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Old 01-21-2020, 06:53 AM   #1
chico68
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3.73 or 3.42 with 33" tires

Hello, K10 short body 350, sm465, np208, 33" tires. Will be daily driver and occasional towing lawn equip. trailer. The question is 3.73 or 3.42, thanks.
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Old 01-21-2020, 10:35 AM   #2
mongocanfly
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Re: 3.73 or 3.42 with 33" tires

What gears do you have now?...
Just keep in mind a well worn 33in tire and 3.42s will give you about the same rpm as the 3.73s and new tires
I'd think 3.42s
Heres a rpm calculator
http://www.csgnetwork.com/multirpmcalc.html

3.42 gears..33in tires...1:1 final in trans ...65mph. =2263 rpm

3.73 gears..33 in tires...1:1 final in trans...65mph. =2469 rpm
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Old 01-21-2020, 10:38 AM   #3
Palf70Step
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Re: 3.73 or 3.42 with 33" tires

Personally, I would go with the 3.42 for the lower cruising RPM since your not towing or working it much.
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Old 01-21-2020, 10:50 AM   #4
Ziegelsteinfaust
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Re: 3.73 or 3.42 with 33" tires

My K20 had 3.42 rear gears for awhile till I swapped in matching 3.73's with the front. My tires were 285/75/16's which worked out to 34" on the pyo rims roughly.

It was fine for driving, and hauling 1000 pounds of junk. After that it got weak pretty quickly. I had a hopped up 260hp 350ci crate motor.
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Old 01-21-2020, 12:23 PM   #5
kwmech
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Re: 3.73 or 3.42 with 33" tires

3.42s or 3.73. I have 4.56s, but I have an overdrive
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Old 01-21-2020, 02:29 PM   #6
Mattchu60
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Re: 3.73 or 3.42 with 33" tires

With no OD I'd go with 3.42s, better for driving over 65 mph (interstate driving).
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Old 01-21-2020, 06:09 PM   #7
chico68
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Re: 3.73 or 3.42 with 33" tires

Thanks for all the opinions, it now has 2.73's they have to go, I don't know why they would even offer those in a 4x4. My 82 k10 stepside had 3.08's and those were awful.
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Old 01-21-2020, 06:49 PM   #8
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Re: 3.73 or 3.42 with 33" tires

During most of the square years of 73-87, mfgs were dealing with the problems of mileage, emissions and changing gasoline formulations. Performance took a back seat. 2.73s kept the engine rpms down. Many of the EPA tests didn't really care if 0-60 was measured in minutes. And the OEM tires may have been smaller which could have bumped performance a bit.
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Old 01-21-2020, 07:58 PM   #9
mongocanfly
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Re: 3.73 or 3.42 with 33" tires

my 85 had a sm465 and 2.73s...what a joke it was
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Old 01-21-2020, 08:22 PM   #10
1976gmc20
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Re: 3.73 or 3.42 with 33" tires

Funny, my 1980 was never a problem, except 4th gear was basically overdrive so you had to be about 35 mph. I just ran the stock tires, though.

It actually worked pretty good for towing my camper trailer. I could shift to 3rd on a hill and roar up the hill at 60 mph. I even towed my Cat on a heavy rented trailer over a high mountain pass. It went up just fine (2nd gear about 30-35 mph) but not so fun going down the other side. Didn't do that again.

Oh yeah, that pickup got 18 mpg at then legal 55 mph, and still 15 mpg at 80 mph Was a bunch better than my 73 that hardly got 10 mpg.
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Old 01-21-2020, 09:12 PM   #11
TwoFiftyShifter
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Re: 3.73 or 3.42 with 33" tires

To each their own. My 82 K10 has the 350/465 with 2.73s and I have no issue. 1st is still low, 2nd to take off, 3rd around town and 4th for 55+ mph. If I were to tow, I would never use 4th. I don't tow any trailers though.

Having had trucks with 2.73, 3.08 3.42 and 3.73, I would ask how often you run 70+ mph. If that is a rare occurrence, then you might like the 3.73s. You should get about 2600rpm at 65. Remember a "33" will measure about 31.5 when mounted. My 31s are 29.5 or so.
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Old 01-22-2020, 08:55 AM   #12
Mike C
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Re: 3.73 or 3.42 with 33" tires

Our 81 Jimmy had 2.56 for all you guys that think the 2.73 were bottom of the barrel.

My 72 Jimmy was 3.07 and with the 4 speed was actually a pretty nice combo with 235 75 15 tires. Was a DOG with the 33s I put on it. 4.10 are perfect with 35s and the SM465.

If you EVER plan to even consider 35s get the 3.73. If 33s are the top of the ladder and no overdrive, the 3.42 works fine.
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Old 01-22-2020, 07:44 PM   #13
chico68
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Re: 3.73 or 3.42 with 33" tires

No I don't ever plan on 35's, 33's are the biggest. I had 33's on my 82 k10 short stepside 350, sm465, 3.08, and it never really got great gas mileage and not great for towing either. Want to do better, trying to find the middle of the road gear, better mpg. and ok to tow.
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Old 01-22-2020, 10:22 PM   #14
Mike C
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Re: 3.73 or 3.42 with 33" tires

In the early 70's they chose gear for good all around performance not just for mileage. And the 3.07 gears with 28" tall tire is what GM thought was best for the 350 powered 1/2 tons.

33-28=5. 5/28=.17857 So the 33" tire is 17.86% taller than the 28.
3.07x17.86=.5483 and 3.07+.5483=3.62 So 3.62 is 17% more gear than 3.07

So to duplicate what GM thought was a good all around gear ratio you would want to go with a 3.62 gear. Since that is not a choice, you can go a little bit more gear with 3.73 or a little bit less with 3.42. With an auto and converter slippage along with 75 mph speed limits common, I would run the 3.42
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