I have a 72 skylark with 455. Rearend is stock 8.5 ten bolt with 2.73 gear ratio. I really don't know what that all means, but I also rec'd a set of 3.42 gears when I bought car. Should I swap them out? What is the difference? Thanks for helping a rookie!
Essentially the ratio; 2.73:1 or 3.42:1 or... is the numbers of pinion (driveshaft) revolutions it takes to rotate the ring gear (attached to the axles) one full time. So 2.73's means it would take 2.73 turns of the driveshaft to complete a turn of the rear tires. The higher the number, the "lower" the gearing. The benefits are that it gets the motor into its power band quicker, helps with torque multiplication, and essentially is better for acceleration. The downside is that it reduces your top speed. You can't just "swap them out" though. There is some setup that takes special tools and knowhow to do it right and keep from toasting your rear.
So the 3:42 gears will give me greater top speed but decreased torque and acceleration? What do you prefer? Thanks again.
No, just the opposite. I like the 3.42's for an all around gear for a car that hits the strip a bit. You can tame the rpm's down using a taller tire for the street (like 28" tall), while running 26" tall tires at the strip. Tire size is another part of the equation. A taller tire will effectively reduce your gear ratio (make it act as if it had higher, or lower numerically, gears). What is the intended use of your car?
It will be mostly a daily driver and for weekend enjoyment. Will either gear give me better gas mileage, although I realize this is not an economy car.
I think the biggest detriment to your gas mileage is attached to the bottom of your right leg... :Brow: But seriously, the 2.73 gears aren't bad. Does it have limited slip (posi)? If not, then you may want to find yourself another 8.5" rear, get ahold of a limited slip differential, and have a shop swap, or experienced person, swap in the 3.42's and the posi. Then all you'll have to do is swap out the drum-to-drum rear and away you go. If you were to just swap 3.42's into an open (no posi) rear, you wouldn't do yourself many favors. You'll just spend all your time buying new right rear tires :shock: This is all my opinion though. I went from an open 3.08 rear to a posi 3.42 rear in my old 71 GS455. It was great! I added headers and a good 2.5" mandrel exhaust system and went mid 13's with it.
if youre not going to use the 3.42 gears i'd be interested in buying them or even trading your 3.42 gears for my 3.08 gears.
I would be interested in selling if I don't use them. In the meantime, if I did put them in it would result in my engine running at higher RPMs when I'm traveling down the road at 60 mph? Wouldn't this cause increased wear on my 455 (i.e decrease its life expectancy)? I won't be racing the car, but I do like it when it goes fast. I did learn that the rear is open and no posi. Bottom line: stay with the 2.73 or go to 3.42?
if u swap t0 3.42 gears you will be running quite a bit higher rpm and of course your gas milage would suck too. youre alreadt getting 13mpg with a stock 455. The higher the rpms you run you will loose life of the engine.
if I did put them in it would result in my engine running at higher RPMs when I'm traveling down the road at 60 mph? Wouldn't this cause increased wear on my 455 (i.e decrease its life expectancy)? that's it, you've grasped it exactly. something else, many people complain that shorter gears (higher number) cause their engines to turn to many RPM and be too loud at highway cruising speeds. so, with the 3.42's you'll have worse fuel mileage, more engine noise at cruise, more wear and tear on the engine, lower top speed and, if you can't restrain your foot, more tires on the RR. the 3.42's will really only help accelleration from a rolling start ... like rolling at 40mph. that 455 has WAY too much tq to be trying put down with a peg leg. maybe if you put a drag slick on the RR. :laugh: getting a posi carrier would make the 3.42's useful at the drag strip / stop light but you'd still have the rest of the issues.
Yes. You can play with a calculator to get an idea of the actual numbers: http://ring-pinion.com/content/calculators/RPM.asp For example, if you're running a 26" tall tire (235-60-15), with 2.73's, in Drive, with a TH350 or TH400 transmission (1:1 ratio in Drive), then your rpm will be approximately 2175 RPM. Now, everything the same but with 3.42's, your rpm will be approximately 2725 RPM. If that's a little much for you, throw on some 275-60-15's on the rear, which are around 28" tall, and your rpm at 60 mph will be approximately 2525 RPM. Remember those math classes we all slept through - damn, we found a use for them. Wait, we have the internet... :TU: Not as long as your car makes good oil pressure, then there shouldn't be a problem. Good means, at least 10lbs of oil pressure per 1000 rpm. Once again, this is all my opinion. You've got a lot of motor, that I'm sure you'll want to do mods to. But until you can get it to hook up, they'll all be wasted. I'd say the important rear mod is to get a limited slip (posi) unit. While you've got it cracked open, swap the gears out too. 3.42's and a posi will really give you a noticeable seat of the pants difference in bottom end acceleration. Unless you like to cruise at 140 mph, the reduction in top speed won't matter anyhow. The increased RPM's aren't a big deal anyhow. Cruise in it at around 70 mph, which will be about the same as 60 mph with the 3.42's and see how the additional ~500 RPM's sound to you.
If you are going to use the car as a daily driver/cruiser, you could go w/ 3.42's and later on, if you decide on an upgrade for your car, a gear vendors overdrive would help increase your gas mileage, decrease the wear and tear on your engine, and give you the best of both worlds...really good take off, and top end! www.gearvendors.com It would cost you a little more to do it that way than to get a good 200r4 overdrive trans if you swapped it in yourself, but the GV overdrive might take less time/and work to put it in than to do a trans. swap. Just a thought, in case you do decide on an upgrade and want to do something like that. :TU:
I used to run 3.42 gears in my 72/455 Skylark then swapped to 2.56, both are posi rears. Now the 3.42s felt great, much better than the 2.56s from a dead stop but the 2.56s gave 2 MPG better milage. Used to cruise at 2400 RPMs at 55 MPH and now cruise at 1850 RPMs. The 2.56s only cost me .04 seconds in my ET in the quarter mile. The 455 has plenty of torque so I'll be no problem running the 2.73s, you can save installation costs and sell the gears you have. :Brow: Dave B
Dave, I'm amazed at how little difference the 2.56's cost you over the 3.42's! When I ran my Skylark with the 2.56's (and spun the rod bearing) last year, I went 13.5 @ 106 mph. That seems like a solid run, but I crossed the line in 2nd gear! It just seems to me that all things the same, had I gone to 3.42's on that same day, I'd be in the 12's. My 60 foot time was really slow, in the 2.2's I think. With 3.42's in my stock, with headers, 71 GS455, I had as low as a 1.855 60 foot. I would also think that using all three gears would make more than just the .35 difference in 60 foot. Do you have complete timeslips from each of your passes?
Steve I was kind of amazed myself, I was expecting to slow down at least 2 tenths. The runs in question were a week apart weather was identical on both days. It was just T/T both nights, with the 3.42s my best time was 13.60 with the 2.56s my best time was 13.64. My engine combo is pretty mild, it's never even run 100 MPH in the quarter, Q-Jet, B4B/then, headers, a Poston GS116 cam and drag radials. My opinion on the cam is that it's a fine cam for a daily driver, lots of torque, but was all done at 4200 RPMs. The only explaination that makes sense to me is, in low gear the 3.42s have a better overall gear ratio than the 2.56s combined with the trans ratio. When the 3.42 car shifts then the 2.56 car has a better overall ratio, combined with the trans, and so on into the next gear. Yeah I was real close to pulling the quarter in second even shifting at 4200 RPMs. By the way from a dead stop the car felt awsome with 3.42s now it just feels pretty good. ou: Dave B
So I have decided to put in the 3.42s. I have an open end right now and I assume I need to change to a posi--is that the same as limited slip differential? What manufacturer do you recommend and how much will it cost (ballpark)?
Yep, positraction is GM's name for limited slip. For a mostly street driven car, I'd recommend going with Eaton. They made the original posi rears for the car and you can buy them new now, although it will be quite a bit more than finding an older used differential. I think there somewhere around $400 new. Go to www.buickperformance.com and there are some articles. One of them tells where to look for used ones in wrecking yards.
