Low gears, high RPM's, and the gas station. Near Portland, Oregon

[longorange68]

1968 C-10
250-6
SM465

Been awhile since I have posted here, about a year to be exact. I would like to drive my truck a lot more since I got it running again, but all I can seem to get out of it is about 53 mph. I probably could get more speed out of her, but my desire to go faster is quickly trumped by my desire not to blow up the motor. In a perfect world I would go with a Gear Vendors overdrive and not look back. Here in the real world, I am thinking more along the lines of changing out the gear ratio to something a bit taller, like 3.08's.

I did get a quote for this work with a drive in drive out option from Body Armor in Vancouver, WA, which came in around $1700. yeah, not so much...
I was told that I could only go posi with this ratio in a 12 bolt, is this correct?? This is going to be a driver, not a street warrior I would rather not if I didn't really have to.

I need some tips and insight, my grandfather used to be the go to guy for that sort of thing so I never really thought twice about rear ends and trannys. He has since passed, and I parked this thing the day after his funeral. That was 6 years ago. I would like to really enjoy it once again, but I don't have the time to do this particular work, and I would rather not be parted from too much of my cash. Need some advice.

JM

[Bob 68327]

Wrong you can put 308's in it . You may have to change the carrier but alot of these trucks came with 308 open diff. Mine had an open 308 in it .

[tll74]

Putting 3.08 is no problem. You WILL have to change the carrier unless you have 3.40 gears which it does sound like. 2.76,3.08-3.07,3.40-3.42 all use a common carrier. The 3.73,4.11, and 4.56 use the same carrier. They may be telling you that do to the availability of the open carrier new. They are plentifull used. This assuming you have the 12 bolt differential. PM me, I know someone in Portland that can help ya.

[67_C-30]

$1700 is crazy expensive for a gear change. If it were me, I'd change the whole rearend out from a truck that already had 3.08's. Of course parts and parts trucks are a easy to find here. They may not be where you live. Either way, new 3.08 gears and all bearings and seals for your rearend + labor shouldn't much over $500.

BTW, Bob is right, the vast majority of 3.08 geared trucks didn't have posi from the factory.

[Longhorn Man]

swap the whole axle... junk yard cost shouldn't even be close to 150 bucks IMO.
Or, you could donate $25 tot he board, so you can post a WTB thread in the parts board, and do a WTB 3.07 rear axle.

[Brad]

You didn't say what rearend you currently have. I'm guessing it's probably a 3.73 or maybe a 4.10. You should be able to get up to around 70 with no problems with either gear. You really didn't describe your concern or issue other than you want to swap out rear gears. Based on my own experience several years ago with 3.08's, I really don't recommend them, especially if you've got intentions of using your truck as a truck regularly. You'll have to use more clutch with the 3.08 gear and unless you regularly use your truck at highway speeds, it's just not worth it. Also, I second what everyone else mentioned- it should cost you no more than about 500-600 in Portland. The place that gave you that price should be strung up by their testicals!

Now, another option to consider strongly is to install a V8 Camaro T-5 in your truck. I've got one in mine and they're very easy to install. I'll have to look for the link another time, but I've written about it on the board a few times. I'd be happy to help if that's the route you end up taking. The total cost is about $700 and you end up with an overdrive gear and a great useable 1st as well!

[shelby987]

I would tend to agree with Brad, I wouldn't want 3.08's behind my 6cyl. I have pushed my truck down the highway at well of 75~80mph with no issues.....keep in mind, you have 7 mains on 6 cylinders.....the bottom end is rock solid on these motors, and the 'relatively' short stroke of the 250 means it can take higher RPM's with lower piston speed when compared to a 292.

If fuel economy is your main concern, I would say buy a motocraft 500 cfm 2bbl, an adaptor plate to bolt it to your current intake, swap an HEI in, give it a decent tune up (plugs, wires, timing, etc), toss some seafoam in there to flush everything out (I actually do seafoam BEFORE the tuneup), and for a quick and cheap gearing change, if the tires are old, upgrade to slightly taller tires. I have driven my truck 400 miles straight with a 3/4t 4x4 on a flatbed behind me moving at ~60mph the whole way (trailer inertia brake constraints, not engine) with no issues (Think I made 15mpg). Even with gears changes, short of fuel injection, an RV cam, and a couple of tricks to make the drag on these trucks a little lower I think 16~17mpg is about the best you will get on the cheap....I hope to one day push 20mpg....but that will likely involve the above listed mods.....along with a GV overdrive.

[mrein3]

Figure out what your rear end gears are. There is a code stamped on the rhs, front of the axel tube. If you pull the cover you can count teeth and divide. There is an RPO code on the build sheet.

Beg, borrow, or steal a tach.
When you are going 53 mph, record what the tach is saying for rpms.

So lets say you figured you have 4.11 rear end gears and are in your top gear. You should be turning 2600 rpms at 52.69 mph.
If you had that same tranny and 3.08 gears you could go 70.31 mph at 2600 rpms.

Factory rear end gears GENERALLY are 10% different between gear sets. 3.73 is 10% higher than 4.11. (4.11 - .411 = 3.699).
3.07 is ABOUT 20% higher than 3.73. And on and on.
Newer trucks have a gear in between 3.73s and 3.07s, something like a 3.42.

If you swapped your SM465 for a modern, overdrive tranny, you GENERALLY get 30% lower r's at the same speed.

As Longhorn Man pointed out, your cheapest bet is to swap in a whole axel that already has 3.08 gears in it.

Be careful. This is where we get in trouble. If you have that new (used) axel sitting there, you may as well freshen up the brakes. As long as the brakes are new you may as well paint it up nice. As long as it is painted up nice you don't want to stick a nice shiny rear end into a rusty frame. You can't paint the frame right without removing the box. As long as the box is off you may as well paint it...

[shelby987]

I always just jack up the rear of the truck (if it is an open rear, only one wheel needs to be off the ground). Rotate the lifted wheel off the ground, and count how many times the pinion yoke rotates (just over 4=4.10's, just under 3.73, three and a half times 3.42, four and a half times = 4.56). It has worked for me many times without fail.
Make sure you get a tach that is 6cyl. friendly, some are 8cyl. specific, others have a dial switch inside the housing that needs to be changed depending on the engine. Again, I think with regular maintenance, these engines are more than happy at 3,000+rpm on the highway....

my 03 saturn turns 3,200~3,300rpm on the highway at 75~80mph.....I regularly drive 12 hours straight stopping only for gas.....and I would say my 250 is probably more stout than the 2.2 in my car.

[Musclerodz]

they are nuts if they think they can charge $1700. I can buy a new Moser rear end for that if I don;t get to fancy. Should cost anywhere from $250-$500 depending on parts and labor.

Mike

[Psycho71]

Yeah, for $1700 you could freight the rear end down here to Tx, I'll take your gears and carrier out (to use in my truck), and set you up with my open carrier and new gears, bearings, and all. Then freight it back up to ya.

I'd bet we could get all that done for $1700, or maybe a little less.

Find someone who hasn't lost their freakin' mind.


Jay