Rear Gear Change

[6xchevy]

My 68 shorty came with a 6 cyl and 3 speed on the column and either a 3.70 or 4.11 rear. The 350 in it now with the same drive train is not happy at 55 or 60 mph, would it be a better choice to change the ring and pinion or to change the entire rear end to get a highway friendly set up? What rear end assembly would be the easiest fit? I am thinking about going to something in the 3.08 - 3.36 range. Thanks for any help.

[aruss99]

I have a 3.08 in mine and the engine runs around 2300 at 65 or so

[LongBox]

Probably very difficult to find, but the three-on-the-tree trucks had an optional overdrive unit. Would be neat if you could find one, and then you'd have the best of both worlds--a good gear for getting up to speed, and a relaxed gear (and a "passing" gear) for highway cruising.

[biketopia]

If it has a set of gears in the 3.70 range I'd leave it well enough alone. The engine is right in the it's power band there, just seems not happy since we are all used to modern cars that are quite at highway speed, I've got 3.73's in my tuck and it's goes down the highway just fine, and gets up and goes a lot better than the 3.08 that were in it. If you are really set on changing them though, if you are not comfortable setting up a gear set properly and don't want to pay someone to then your best bet is to find a whole rear diff to swap in. But you could get away with picking up a used set of gears in good condition. I see your in VA as am I, I am going through a truck I just got and finding parts to sell, might have what your looking for, once I set up my membership to sell I can let you know if it's something you could use. Do you have a coil spring or leaf rear? I'm assuming since it's a 68 it is 6 lug?

[TwoFiftyShifter]

They came with 3.08s. you can find one if those to know k your rpms down. My 71 is a 3 on the tree and dies 70 at 3000 rpms. Seems fine to me.

[GASoline71]

Trust me... that 350 is just fine at 3,000 RPM's. Most people are used to newer rigs with overdrive transmissions and FI.

Gary

[demian5]

3.08 or 700r4

[WorkinLonghorn]

I agree with the guys saying 2300-2500 is good on the freeway.

I have 31 inch tires (235/85/16) and 3.73 rear axle and it is fine on the freeway, about 2500@60.

My last rig had 33 inch tires and 3.07 axle (these trucks never came with 3.08, it is a different ring and pinion) and it was a dog on the freeway and got 10MPG freeway at 60 MPH. Cruise was 1875 RPM. Super low rpms are great with fuel injection but suck a lot of gas with a carb.

Both trucks were 350ci 4v with "RV" cams

[demian5]

Originally Posted by WorkinLonghorn

I agree with the guys saying 2300-2500 is good on the freeway.

I have 31 inch tires (235/85/16) and 3.73 rear axle and it is fine on the freeway, about 2500@60.

My last rig had 33 inch tires and 3.07 axle (these trucks never came with 3.08, it is a different ring and pinion) and it was a dog on the freeway and got 10MPG freeway at 60 MPH. Cruise was 1875 RPM. Super low rpms are great with fuel injection but suck a lot of gas with a carb.

Both trucks were 350ci 4v with "RV" cams

Dude were you running 44" tires or a 700r4?

I have 31" tires on mine with a "3.07" and a turbo 350 and it spins 2600 at 65 MPH. With the 3.73 it had before, it was 3400 to keep up with traffic.

Its not a dog with 3.07 and it hauls ass with a stock motor. Yes it was peppier with the 3.73 but now its got some long legs and can cruise comfortably.

[WorkinLonghorn]

Originally Posted by demian5

Dude were you running 44" tires or a 700r4?

I have 31" tires on mine with a "3.07" and a turbo 350 and it spins 2600 at 65 MPH. With the 3.73 it had before, it was 3400 to keep up with traffic.

Its not a dog with 3.07 and it hauls ass with a stock motor. Yes it was peppier with the 3.73 but now its got some long legs and can cruise comfortably.

Dude, Go to this site and enter the ratio (3.07) tire size (33) and speed (60).

I actually did the math without this site 25 years ago and got the same answer; 1875

https://www.ringpinion.com/calculators/Calc_RPM.aspx

[jocko]

Highly recommend an overdrive trans over the gear or whole rear swap. Rear ratio changes only will always be a tradeoff. True that the 350 is fine to cruise at the rpm it did back in the day, but od transmissions are a good thing. Better low end torque and lower highway rpm. Save up, but it's not a lot more than a rear end or gear change. Worth the difference if you can afford it.

[weq92f]

Since we're talking tranny swap... I think a new 700r4 can be had for ~2000 bucks with converter. Might take a few hundred more to get it installed and integraged ( X-members, shaft, shift, linkage, etc ).

New 3.73 posi ( Eaton ) can be had for ~650 shipped.

Just sayin.

-klb

[67chevy1]

Originally Posted by weq92f

Since we're talking tranny swap... I think a new 700r4 can be had for ~2000 bucks with converter. Might take a few hundred more to get it installed and integraged ( X-members, shaft, shift, linkage, etc ).

New 3.73 posi ( Eaton ) can be had for ~650 shipped.

Just sayin.

-klb

This is the exact route I chose, but haven't got the rest of the truck put together to see how I like it yet.

[weq92f]

My speedo indicates about 10MPH faster than actual speed here... BB402/700r/3.73/20" rear tires at 295/40 ...

[6xchevy]

I don't have a tach installed but at 50-55 mph the engine is turning way over 3,000 rpm. I have a couple of other cars that run in the 3,000 rpm range at highway speeds and this one is turning a lot more with 235X70 15 tires. Probably a 4.11, I will do the drive shaft rotation count to make sure. Truck has 6 lugs and coil springs, custom seats with console and no clearance for floor shift lever. Would like to stay with column shift 3 speed so a complete rear change is a definite possiblilty, what rears will work with the least amount of adaptation? Thanks Again

[1972RedNeck]

Originally Posted by 6xchevy

I don't have a tach installed but at 50-55 mph the engine is turning way over 3,000 rpm.

3100 RPM @ 55MPH with 29" tires means you would have to have 4.88:1 gears. While this is possible, it's not likely.

Get a tach and figure out what your gear ratio actually is, and go from there.

[MikeB]

My 69 C10 came from the factory with a 307 V-8 and a 3.08 axle. Even with the 3.08, I always wished I had an overdrive at anything above 60 mph, especially after I installed a 350 V-8.

How fast and how often do you drive on the highway? And how tall are your rear tires? Also, does the engine pull happily from 2000 RPM? If you like to drive 65-70 and have 28"-29" tires, I'd say a 3.08 would work for you. If you don't spend much time at those speeds, then a 3.42 would work well. Assuming it's a 12-bolt truck rear end, you don't have very many ratio options. Also, if you drop to 3.42 or 3.08, you will need a "3-series" carrier.

My 82 C10 has a mild 350 and a 2.73 axle (8.5" 10-bolt) and 29" tires. It's great at 70mph, but definitely hurts acceleration. I'm thinking about going to a 3.23 because the truck doesn't spend much time on the highway.

I like to deal with these guys. Their YG gears are quieter than the ZG.
https://www.ringpinion.com/b2c/Parts...RWD&DiffID=192

[LongBox]

Just wanted to add two points to this thread: Without actually using a tach to see the rpm, you are judging this just by sound. I would check that you have a clutching fan, which reduces the fan revs substantially. I know from experience that the fan noise gives the impression of high revs.

And, while it is true that the old 350s and 307s would cruise along happily at 3000 or more rpm, they did so at the expense of terrible fuel mileage. I used to own a 67 Chevy G20 van with a 283, powerglide, and a 4.11 rear axle. It was lucky to get 12 mpg.

Agree with the posts suggesting an OD, either a 700R4, a Gear Vendors unit, or an old OEM OD, if you can find one.

[TwoFiftyShifter]

I am getting 13mpg commuting to work at 70 (3000) rpm. 350 Qjet. I am thinking that the 3.08s would lower me to 2500rpm and probably net me 2-3 mpg. OD transmissions are a but of work to swap in for a durable manual tranny.

[88Stanger]

I had 3.08 gears with non-posi in my truck and just installed a new set of 3.42 gears and new Easton posi. I am running a turbo 400 with V-8 built and basic stock style height tires. Not sure of the RPM yet as I just got the gears installed, but I personally did not buy this truck to be a "gas mileage" rig, but rather for a fun rig that goes really good but yes does ok on gas on the highway, but then when that person rolls up next to you at a light and jacks ya, well you know the rest. I can tell you that going to an overdrive tranny you may have a lot of things will need to be converted over, ie. driveline length, shifter, electronics depending on tranny type. Gears on the other hand, well you either have the equipment and knowledge to install them or have someone do them for you. either way, there is work and or money to spend.

[James the III]

Originally Posted by 6xchevy

My 68 shorty came with a 6 cyl and 3 speed on the column and either a 3.70 or 4.11 rear. The 350 in it now with the same drive train is not happy at 55 or 60 mph, would it be a better choice to change the ring and pinion or to change the entire rear end to get a highway friendly set up? What rear end assembly would be the easiest fit? I am thinking about going to something in the 3.08 - 3.36 range. Thanks for any help.

How short are your tires??
you might try, putting a used set of taller tires on the rear first.. then if you like it, buy new tires..
you might be surprised what a change from 26" tall tire and a 28-28.5 tall one will do..
and is easier and cheaper..


to give you an idea.
a 370 rear geared 26" tall tire with a high gear on one to one (no o/d)
at 65mph is 3108 rpm

same thing with a 28" tall tire is running 65mph at 2886 rpm
31" tire =2607rpm



308 gears and 26"tall tire at 65=2520 rpm
""""""""""""'''''''28" """""""""""""""=2402

[TwoFiftyShifter]

How the heck can you determine rpms without a tach?

[1972RedNeck]

Originally Posted by TwoFiftyShifter

How the heck can you determine rpms without a tach?

Speed, gear ratio, and tire size.

[TwoFiftyShifter]

Originally Posted by 1972RedNeck

Speed, gear ratio, and tire size.

I meant while driving...

[LongBox]

Yep, there are sites on the net that will give you the revs per mile when you plug in the tire size. Just google tire size calculator, or something like that.

A typical tire will run 720 revs per mile, which is the same as 720 rpm at 60 mph. Multiply by the rear gear ratio, say 3.73:1, and you get 2,686 engine rpm.