83 K20 GEAR RATIO

[Old60Driver]

G'mornin y'all!

So, after some calculatin' this morning, I'm pretty sure I have 4.56 gears in her. Going out to confirm later this morning, but it's just too danged cold right now. I'm 99% sure the original owner (late father in law), never had anything modified on the truck, but it was used as a farm truck (hauling horses, trailers, etc..), so maybe he did for the extra low end grunt? I didn't even know that 4.56 gears were an option for a 350 K20 stock

Anyway, I very rarely get on the highway, but when I did yesterday, I noted the following:

60 mph (gps confirmed)
2950-ish rpm (digital)
285/75 R16 tires (32.83" tall tires)

That led me down another rabbit hole looking at gear ratios. I couldn't believe how much of a difference the gear ratio could make! It sounds like I could reduce my highway RPM @ 60 mph by 500 or so by going to 3.73's. Does that sound right? And if so, how much of a hit will I take on the 'around the town' cruising performance? She runs great, and I can bark them if I'm feeling frisky. And the roads are wet. LOL

I don't do any towing with her, but dang that highway RPM is insane so I'm thinking of making the swap.

Thoughts y'all?

 

[75gmck25]

Based on my references, the 4.56 was only a factory option for the K30, and 4.10 was the lowest ratio for a K20. However, it may have been swapped.

 

[WebMonkey]

my 85 k20 ultra hd has 3.73 with stock height tires.
i think it's a pretty sweet spot for off the line vs 60mph state highways.


'monkey

 

[SquareRoot]

Excellent gears! The issue is the 3 speed transmission. Put an overdrive in it and start living the dream.

 

[Buck69]

I have the 4.56 gears in mine. It is my understanding the carrier also requires changing if going to 3.73's. Something to include in your thoughts if you haven't.

 

[K5 Blazer]

I agree, the transmission is the problem, I would install a transmission with an overdrive and keep the 4.56 gear ratio. I just did an LS swap and installed a 4L60E transmission into my 73 Blazer. The Blazer has 3.07 gears which were way too tall for the new transmission. I am spending $4100 to have 4.56 gear ratio installed front and rear.

 

[Grit dog]

Based on my references, the 4.56 was only a factory option for the K30, and 4.10 was the lowest ratio for a K20. However, it may have been swapped.

According to the vehicle info kit with all the options, 4.56s were available in 80s K20s.
Plus I recall having one, back in the day. Was an 81 or 82. Base model K20, 350 and 4.56s. Not swapped.