making my truck drivable

[jewels.]

Hi guys! I haven't posted in well over a year. But my daily-driver
car broke down, and I am now forced to get my chevy drivable.

I have a 1968 C-10 Stepside.
It originally had a straight-six under the hood.
(I replaced the six when it blew with a 350 V-8 small block.)
The truck has a three-on-the-tree shifter.
(Three speed on the column.)

The problem is that the truck tops out on the highway at about
55 or 60 mph. It is just racing at that speed.

After searching the boards, I found two possible improvements
that I can make to get more speed out of my truck:
1. Increase the tire/wheel size. (I am in the process of digging
up some original rally wheels to replace the small rims on her.)
2. Re-gear the rear end by changing out the ring & pinion (or the
full rear end).

My question concerns #2. I was trying to find out my current
gear ratio. After searching the boards, I found three possible
ways to determine my gear ratio:
1. Count the teeth on the ring and pinion gears and divide.
2. Read the stamped gear ratio off the side of the ring gear.
3. Rotate a rear tire and count the simultaneous rotations of the
drive shaft.

So, I didn't want to have to pull the diff cover quite yet
(I just did that recently anyways)
So. I tried #3.

When I turned my wheel:

1 full turn of wheel == 1-7/8 turn of drive shaft

Does this sound really super low?
What is wrong here?
I thought that if I had a 3.73 gear ratio, I would get 3.73 turns
out of the drive shaft for every turn of the wheel.
Is my truck geared really low?

Any suggestions you have on this, or anything else I have
commented on above would be greatly appreciated!

Thanks so much!

jewels

[COBALT]

... multiplied by 2 ~ 3.73 (3.75). Remember you're only turning one wheel. Think about what's happening inside the case and how it works with both axles.

Check your options sticker too, but the only way to make sure is to pull the cover and count, but your poor man's way gets you a pretty good idea.

[cdowns]

what you really have to do is jack up the rear on jackstands crawl underneath and rotate the driveshaft with tranny in neutral, you'll probably find that 4 1/2 turns on the shaft will tell you you've got a 4.56reargear

[JimKshortstep4x4]

When you only rotate one wheel, you can get inaccurate results.
I think that it is better to leave the wheels on the ground and roll the truck for 1 revolution of the tires. Doing one tire kind of works but you need to multiply the results by 2 because of the effect of the spider gears.

It seems as if you have a 3:73, so going with a 3:07 will be beneficial.

Jim

[Roberty]

Try finding a '71 or '72 5 lug rearend out of a V8 automatic truck. They tend to be geared for the highway (most are 3.07 geared). This way you get more top end speed and better fuel mileage and if you ever swap over to a disk brake front end you already have the 5 lug rear end under it. This would be the quickest/easiest fix IMHO.
Robert

[COBALT]

CDOWNS: He wouldn't have been able to rotate the wheel unless his transmission was in neutral to begin with. If his driveshaft turned 2 1/4 turns it would roughly be 4.56, but 4.56 gears in a 1/2 ton??? That's a stretch. Why not just buy a 3/4 ton truck to get the heavier suspension if you're going to tow heavy weights.

[jewels.]

Thanks to all the replies!
---------------------------------
Cobalt, thanks! As soon as I read your reply - I had an "oh duh" attack. I should have thought about it some more, and I would have figured it out.

(1-7/8 turn of drive shaft * 2 wheels) = 3.75 (approx 3.73 as expected)

---------------------------------
I probably will not change out the whole rearend because I am looking to stay with originality here. I would rather spend the money and change up the ring and pinion internally.
---------------------------------
I don't believe that I would have a gear ratio of 4.56 (implied in the reply). That sounds a little too powerful for my little truck.
---------------------------------
Overall, I think I have a 3.73. I am now off to find a good ping and pinion shop to do the swap for me. If anyone has suggestions for places in Northern Indiana, let me know. I will also have to do some shopping for the new gears. But I will probably try Randy's Ring and Pinion. (www.ring-pinion.com)

Thanks again!

jewels

[Roberty]

Originally posted by jewels.
But I will probably try Randy's Ring and Pinion. (www.ring-pinion.com)

jewels

I checked out your link, Jewels - They carry gear ratios from 2.56 all the way up to 7.17!! And I thought a 4.56 or 5.11 was extremely low geared - LOL.
Robert

[jewels.]

yeah - i saw a lot of suggestions on the board in the past for Randy's ring and pinion - especially for technical info about rearends

plus, my brother (who isn't a chevy person -- but he is a jeep 4x4 fanatic) swears by randy's as well ---- he has had lots of rearend changes (too much four-wheeling IMHO) LOL!

jewels

[COBALT]

Randy's Ring & Pinion (www.ringpinion.com) rebuilt my Corp. HO52 axle back when I knew nothing about doing it myself (They're local to me: 30-45 minutes north of Seattle). They did excellent work, even though it cost me an arm and a leg. Their parts selection is really good.

Good luck.

[gr8scott51]

They sold a lot of the short wheel base, 1/2 ton, 6 cyl, 3 spd C10s as fleet trucks. Mine was set up the same way, with 3/4 ton coil springs! I wanted to keep the pulling power, so changed transmission. Put in a 5 speed (w/0.72 overdrive) - now I can roam the freeway with the best of 'em.

[jewels.]

Originally posted by gr8scott51
... so changed transmission. Put in a 5 speed (w/0.72 overdrive) - now I can roam the freeway with the best of 'em.

Yes - a transmission change would probably be a good solution, but I don't really want to loose my 3-speed-on-the-column. Call me nastalgic, but I really like the column shifter. I am not sure how different transmissions would handle the shifter, so I have been reluctant to change the tranny.

jewels

[Long Knight]

What tires are you running?

[COBALT]

Yes, but third gear is still 1:1 isn't it?

[jewels.]

hmmm - i know that the rims are 15s, but the tires, hmm - i would have to go look at them ---

still, i *do* know that increasing the wheel and tire size would be beneficial

i will check the tires later this afternoon, and post a reply for you

jewels

[jewels.]

Originally posted by COBALT
Yes, but third gear is still 1:1 isn't it?

cobalt --- hmmmm, i'm not sure what you are asking

i guess i am hesitant to switch transmissions because i just simply don't know if a newer style transmission will hook up to the 3-speed linkages in the same manner. will it? i am no expert on tranmissions.

is that what you are asking about?

the transmission i have is most likely the original 3-speed tranny.

jewels

[Blue68]

With a 3 spd tranny (1:1 3rd gear) and 3:73 you should have no problems cruising at 70mph assuming all things are good. You should be able to do faster if you don't mind the noise. I ran the original 307 with the original 3sp for many years with no real problem going as fast as I wanted. I was also running about a 30 inch tire as well which helps some. I'd look at the other areas first before dropping a load of money to swap a rear-end that may not solve your problem.

On the counting the revolutions, the solution is to have both tires off the ground and turn them both IN THE SAME DIRECTION together to get an accurate count of revolutions of the shaft, or roll backwards while someone counts revolutions.

[jewels.]

Originally posted by Blue68
With a 3 spd tranny (1:1 3rd gear) and 3:73 you should have no problems cruising at 70mph assuming all things are good. You should be able to do faster if you don't mind the noise.

hmm - perhaps I am being a little timid with the engine.
I am not sure.
It just sounds really loud and noisy at 55-60mph, so I just assumed that it was struggling at that point.
Is there an easy way to hook up a temporary tach, so I can get an RPM reading? Perhaps that would help.

Oh - keep in mind that I swapped out the original straight-6-cylinder for a 350 V-8 small block. Would that make any difference here?

Oh - another thought. Perhaps the 2-bbl carb is the limiter here? Maybe?

Boy, I have really confused myself now.

jewels

[Blue68]

Maybe we can alleviate some of the timidity with some math. Let's make some assumptions about tire height and rear end ratio and that the speedometer is accurate. You can use the following link to get your engine RPM at a given speed.

http://www.700r4.com/speedocalc/rpmcalc.html

You should only be turning about 3000 rpm at 70 mph assuming 3.73 and 29 inch tires (which are a little tall, but barely). Hey, at 100 mph you are still only at 4300 rpm.

If you were to step to a newer tranny with overdrive you would drop that by over 800 rpm. A rear swap will buy you 500 or so RPM gain.

As for the noise, those 3 speeds can make quite a racket, I'll give you that. Especially if they've been driven without the clutch cover or the mounts are worn out.

If you have a handheld tach that you use to set the timing/dwell, you could run a wire from engine compartment into the cab and hook it all up temporarily. I'd use some zip ties or tape to make sure things stay put and out of harms way though. If you have a tach handy, hook it up too.

Oh, and the 2bbl won't restrict your speed to that low a number, perhaps your ability to get there quickly though.

Have fun.

[c-tenkid]

My truck has a 350 with a 3 on the tree as well.At 70mph I am turning 3500rpms.I normally cruise at 55 in 3rd which is 2500 rpm.


Does this sound right?


My tire size is a 255/70/15

[Dropt72Shortbed]

tops out at 55-60mph? what size are your tires? does anyone else think thats a little strange?

i have 3.73s also as my truck used to be a 6cyl. its also a v8 now, but mine has a th350 (not sure how much that tranny comes into play on this) i have 28" tires and although my rpms are kinda high on the freeway, my engine isnt racing until i go over 80mph. driving 55-60 is a comfortable low rpm for me

[jewels.]

Originally posted by c-tenkid
My truck has a 350 with a 3 on the tree as well.At 70mph I am turning 3500rpms.I normally cruise at 55 in 3rd which is 2500 rpm.


Does this sound right?


My tire size is a 255/70/15

yes - actually that sounds about right ---- it runs fine at 55-60 mph --- but it just sounds loud

i think i have had it as high as 65mph, but i didn't feel comfortable running at that speed - it felt like i had to "push" to hard to maintain that speed.

my best guess on tire size is close to what you have there. i will double check it in about an hour, and repost here

jewels

[jewels.]

Originally posted by Blue68
[B]Maybe we can alleviate some of the timidity with some math. Let's make some assumptions about tire height and rear end ratio and that the speedometer is accurate. You can use the following link to get your engine RPM at a given speed.

http://www.700r4.com/speedocalc/rpmcalc.html

You should only be turning about 3000 rpm at 70 mph assuming 3.73 and 29 inch tires (which are a little tall, but barely). Hey, at 100 mph you are still only at 4300 rpm.[B]

ok guys --- i measured my tires

i have 255/60/15's
and if i use the calculator suggested above by Blue68 (assuming a 3.73 gear ratio), then i come up with 2500 rpm at 55mph and 3500 rpm at 75mph. so, now i am beginning to think that it is just running super noisy at the high speed, and not super rpm's. and if a ring and pinion change will only gain me approximately 500rpm's then maybe i need to start looking at some other options.

Originally posted by Blue68
[B]
As for the noise, those 3 speeds can make quite a racket, I'll give you that. Especially if they've been driven without the clutch cover or the mounts are worn out. [B]

i don't think that the engine mounts are worn out. those are fairly new. i haven't touched the clutch since i bought it, so i think the clutch cover is still there. but a factor that i haven't mentioned (when it comes to noise) is that my hood is louvered with no noise padding. so, i hear the engine racing at high speeds right into the cab. there is also no noise insulation under the dash either. both the hood insulation and the dash insulation were removed by a former owner to make customization a bit easier (i assume). perhaps that is adding to the noise. i think i will take a minute to drive it around tonight and test some speeds again.

any reactions?

jewels

[oldsub86]

I think these old trucks are a bit loud at speed if you have not patched up the holes in the body and changed the weatherstipping etc. My 68 with a 292 and a powerglide is pretty noisy at highway speeds. I don't think there would be much point in my having a radio unless I do some body repairs etc. because I wouldn't be able to hear it. I think you need to get a tach before you start changing the rear end or the transmission. Buy an inexpensive aftermarket tach and put it in to see what rpm you are getting at highway speeds. It may be within spec and you need to do other things to make it quieter if that is your goal.

[shortbed70]

I've got a 4.56 gear and turbo 350 295-50-15 tire and turn about 3600 @60. Not real fun to drive on the highway but a blast in town until you hit second . Swaping out for a 3.42 sometime soon.