im looking to upgrade rear end assembly on my 1978 monte, I recently installed a rebuilt corvette motor and everything else under hood as well as headers, the car has a 3 speed automatic transmission and im looking to upgrade rear end assembly, thinking about 8.5 10 bolt with posi and new rear drum setup but unsure of what is a good highway gear because I plan on driving car from connecticut to north carolina, ive heard 323 gear might work, some say a 308 gear, any ideas or help is greatly appreciated. I really want to get it right the first time.

 

your gear ratio should match your camshaft. i dont know what corvette motor you have so its hard to say. if the car is going to be a weekend warrior than the 323s are ok. if its a daily highway driver with a 3spd your going to want the 308s. the issue with running 308s is that if the cam doesnt come to life down low the car is going to feel like a dog off the line. this only applies to drag/street racing. on the 8.5 rear thing its an excellent rear end and can be upgraded with 30spline axles and carrier to make it pretty stout. i myself have yet to encounter an affordable one in my 15 years of owning a monte. you can also upgrade the 7.5 with a 28 spline posi carrier and axles. theyve been known to survive behind many a 12 second car. there are also companies like dts that sell ready to bolt in dana 60s and 9" rears for your car. obviously your motor trans combo should dictate what rear to run. you could have an ls7 corvette motor or a crossfire 350 each will demand a different strength and gear rear.so post some pics and some specs on your motor. oh and your cars a 3rd gen not a 4th gen, be proud its part of a rare breed. flexible drivetrain options like the 4th gen, itll take any sbc or bbc, but its shorter length and lighter weight make it a better choice for street/drag racing.

 

Just to add to what Monte Man said, you rear gear will also need to factor in what size (overall height) tire you will be running. I have 3.73 gears with an overdrive of .67 and 25.7 inch tires and at 65mph, I am turning 2350 - 2400 rpms. This is on a stock L69 motor with TPI fuel injection.

My corvette runs 28" tire, 3.70 rear gears and a BW super t10 4speed with 1:1 4th gear and it turns 3000-3100 rpms at 65. This has the L82 motor with stock cam. This setup is NOT real highway friendly, but it is not terrible either. The Monte is much better staying in the 24-2500 range as it does pretty good on fuel and keeps the motor near its torque/power curve.

Just some more to take into consideration. There are some pretty good rear gear calculators on line to help you figure what gear you want by plugging in the info I talked about above.

Good luck!

 

thanks guys for all the helpful info, since my last post I upgraded to 323 rear gear and had posi unit added, to date ive removed the original 305 small block and replaced with a rebuilt 350 chevy corvette motor, added posi unit to rear and swapped gears to 323 ratio, my new question is because I want to drive this car on the highway what changes do i need to make transmission wise or do I have the wrong gear set up in rear end. ive been told that if I add a different transmission with 4 gears and overdrive that my monte will drive alot better, right now in the city it drives fine, but on the highway it simply feels like a very hard ride, Ive had a dyno performed on vehicle and its reving at about 3000 rpm's at 75 mph, which isnt 75 mph because the speedometer is off by about 10 mph. any help in the right direction would be helpful, I simply want to have a vehicle that performs well on the open road for long distances but has the power to get up if needed. thanks again guys

 

glad to hear youre still fighting the good fight.

onto the topic at hand. first question is, is your speedo 10 slow or fast? ill assume 10 fast. this is part of the reason why knowing your cam specs is important. if the motor stops making power at 5k then its screaming its head off trying to cruise at 3k. if it makes power to 7k 3grands not bad. getting a 4 speed trans for the car would be a good idea regardless. the 4 speeds usually have a deep first gear which will help with your lack of gearing to get you out of the hole, and the overdrive will help by reducing your gearing when in 4th and cruising. the down falls to overdrives are they arent really performance oriented, theyre heavy, they usually require driveshaft modification since they tend to be longer than 3speeds, and convertors are more expensive. there are several options you can go with, but ill touch on the 2 most popular and affordable. they would be the 700r4 and the 200-4r. the 200 came in the g bodies so are easier to source all needed parts from a junkyard. the 700 came in fbodies and bbodies, so youll have to have your driveshaft cut or buy a new one. the 200 also has the better gearing from the factory. 700s have a deep first for hard launches, but a huge drop in rpm from the 1-2 shift. the 200s 4th gear is better i think its .63 compared to .75. both are pigs from the factory requiring shift kits and convertors along with a few things here and there to make them perform. i personally run a 700 and if i were you would pick it only because of the super deep first gear, but my brother owns a shop and builds race cars so i get a good deal.

i dont know what size tire youre running but that also matters. the taller the tire the more your gearing is reduced and vice versa with a short tire. its part of the reason why you see drag cars running 30"+ slicks. gear ratios are usually applied to a 26" tire. i run 3.73s with a 28" tire so its more like 3.60s. if you run a 24" your gearing could be close to 3.40s.

good luck with the car and keep us posted, and dont forget to post some pics of your car.