I'm just curious as to what the highest (numerically lowest) gears are that ever came in or were ever available in Chevy 12 bolt rear ends. The reason I ask is that the '70 has a Chevy 12 bolt rear (I was told by the previous owner its a posi but I have yet to confirm that), a 200-4r trans (overdrive is .67 I think), the car has 245/60/15 BFG Radial T/As for back tires, and in overdrive at 70 mph the tach reads in the neighborhood of 1600-1700 rpm. I don't think the tach is messed up because at that speed you can hardly hear the engine at all (top up or down) even with the 2.5" dual exhaust :laugh: I haven't had the chance to do that trick where you jack up the back end of the car and count the driveshaft rotations to make the tires spin once, and I was curious because those numbers lead me to believe the rear gears are on the very high side.
2.73 is the most common high ratio 12 bolt 2 series gear. Very common gearset in 12 bolts from auto a/c 350-4bbl Elco's & Montes. Believe 2.56 was also avail, just can't remember having a set. Hope this helps. Got Posi? :3gears: Roger
200 4r I'de do the rear end check. If your running the 200 4r overdrive Trans. that will change everything,
2.56 and 2.73 are pretty common, but there is a 2.29 as well. That would be a bit much with an overdrive! Carl
What is the biggest rear you would want to go with? Not for racing, or anything, but as a cruiser? I've thought about going with a built 200-4r and my 2.93's in boatttail, too much gear?
overdrive and gears well most stock motors in the muscle car era ran best at the 2000 -2400 rpm range. When overdrive and fuel injection came along the rpms were lowered because the injected motors could hand the throttle responce at a lower operating rpm and pick up quick. with overdrive trannies the 3.23 was about as low as they need to go anything underthat will be very doggey if you try to pass on the freeway. overdrives do not kick way down like a properly set up 3 speed hydramatic tranny. I have a Overdrive in my monte with a 454 and 3.31 gears. I think I am going to switch to 3.42 with the cam I have being 272 crane HMV when the converter locks up it is almost too low rpm for the motor to work at its most efficent area of 2200 rpms. Youu need to figure out what cam you have and the area your car need to cruise at for optimum gas power and throttle response at cruise. Then pick that gear. cruise MPH X gear ratio X 3.36 (pie) divided by tire size (inches) = rpm of motor. Jim ask more questions at jdrace.com too.
I believe the engine is a stock (except for the carb) '72 or '73 455 buick motor, so I imagine the cam is stock. I don't know how low of gears I would want to put in the rear, but I just know I think they should be lower because from a dead stop the car totally lacks that lightly tap the gas break the tires loose and go sideways pin you in the seat power, but I also don't want the thing to scream on the highway either. I'm thinking 3.31 or 3.42 would be a good size.
problems Well to tell alot of you the truth. I want to make money sellng gears and doing rear rebuilds...do get me wrong. I have noticed over the years of racing at the track when a guy brings a muscle car out and goes down the track and is totally disappointed. we figure well i got a 455 compared to a little four banger i should kill them attitude. Most of the time all the parts on your 455 distributer and carb have not been checked in years and will all add up power robbers. Another problem are combinations we tr to put together. big cams low compression converters trick parts...etc. anyone of these dipsedottles put in the wrong way or combo will mess a cars performance fast and make you buy a ton of parts you never needed in the first place....lol. So what am I getting at here. PLease check all the simple items on the car first before wondering why the car does not have enough power it wust be gears or a cam or too small of a carb etc. I would make sure your distributor is advanving correctly ALWAYS. Biggest over looked item on a car performance wise. example well it sparks it must be fine....lol. You could have the total advance way to much or not enough. the weight could be rusty and not operate smoothly weak spark allthese thing will make a car sluggish at low end or top end. Carbs- dirty carbs, OVER adjusted carbs, too big of carbs more trouble here. tinker with that carb so much you don't know which way is up down left or right anymore. Most new carb out of the box will work almost everything perfect as is. If it does not look else where first back to the distributor or vacuum leaks most likely. Gears maybe the fix you need all i am saying is make sure your cars if functioning to the best of it's ability before change out high dollar labor and parts. If your car came with those gear originally I am sure that combo work very well or GM just plain would not have put them together. I had a 1970 350 monte with a stock 929 cam and it pulled hard in every gear and would put you in the seat. stock engine heads 441 76cc 2bbl carb burned rubber like a mother too. stock 350 th tranny 2.73 geared rear. Did have duel exhuast but with stock manifolds. remember you have 100 more cubic inches too. Good luck on the car...I have those gears if you like 3.42 is going to be the way you want to go because 3.31 gears are not reproduced by any gear company (GM SS ONLY) 3.08 might be an excellent choice too. Jim www.jdrace.com
2 cents Sorry to all you received 2 cents and at least 3.00 more....lol. Take it or leave it i am just really trying to help out everyone . Take care, Jim
The carb on the car is a Holley 750 double pumper. When I got the car a year ago it came with the Pertronix Flame Ignitor electronic ignition, which it still has, so there are no points to deal with. As far as the timing goes, I don't know what kind of advance it has. My cousin did all the tuning on the engine at his shop, and right now it runs awesome, so I don't think the engine is the problem. I will say the car does have a good amount of grunt off the line, but its not spectacular, which is what lead me to suspect the gears.
power 455 I am alaways just making sure we check everything before the money is spent and more head scratching goes on....lol o No: So what ever gears you need I can most likely get them to you. $169.00 for the 3.42 Yukon Gears. I also have 3.55 if you want a bit more grunt. Alot of guys like the original ratio GM produced for these rears. 3.42 is more of a 8.5 7.5 ratio but nice too. All depends on your wants. Take care, Jim www.jdrace.com
have friend with a 67 elky with a done up 350v8 & 700r4 tranny. after experimenting with various rear gears (chevy 12 bolt) he settled on 3.43 . works well for him as he does a lot of hwy driving.
gears 12 bolt I have the 3.31 and think the freeway with a 454 272 hmv crane cam is still a bit doggy when it locks up 4 gear . My system is a B&M lockup to control lock-up not a quick in and out as the vacuum controled GM unit. I mainly use it to cruise or slide it over to drag race. Non lock. So Yes I will moving up the scale to either 3.42 or 3.55 also. I agree. Jim
