So, I will be in the process of dropping a 455 in my skylark. I have a 3.73 gear set. But I believe that will be to much for my 3 speed TH400. So what gear would be good that I can use to drive to car shows and also burn some rubber at the track, 3.42?? This will be more of a street car that can show its muscles.
You could try a 28" tall tire and see if that tames down the rpm enough. It will bring your effective gear ratio to around 3.42. It will depend on how much highway driving you do and what speed you travel.
so, what you are saying, is having a 28'' tall tire with a 3.73 gear ratio. It will, in actuality, be a 3.42?
Close enough that you can make a decision as to whether you find it to be too much/ not enough gear. A couple of driver quality rims and cheaper 28" tires costs less than installing a set of 3.42 gears and finding out it's still too much gear for your uses. You can use the taller tires for cruising and highway and switch to the short tires with stickier rubber for trips to the drag strip or street shenanigans.
The difference between a 28" tire and 3.73 and a 26" tire with 3.42 is less than 100rpm at 70mph: 3133 vs 3093 respectively. If you have a 26" tire with 3.73 you'll end up around 3374rpm at 70mph.
Gear choice is really a driver choice. You can ask others as many times as you want will all have different reasons why a ratio will be good. YOU have to figure out what is good for you. Ask your self questions of how much freeway or distance driving will be done with the car...Or how much track time you will do.... or what RPM you prefer your engine to be at. OR it may come down to efficiency of the motor and the camshaft choice you have. Usually the best choice is a overdrive transmission... gives you everything. Jim PS a 455 should not have any issues with any ratio getting burned up.... So do not worry about burn outs and acceleration...UNLESS that is the top of the list for you I hear a lot of guys using 3.08 on this site saying with all the torque the 455 produces it wikll only loose a 10th in most cases at the track. Another quick thought is a smaller SLICK at the track will give you more ratio for fun so choosing a higher 3.08 or 3.42 with a larger street tire will kill ratio for freeway driving and the 26" tall slick will give you back the ratio where you want it at the track...
No, it's actually about 3.55. Tire circumference will vary directly with a diameter (tire height) increase, the constant being pi. Stock tire height for our cars was about 26.6" 26.6/28= .95. .95 X 3.73 = 3.5435. If you want an estimate of what your highway RPM will be with various ratios, 336/tire height X gear ratio X MPH = RPM with no converter slip. Add 150-200 RPM for converter slip. So as an example, for a car with a 3.73 gear, and 28" tires, at 60 MPH, 336/28 X 3.73 X 60 = 2685. Add 200 RPM for converter slip and you are at 2885 RPM at 60 MPH, too much?
You are kinda on my way of thinking. I'm not looking at a big cam because I am not making this car a drag car, but more of a drive around and show it off car. With the occasional burn the rubber at the track and still get my heart pumping.:grin: I hear that 455 don't like to run at high rpm's. But on the other hand, I have this gear set for free. That is 0 money that I have to worry about making my buick's performance higher than stock. The main thing is having to worry about breaking my motor if I do drive 20 miles at 70 mph with a rpm pushing 3300 to 3700. Will that be ok?
You could drive it hundreds of miles at 70 MPH and not damage anything except your gas mileage. A 3.08 is a great gear for a street driven 455. If the 455 is correctly built and tuned, you will burn the tires off that car easily.
Here's a 'Gear ratio speed calculator'. Try some different combinations. http://www.apexgarage.com/tech/gear_ratios.shtml For tire size, a 245/60-15 is 26.6", a 255/60-15 is 27.1, and a 275/60-15 is 28" (BFG Radial T/A) ST400 trans has ratios of 2.48, 1.48, and 1.0 I still have 4.11's in my blue '66. 3000 rpm at 60 mph is fine. 70 mph is getting up there at 3450 rpm. Worst part is that first gear is relatively useless in normal driving. The 1-2 shift happens after getting less than halfway through an intersection. I'd suggest leaving what you have until you get the 455 in there, then decide later.
While I was doing some research It dawned on me. I have this whole rear end axle housing, diff and all parts in it. Its not on the car but one of the parts the seller had given me. Only thing is.....It's a 12 bolt rear end. Ran the numbers, looks like its out of a 70-72 chevelle. Will this work on the skylark?. Is it better, worse, same as the 10 bolt I have? Remember, this was all something I have, so I don't have to spend any money, other than install.
The 12 bolt will have C clip axles, and it uses a different U-joint than the Buick rear. Those 2 things are undesirables. The 8.5 10 bolt that should be in your car is a very sturdy rear, and parts availability is excellent. The Buick 8.5 10 bolt rear has bolt in axles, the same pinion shaft diameter, same pinion support as the 12 bolt, and it uses a bigger U-joint. The 12 bolt rear will bolt into your car if you use a hybrid u-joint. I think it would be a mistake for you to do that, but if you want the 12 bolt, you shouldn't have any trouble selling the Buick 8.5. I have a 1971 Buick 8.5 10 bolt in my car. I upgraded it with new 30 spline Moser axles, Eaton 30 spline Posi, and 3.73 Yukon gears. For your purposes, the stock 28 spline axles are plenty strong.
Interesting. I'll need to do more research, but will the Gears be the same? I do want to keep the Buick rear end. I guess, after doing some research, I can sell the 12 bolt and all components and maybe get a higher gear ratio with posi trac. I'm trying to keep it simple as possible.
No, 12 bolt gears (8.875) and 8.5 10 bolt gears are not the same. You can buy whatever you need for the 8.5 out of Jegs or Sumitt. Check the rear in the car right now . Make sure it is an 8.5. Someone could have swapped it out. If you do decide to buy a Posi and gears, check with Jim (Monzaz on V8) He may be able to give you the best price. http://jdrace.com/diff_id/10b85.htm http://jdrace.com/diff_id/12bCHEVY.htm
if you have an iPhone download tremec for free it gives you an easy to use app for exactly what you need and more
I guess what I am trying to get at is....Will it be difficult to use this 12 bolt rear end to using my 10 bolt rear end and buy gears with a posi trac. o No: What will be simple and cheaper??
As stated change the gears in your 10 bolt 8.5 rear ... it is more than strong enough for what your doing.... gear choices are 2.41 2.56 (2 series gears) 2.73 2.93 3.08 3.23 3.42 3.73 3.90 4.10 4.30 4.56 4.88 5.13 5.38 5.57 (3 series carrier) You just need to google on line the tire calculator that will give you the rpm at certain speeds...Here is the calculator version if you need it. take the mph x 336 (PI) x gear ratio and divide all that by the tire size (inches IE 25 26 27 28 29 inch tall tire.) IE 70mph x 336 x 3.42 divided by 26" tall tire = roughly 3093 RPM remember this will vary as torque converter slippage in a auto trans will be tighter or looser. standard shift should be pretty close. IE 70mph x 336 x 3.42 divided by 28" tall tire = roughly 2872 RPM so about 100 RPM per inch which is really NOT much of a difference. But can help some if you want to tweak the rpm some. gear ratio swap will be the best way... OVER DRIVE trans is even BETTER!!!!! SO use it and calculate the RPM you want to be at and get those gears installed before the spring season HITS... Jim J D
Over drive figures IE 70mph x 336 x 3.42 divided by 26" tall tire = roughly 3093 RPM THEN multiply that by your over drive percentage (IE 700r4 is .70 i think) you will get 2165 rpm drop .....WOW what a difference.... so your talking about roughly 1000 rpm drop that means a 3.73 would run more at 2400 rpm real nice...you get all the low end you want and do not have to sacrifice freeway driving.... Think about it.... save your money.... Jim
Sell the 12-bolt Chevy to fund the rebuild of your 8.5" 10-bolt. They are plenty strong,and you don't have to mess with changing driveshaft lengths or different u-joints. I run a 3:73 with a 28" tire,and it spins 2800 at 60mph. This can also vary a little with different converters. If this sounds a bit steep for you then I would go with a 3:42.
