worse than i thought

[ezz16]

I knew the donor car for my truck was a 77 Monte Carlo. I knew the rear end was a low numerical gear. 2.73 I assumed, but today I decided to check it. counting the rotations of each and it turns out to be 2.56. the truck runs great above 45 mph.while the width is great and it is in and working. I think a 3.31 3.42 would work better maybe even 3.08 I understand I can change the ratio if I buy a new carrier also. is this right? and has anyone done this? or are you just buying a rear with the ratio you want. the truck has a 283 mild cam and th350 so its not a hot rod.

[mongocanfly]

I'm guessing you have a 10 bolt rearend..you don't have to change the carrier to change gears..you can buy just a ring and pinion set..then get a knowlegable installer to set it up for you..before you just pull a ratio out of the air I'd recommend you use a rpm calculator to determime what will work best for you. But it may be cheaper to find you a complete new rear end with a ratio you like..http://www.csgnetwork.com/multirpmcalc.html

[Coupeguy2001]

Why don't you install a 700R4?
first gear is 3.06, and is what that rear end waas designed to use.
With the 700R4, and 2.56 rear, your 283 would be pushing 1700 RPM at 70 mph, giving you about 22 miles per gallon .
Your biggest problem would be finding every foot pound of torque you can get out of that engine. An RV cam would help a lot, as would heads and an intake or a stroker kit.
But it's a 283, those pieces would be expensive.

It's a big deal to change the ratio on that rear end.
You need to replace bearings, pinion bearings, shims to align the ring gear with the pinion, and while you are at it, pinion seals, and a fixture to hold the rear end from moving while you compress the crush sleeve to preload the pinion bearings.

Other than that, it's just buying a ring and pinion with the ratio of your choice and putting it in.

[mongocanfly]

700r4 with a 2.56 rear???..that engine would be barely above a idle at highway speed..the 700r4 4th gear is .70..you've got to factor in tire height as well

[joedoh]

should be an 8.5 10 bolt but could be a 7.5 10 bolt. look at your rear cover, if its round and has 10 bolts it will be a 8.5, if it is roundish but squared off and has 10 bolts it will be a 7.5. either one is ok for a mild engine although the 8.5 is much stronger. saying that, lots of guys will complain about 7.5 but it was the rear in lots of full size cars trucks and even camaro/firebirds.

changing rear gears is a bit of an art, and the artists charge well for it. you will want to order a "master install" kit with your new ring and pinion, it is bearings and shims that you should replace while you are in there. most shops require the kit because they are warrantying the work.

if you want to tackle it yourself, the master kit will come with some marking grease, you will need to make certain the pinion gear is shimmed correctly to the ring gear by wiping on the grease and assembling it and turning it slightly. there are good how to's online so I wont go into exhausting detail.

my advice? find another rear end. much the same way that "observation changes the outcome" (and conversely how schrodingers cat is still "alive" if you dont open the box), opening a perfectly working part is a great way to ensure problems with either it or something you take apart to get to it. just me though, lots of rear ends in junkyards.

[1project2many]

Lotsa stuff to go over...

joedoh is right on with differential sizes for 77 Monte. Almost always 8.5" but 7.5" is possible. Easy way to ID axles is with pictures. Bottom axle in photo is 8.5", top is 7.5". If yours looks different it may be an 8.2 or it may be from an older Pontiac, Olds, or Cadillac which are all special cases. Take a picture and post it here if unsure.


If axle is 8.5" then you are in a bit of a spot with 2.56 gears. As a rear axle gear ratio gets numerically lower the pinion gets larger. Eventually the pinion can get so large that the ring gear would be too thin to survive. The solution is to move the mounting flange for the ring gear farther away from the pinion. You get into trouble when you want to switch from a 2.56 to a gear ratio with a smaller pinion. Originally 2.56 and numerically lower ratios used one carrier while 2.73 and up used a different carrier. At some point the aftermarket realized people didn't want to shell out the $$ for a carrier to do a gear swap so they began to manufacture spacers to fit between the ring gear and the carrier. The spacers were supplied with longer bolts and if installed properly in a car with a reasonable power to weight ratio they work well. At one point Zoom offered numerically higher ratios with ring gears that were thick enough to work with the 2.56 carrier but I don't believe anything like that is available new these days.

I learned to change gears working in a gear shop in the '80s and I believe you could change gears yourself. You need an eye for detail, patience to clean and set up the differential, a couple of special tools, and it really helps if you know someone who can provide tech support. But it's not going to be fast or easy. I usually figure 4-6 hrs of labor when I'm doing a GM gear swap (DANA axles take longer). Much of that time is spent cleaning! Folks never, never, never seem to realize how much garbage accumulates in a rear axle. Without proper cleaning your new gears will fail prematurely.

For an average guy the best answer may be to buy another rear end. You need to consider how much modification has been done to your axle and truck to make your current axle work. The downside is you're going to be buying used parts! Are the axles any good? Is the carrier or pinion damaged? Has the center pin bolt sheared? You may end up spending as much fixing a used axle as paying for a gear swap.

I'm not sure a 283 with OD transmission on that final drive will work out for you. GM made plenty of 80's A and G body cars (Malibu and Monte Carlo) with crazy low gears (2.29, 2.42, 2.56 were all common) and low hp engines but those vehicles had a lower frontal area and were lighter (2300 - 2700 lbs) and the engines tended to make more torque. A good low end cam would help considerably. If you need more displacement a factory 283 "stroker" consisted of a 327 crank in a 283 bore to make 307 cubes. If you're set on keeping your block it's possible to make a 328 by using 400 rods under 307 pistons with a turned 350 crank but when you get done with this project you would spend far more time and $$ than switching to a 327 or 350.

Another solution might be to keep the stock rear which provides a nice highway ratio, keep the THM 350 as overdrive isn't needed, but change out the 2.52 1st gear stock planetary for a 2.75 1st gearset. It's not cheap at about $500 but it may be an economical solution if you think of the time invested in either a rear axle or engine swap. And although many people think a transmission disassembly is difficult, the fact is changing the planetary set in a THM 350 will be easier than changing the rear gears!

Keep us posted!

[_Ogre]

lots of good info there 1p2m!

truk has 700r4 and 3.27 gear, it shifts into od at 45 mph
good gas milage but a little luggy on 45 mph roads around here
i imagine a 2.56 gear would barely idle at 55 mph

[mr48chev]

Most 73/77 Chevelle, Regal and Cutlass rear axles will bolt in as a unit. You might well be money and ease of getting it done to hunt down a rear axle assembly with the right gear ratio.

[ezz16]

thanks to everyone
1p2m is right where my thoughts are headed. MY truck is a low buck build mostly ride around every chance we get then take on upgrades as possible. the rear is an 8.5 according to the pic I took. I get a good discount on auto parts and have someone that can setup the gear. as I thought I will have to have a new carrier also. Ideally I would go with a posi carrier. it is a great cruiser but not much fun around town

[ezz16]

for some reason I can't upload certain pics

[Coupeguy2001]

What mine runs at on the freeway.
4+3 with a .7 overdrive , 350 small block, 3.42 rear, 27 inch tall tires. Look at the tach, double the reading, it should do 140 mph
I have never had it over 100. I'm afraid of the driveshaft if it spins any faster. It's a very long driveshaft, and it's original.

[mongocanfly]

Coupeguy...with the numbers you gave. .7, 3.42, and 27in tire you should be turning 2234rpm..not 2000
With the 2.56 gears that ezz has and all else equal he would only be turning 1673rpm at 75

[1project2many]

ezz16, nice truck. Of all the AD trucks, I like the ones sporting that grille the best.

Coupeguy2001, are you using a computer with that engine / transmission combination? I was really excited about that transmission as a youngster and worked with several during my time at a Corvette dealership.

[Farm91]

Nice- truck, what is your MPG City/Hwy?

[Coupeguy2001]

Mongo, There may be an error in the tach, but it is only a couple years old, and I got it new, but at least the thing goes 75, lololol
I will have to check the speed with my GPS.


1 project,
no computer. I have an engage switch with a couple relays. an arm relay off the ignition switch and an engage relay from the engage switch.

I had determined that the computer upshifted and downshifted with no regard for engine RPM and clutch engagement, only speed, and throttle position.
I had talked with a lot of people on the corvette forum, and their opinion was the same as mine, the computer aggravated clutch wear and the tanged ring damage.

I use the trans mostly as a 5 speed, clutching the overdrive like another gear. 3rd overdrive is 200 RPM less than straight 4th.
I have had that thing in there for 8 years, and the last time I pulled the pan, there was just a haze of clutch material in the bottom of the pan. No real accumulation of clutch material.

It has around 19,000 miles on it since I put it in, and lately, when the trans gets hot, the overdrive disengages
i think it is either the solenoid or the pressure switch that stops working. Could be the first gear lockout switch too.

I have the internals of 3 transmissions, so I intend to change the whole valve body, and play with the old one later.

Anyway, on the gas mileage, with the old muncie, with a 327, I would use 4 gallons gas every 50 miles. Now I use 2 gallons of gas every 50 miles with the 350/overdrive.
Above 75, the gas mileage falls off really fast. uses a ton of gas at 90. Closing the windows keeps it from rocking side to side at 90, so I guess the rear window catches a lot of air.

[ezz16]

farm 91 I have no clue to mileage speedo hasn't worked in a while it was 10mph slow before it started growling so bad I unhooked it. I have another speedo yet to install and have went 2 teeth lower on the gear
I have gotten used to using a gps for the speed

[1958Warrior]

ezz16 what size tire are you running on the rear?

[ezz16]

1958 they are 215/70r15

[dsraven]

try this site to figure out rpm at speeds given gear ratio and trans ratio.maybe help you figure it out.

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