire?
 

theblindchicken

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Hmm.. I better go measure my old spare tire. Maybe it was a little smaller for the 1/2 ton 4x4's?
1/2 ton did get a smaller tire, 28.9"
3/4 received 31.7"
 

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The upside is you can bump up to a 33 pretty easily and for normal cruising it should be pretty close to the sweet spot, but with the granny gear you can still tear down a house.
 

bucket

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My 78's stock tires only measure out to 30", and they are even 95% tread.
Mine is right around 31".All I did is calculated the height from wheel and tire size. Possible that earlier squares had a bit smaller tire.
 

bucket

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Mine is right around 31".All I did is calculated the height from wheel and tire size. Possible that earlier squares had a bit smaller tire.

Yep, 9.50/16.5's opposed to the 235/85/16 found on later trucks.

So even if the "31's" in question measure smaller (highly likely) then it's still in the ballpark of what the factory put out.
 

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