How to determine gear ratio?

[sixtyfour]

I think my next big project will be re-gearing the rear axle. This will be on my 64 with 4 on the floor (1st granny gear) and a 12 bolt rear axle. Not sure what gears are in it now but I wouldn't be surprised if it had 4:11's. Motor is pretty much screaming (4000 RPM) at 75mph.

So my main question is how do I determine what gear ratio I need? I tend to drive pretty hard and fast, would like something with good acceleration on the street and good cruising speed on highway. Would I need any special tools for something like this?

Thanks
Mike

[kevindtimm]

A better solution would be an OD transmission. You can go auto (700R4/4l60 for example) or manual (like a T56).

One other thought though, with the granny 4 speed you might be able to put a lower numeric gear in and then use the granny gear rather than starting in 2nd gear. Something like a 2.70/3.10 range might be nice. Don't know about finding one for a truck though, I think these 10 bolt carriers won't fit a number like that.

Personally, I would look at the OD transmission first unless a strong rear end made itself available to me with a lower number ratio very cheap.

[LILRED66]

Factory gear ratio of the '64-'66 Chevy, Series 10 (1/2 ton) trucks was 3.73:1. The surest way to check gear ratio is to pop the cover and count the ring gear teeth and pinion gear teeth. Divide the number of gear teeth on the ring gear by the number of teeth on the pinion gear. The result is your rear differential gear ratio.

There are other methods, but they are not as accurate.

[jchav62]

There is usually a number stamped on the ring gear. ##-##. This is the number of teeth on each gear. Divide the two numbers and this will give you the gear ratio...

[sixtyfour]

Not really trying to figure out what's in it now but rather what to put in it next for better drive ability? How do I figure what to go with?

[Hillbilly Deluxe 65]

i like the 3:73 gear for street useage

[04Xrider]

You did not mention what engine you have. If it was mine and I had the 230-250 six, I would go with the 3.73, if it had a 327-350 I would go all the way up to a 3.50 or better yet 3.08 With the 4 speed tranny you should enjoy everthing you want with the higher gears including a lot better MPG's.

[MikeN]

Originally Posted by LILRED66

Factory gear ratio of the '64-'66 Chevy, Series 10 (1/2 ton) trucks was 3.73:1.

Agreed...almost all 64 C10'shad 3.73's...very difficult to find a factory rear with anything but a 3.73

[jchav62]

With a non-overdrive tranny, I would suggest 3.50's or 3.08's if you are doing alot of freeway driving. I have 3.73's in my 62 and with the overdrive on it's okay, but without, it's screams 3700-4000 @ 75-80.

[neto]

Since we are talking rears. I couldn't find bolts on the outside of the backing plate of my 63. Is this correct? I got the disc brake setup from CPP for the 63 and was going to use the backing plate for my 66 since one side is damaged. Thanks.

[sixtyfour]

Originally Posted by 04Xrider

You did not mention what engine you have. If it was mine and I had the 230-250 six, I would go with the 3.73, if it had a 327-350 I would go all the way up to a 3.50 or better yet 3.08 With the 4 speed tranny you should enjoy everthing you want with the higher gears including a lot better MPG's.

Running a 350ci.

[greg64]

Hey sixtyfour,

With a 350 and an SM420/465, I'd go for 3.08. I have an SM465 in my 77 and 3.42 rear gears. It runs about 2800 at 65, which is faster than I'd like, but I only have a 250 six, which wouldn't match the HP of the your 350. I'm sure the 350 would have the torque for 3.08s, plus if you're pulling a heavy load, you can always use the granny low (6.55:1 in the 465). As they say over at Novak, it's the gear span that's important, not how many gears you have. The SM420 & 465 both have more gear span than any O/D tranny (manual or auto), and they are damn near indestructible.

Heck, you could probably make 2.76:1 work, but that would require a different carrier. If I'm right from memory on my 12 bolt knowledge, the 3.08 is the highest gear you can run on the carrier you have now.

3.08 with a 235/75R15 tire would give you 2850 RPM at 75 MPH.

[blackedoutharley]

My '65 has the Sm420 trans and was originally equipped with the standard 3:73's
This set up with a 30.5" diameter rear tire was a blast to drive but was murder on the freeway; especially when I commuted to the bay area daily.

When I finally blew up the rear end beyond repair I swapped in a used rear end from what I believe was a '69 or '70 that had 3:08's.

This thing rocked on the freeway! and with the stroker motor was still fun to drive.
When I launched the pinion yoke (and the rear driveshaft) I installed a new set of 3:42's and this is the best of both the 3:73's and the 3:08's.

Don't forget that tire diameter will affect final drive ratios so take that into account. There are no "special" tools required but if you set up the gears incorrectly you will have to do it again with most likely alot of new parts (=$$$).
If you are uncomfortable it may be best to have somebody else do it.
There are several online gear calculators that will give you info on what ration is best but the intended purpose of the truck will ultimately dictate which ratio you choose.
This is not a quick fix if you pick wrong so invest some thought before dropping the dollars on it.

At 75-80 mph I am seeing about 2700 rpm

FYI my truck is as follows for your reference:
'65 C-10 LWB with SM420 trans, 31x10.50R15 Tires, 12" clutch, 391" small block.

good luck!

[koolkat]

Originally Posted by Hillbilly Deluxe 65

i like the 3:73 gear for street useage

I have a '65 c-10 with a 3.73 rearend. Right now it is 3-on the-tree that I want to convert to a 4-speed. What is a good tranny to get?

[PrerunnerRob]

What tire o.d. are you running?
http://www.rocky-road.com/calculator.html
http://www.dirtroad.com/gears.htm

[koolkat]

P255/70R15's