12 bolt gear ratio

[69gumby]

Can someone tell me the stock gear ratio for a 69 with a 350/350? I'm getting ready to swap out my rear end that must have something like 400s or so in it.
I run 65 MPH max. The rear end I am installing has 307s. I was just wondering how these were going to work and if they were the stock gears for the truck.

[71tahoe]

If you're running 65 max, you either have something closer to 5.13:1 or your tranny is slipping... BAD. 3.07 or 3.08 came stock in v8 trucks

[69gumby]

No slipping this big old truck flies from 0-65. The man I got her from used her to tow a camping trailer back when the speed limit was 55 MPH. At that time this was perfect, tons of torq when your taking off from 55 and towing, I tow a boat and it is nice. This is California and I get run over @65. I just want to go faster at a lower RPM. Thanks, I was hoping this was a stock ratio.

[glock35ipsc]

3.73 was also an option.

[kcjones]

My 67 C20 currently has an Eaton HO52 with 4.11. I run 3100 or so RPM on a 30" tire at 65.

If you're running higher than ~3000 RPM at 65 with normal size tires (between 27-30"), you've got gears lower [numerically higher] than 3.73.

[Longhorn Man]

if nothing was chainged, and no options listed for gearing, your truck should have the 3.07 gears.

[kcjones]

I'm guessing a 2,100 rpm at 65 with 3.07 and 29" tires? That seem right?

[glock35ipsc]

What is your tire size (P235/75-15, etc.), and what RPM are you running at a given speed?

[Psycho71]

Mine, with 3.07 gears and 255/70/15 tires runs 2500rpm @ 65mph. Don't know what it runs with the 275/60/15's that are on it now, since my cheap tach took a crap. Sometimes it's better just not to know.


Jay

[glock35ipsc]

With the 255's, you are right at 70 mph instead of 65 mph.

With the 275's, it should be 73 mph at 2500.

[kcjones]

A 255/70/15 tire is 29" tall
A 275/60/15 tire is 28" tall

A 29" tall tire will give you 2576 RPM at 60 mph with a 3.42 gear and 1:1 final gear. With a 3.07 gear, you'll spin 2312 RPM.

3.73 gears: 2809
4.11 gears: 3095

If you ran a 27" tall tire, 1:1 final gear, 3.07 gears, you should spin 2292 RPM at 60 MPH.

[69gumby]

You guys have been great. I am almost finished installing my new rear end with 307s. I may have to think about some gears in the 350 range, they sound like the best of both worlds. I can tell you I will miss the off the line performance I have been getting form my old gears, those little Imports are amazed to see this big old truck in front of them, but I can't wait to cruse down the freeway at 70 MPH.

[kcjones]

Maybe it's time for a 700R4 and 4.11's!

[69gumby]

That is something I have had on my mind but for now this will have to do. I have more work to do on the truck and I have this other project.

[leddzepp]

Originally Posted by kcjones

maybe it's time for a 700r4 and 4.11's!

yes!!

[leddzepp]

Is that a Chris Craft 69gumby?

[69gumby]

Yes Sir, a 1953 19' Chris Craft Holiday and I must say she looks real nice being towed by my truck.

[leddzepp]

I thought so...love the classic boats too. My dad has a 59 Biesemeyer; we also had a Chris Craft growing up, but it is long gone now.

[69gumby]

I found some axle codes. The code is stamped on the top passenger side axle tube. The code on my rear end reads HB 03 25 W. The HB is the gear ratio, 03 is the month it was built and the 25 is the day. These codes are for the
C-10 series.

HB,LL,RA.TR,TT,TU are 3.07s

HA,HJ,HQ,MF,TQ,MI,MK are 3.73s

HK,HL,HR,ME,MG,TS are 4.11s

HC,HN,MU are 3.73 Posi

HO,HP,JQ, are 4.11 Posi

[69gumby]

OK. The new rear end is in and now I can fly down the freeway. I found a rear end with 3:07 gears at the pick and pull for $50.00 on half price day, brought it home set it on some saw horses and went though it. I took it all apart (except for the gears) new wheel bearings and oil seals, cleaned and painted, rebuilt the brakes and put it all together with new bolts. Then just made the swap. Doing all the work on saw horses sure made things easy.

The only problem is the new rear end under there make the rest of the underside look like... well you know what.

Thanks for the help everyone.

[70GMCer]

You better get busy on the frame. That pretty rear end & exhaust makes the rest look bad......

[jvs99560]

What torque converter stall speed did you use?