Getting ready to replace the 2:73 gears that came in my 8.5" 10 bolt. Engine is 406 SBC, Muncie 4 speed transmission, 29" rear tires, 235 @ 50 roller cam, single four barrel. Car will never see the strip. Will only be driven occasionally. Thinking 3:42 or 3:55 gears, would like to hear what your experience. Yes, I will do the speed vs rpm calculations, but want to hear user comments. Thanks.
Agreed - 3.73 would be ideal. Making the engine struggle while leaving stop signs, or accelerating while going slow, while it's hard on the engine, it's no fun in the seat either. When 3.73 was a highway gear..? Hell, back in the early 70's, my girlfriend lived 30 miles away. My 56 Chevy had 4.11 gears in it for about a year and a half, then I finally found a GOOD highway gear...3.90's. I say "finally" found, at the time, no one was making the big Pontiac/Olds (I put a 57 Pontiac housing in the car) parts at that time. The 3.90 ratio was like having 10 pounds of platinum. Now a days, all of those parts are easy to find for the big GM axle. Mike
My '56 Chevy had 3.08s in it when I got it. It felt mighty lazy, even on the freeway at 65-70. At the first opportunity I bought some 3.55s and it woke it right up. Small block Chevys seem to like to stretch their legs. It pulls nicely from a dead stop and at highway speeds. If I run across some 3.70s for a reasonable price, I'd like to try them. The increased RPM takes a little getting used to when most of our late model drivers go quietly down the road at 1800 rpm, but with a full length exhaust system and quiet mufflers it's fine........and a lot more fun. I guess that's why 5 spds and OD automatics are so popular these days, you get the acceleration in the lower gears and the OD keeps the highway rpm down - the best of both worlds.
2800 rpm at 65 mph = 3.71:1 which is close ^^^ [245 deg @ .050 is pretty stout for cruising] 2500 rpm at 65 mph = 3.31:1
I have 3.00 in my 8”, t5, 350 in my 32 sedan. During The build stage I was sure I was going to change Gears but waited to get it running and driving. I’m glad I never changed it has enough poop to get it rolling and it will Cruze at 80 mph all day long.
I've got a 2.73 with a 350 turbo and 29in. tire in my T. Just like rusty rocket I planed on changing it but it works better than I thought it would. It's a pretty light car though.
My Mercury came Stock with 3.73 Standard Shift/ when I put the SBC in it with the Automatic, I didn't change the Rear it is Great on the Highway and Coming off the Line. Just my 3.5 cents Live Learn & Die a Fool
I put 3.25’s in my sons 40 Chevrolet with a 302 GMC. I told him to get a 4 speed with a lower 1st gear. He bought a Richmond with a 2.88 and it’s perfect. A transmission can dictate the rear ratio.
With that duration, Id say 3.55 is minimum, 3.73 or 4.11 is better. Itll **** on the highway but thats the tradeoff with a healthy cam. Be honest with yourself how you wanna use the car. If you want more of a driver, higway cruiser, swapping cams would be worth it. Or just spin it hard on the highway and throw rings and bearings in when needed. Its not like its a daily driver, use it up, beat on it, SBC parts are cheap.
1946 Studebaker coupe. Occasional drive/cruise, but wont be beating on it. Will open it up to hear it through the gears every now and again. Will not see many freeway miles.
If you have an engine that makes power down low, under 5,000, then you need a higher gear like a 3.50. Or a 3.08. Lippy
350 Chev 3.25 gear 9 inch. 350 turbo 2200 rpm at 110Ks gives me a bit of performance and good fuel milage
There's a lot to having a 2 series rear end with a top 1:1 gear and a torque cam. The stator will multiply torque to a higher degree with a higher (lower numeral) rear end. On the street, you need big torque right off idle not up in 5 or 6 thousand rpm.. I ruined the performance of many good running motors putting long duration, short lobe separation, rump rump, cams in them and then going into a state of denial about how lousy they really ran.. My present set up is a 1978 454 stock compression, stock cam, th400, 2;73.. This set up screams out of the hole and cruises at 90 all day long with ease... Tuning it made a world of difference, 20 initial, 16 mechanical all in by 2600, no vacuum.. You can add a lot of initial with low compression and still run 87. Im running a 180 t stat, I might go to a 160 if I get any ping.
I had the stock 3.56 rear in the car and it lugged badly around town unless I left it in 3rd. I did some calculations and came up with the 3.89 ratio. I have tires that are close to 29" tall and at 70 I'm turning around 3200 RPMS.
The most important lesson I have learned about gears in hot rods that are regularly driven for pleasure is that the factory set-up is usually pretty close to ideal. Meaning a final drive ratio of something between 2.75:1 and 3.5:1 is going to give you the most satisfactory compromise between performance, mileage and comfort. I recently removed the factory 2.75:1 gears in the 8-inch under my tub (30.5" tires) and replaced them with 3.80:1 posi gears thinking that with a TH350 and no overdrive that this would be a really great combination behind a relatively mild SBC. After all, I reasoned, this has been the "go-to" gear ratio for generations of street-driven performance cars, hasn't it? Well, maybe so, but I was very disappointed in the driveability of the car with the 3.80:1 gears in place. The combination was oddly uncomfortable to drive, always at odds with itself at low speeds and very busy at freeway speeds. Not at all what I wanted in a car that gets driven almost every fair-weather day. So, I pulled the 3.80:1 gears out and replaced them with 3.25:1 gears - half way in between the former 2.75:1 stock gears and the 3.80:1 gears I had put in to replace them. This has proven to be ideal for my car and my driving conditions. The car is much happier and I am much happier now as well.
I have to add my 3.5 cents.! If you Change your Rear Tire size you change your Rear Size.! Just my 3.5 cents Live Learn & Die a Fool
For a way to visualize how tire size affects "gearing", think of a car with 27.5" tall tires, and 2.75:1 rearend gears. If you went to 30.8" tall tires, you'd have to go to 3.08 rear gears to keep the same RPM at MPH. Going to 32.5" tires (you're getting into 4x4 territory here), you'd need to move up to 3.25 gears. Dropping to 25" tires, you'd need a 2.50 rear gear. You can see that drastic changes in tire size, equate to relatively mild changes in overall drive ratio. So, usually changing gears by 10% or so will get you noticeable results, while changing tire size a little won't really do anything.
Squirrel, I've never seen that proportional comparison, tire size/gear ratio. That provides a quick reference that I will definitely remember. My mentor (Joy Fair) in oval track racing and the guy that got me started in QCs said that the average race car driver can't tell the difference in ratio changes of less than 1/4 ratio. He proved this by the following story. Joe Ruttman was driving for Fair while he healed from a broken back. Big Joe insisted on a .1 gear change. Fair got under the car and banged around for a while. Ruttman went out and improved his lap times. Came in and said, "See!" Fair never changed the gear
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