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Hot Rods Gear ratios and diff ratios - what's the ideal combination?

Discussion in 'The Hokey Ass Message Board' started by Jasper6120, Apr 19, 2017.

  1. Jasper6120
    Joined: Jul 18, 2007
    Posts: 502

    Jasper6120
    Member
    from Australia

    Aloha Hambers


    I have a question regarding diff ratios. I'm in the process of replacing the whole drive train and engine in my Chevy daily driver. My plan is to build up a super mild 283 Chevy and run either a 4 or 5 speed box behind it. I've come down to two probable setups that I could go with.

    283 -> super T10 -> 2.78 ratio diff = 2400rpm @ 70mph (Australia speed limit)

    Or

    283 -> T5 -> 3.08 diff ratio = 1800rpm @ 70mph

    I don't have a boat load of cash to throw at this project. My question is essentially are these two ideas okay and, if so, which one would you guys prefer? Gas mileage is a big one for me as I spend a lot of time driving. But reliability is of course also important for the same reason. I could also go a 2.60 diff on the super T10 - bringing down the cruising rpm to about 2300. But is that a bad idea? 283s aren't too torquey but they do rev... My thoughts are that a diff ratio that is too tall could place a lot of torque on the universal joints and pressure on the diff gears. What's the scoop in all of this?

    This has been plaguing my mind for a few months now. Keen to know what people's thoughts are?

    Cheers

    J


    Sent from my iPhone using The H.A.M.B. mobile app
     
  2. Torana68
    Joined: Jan 28, 2008
    Posts: 1,445

    Torana68
    ALLIANCE MEMBER
    from Australia

    cam? then work out at what revs peak torque is likely to be, gear to that RPM , 1800 may be too low, depends...
     
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  3. Hot Rod Grampa
    Joined: Apr 25, 2011
    Posts: 83

    Hot Rod Grampa
    Member

    5 speed for the overdrive. Do you have flat land or a lot of elevation changes? Cam grinder can work with what gears you choose and have torque even at low rpm. Shock loading will damage parts quicker than low rpm torque. In my opinion. Good luck.
     
    Jasper6120 likes this.
  4. The cam sets the plan, or the cam needs to adjusts to fit the plan.

    Too low of Rpm below the torque curve set by the cam will lug the engine and cost you fuel Mileage.

    Three factors, The weight of the vehicle, available engine torque, and rear gear ratio make an equation that will tell what the drivability performance will be. Rpms that stay on or the torque curve will be better at everything. Again, The cam sets the curve


    It sounds like you want to build Low rpm torque with this set up. 283 wouldn't be my first choice for low rpm torque but you could take the 283 and maximize all you can get out of it. Keep the velocity of the gasses up with small intake runners and small primary headers. Using parts optimized for SBC 350 on a 283 kills low rpm torque but makes for high rev power.
     
  5. Rex_A_Lott
    Joined: Feb 5, 2007
    Posts: 1,158

    Rex_A_Lott
    Member

    You will have better take off with the T-5 and the 3.08 and will most likely not use overdrive very much. A 283 w/ a 4 speed and a 2.78 gear will be a dog unless its in something very light. Good Luck
     
  6. Glenn S
    Joined: Jun 22, 2015
    Posts: 44

    Glenn S
    Member

    Go with the t5 and either 3.08 or 3.36 (sounds like holden ratios). I've got a 301 with a Supra 5 speed and 3.36 gears and only use up to 4th around town and 5th after the 80k signs have finished. The little motors like revs. Had a HG with 283, Muncie and 2.78 gears, OK on the open road but very hard on the clutch around town with the 2.54 first gear.
     
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  7. Mike51Merc
    Joined: Dec 5, 2008
    Posts: 3,855

    Mike51Merc
    Member

    What gear set is in the T5?
    When I researched this question, a common answer is that you want a combined first gear ratio (multiply first gear by rear ratio) close to 12:1, and a top gear ratio around 3:1.
     
    34toddster likes this.
  8. Mike VV
    Joined: Sep 28, 2010
    Posts: 3,329

    Mike VV
    Member
    from SoCal

    2.60 ????????????
    Ha..."ideal" is the same as "best"...both useless words in the automotive world. Talk to ten people about one subject, 7 will have a different..."ideal", or "best". Everyone's car is different, everyone's driving is different, everyone's circumstances are different.

    Remember...MOST American engines, especially prior to 1995 or so, were not designed to putt, go slow, little more than idle...AND make any power to drive said car at reasonable speeds.
    The cams just aren't designed that way.
    Minimum gear would be in the 3.23 range, and even that would depend on the rear tire diameter and the cam design.
    If you have a 26" tall tire and given gear ratio, install a 30" rear tire...see just how sluggish the engine/car becomes. Won't get out of its own way..!

    This a be careful what you ask for..!

    Mike
     
    Rex_A_Lott and i.rant like this.
  9. 4-Speed and I wouldn't run anything higher than a 3.73. My grandfather ran a 4.88 on the street in his 327/4-spd daily. My pickup has 3.73s and 29" tires, when it's running decent it's my daily. Real men don't need overdrive.
     
  10. Joe H
    Joined: Feb 10, 2008
    Posts: 1,831

    Joe H
    Member

    My truck has a 250 inline six, basic stock engine with 110 HP when new. Standard T350 automatic with 1 to 1 final drive, 3.08 gears, 28' tall rear tires, and weighs 3300 with two on board. 2400 @ 70 mph, which is what this engine really likes. Fuel mileage went up when I switched gears from a 3.42 to the 3.08, but performance went down. It's really noticeable from a standing start. I don't have enough engine to pull any more gear.
     
  11. Johnny Gee
    Joined: Dec 3, 2009
    Posts: 14,214

    Johnny Gee
    Member
    from Downey, Ca

    Your talking end result. Whats 1st gear ratio in your options of transmissions ?
     
  12. Mike51Merc
    Joined: Dec 5, 2008
    Posts: 3,855

    Mike51Merc
    Member

    That unneeded overdrive sure comes in handy on 100 mile interstate rides.
     
  13. 34toddster
    Joined: Mar 28, 2006
    Posts: 1,482

    34toddster
    Member
    from Missouri

    A long time ago at a far away place I had a 283 4 speed in my 63 Impala, being 17 I thought and all of my buddies thought I needed a really big cam, and since the rear gear was a 3.08 I thought I could get by with it for a while...Nooooo I got my lumpy bad ass 283 out ran by a F'ing Falcone 6 cyl station wagon.
    Lesson learned, there is a reason the factory put 3:31 under GM 's with 4 speed
    Sooo I did what any Midwestern youth of the day would do and I put a 396/425 out of a wrecked corvette in the old 63 it ran then, between replacing spider gears carrier bearings
    I sure wish I still had that BB I could live a year off of the proceeds.
     
  14. Jasper6120
    Joined: Jul 18, 2007
    Posts: 502

    Jasper6120
    Member
    from Australia

    I'm not savy with cams at all. I just want to have something that doesn't use a lot of gas. I'm happy to sacrifice a bit of HP for that. And considering the light build of the T5 that's probably not too bad an idea. There are a lot of hills where I live, but lots of long flat road out to the city - where I head twice a week.
    The car is 1400kgs and has a 4.5 inch roof chop, rather skinny tires so it rolls well. Currently has a Holden 253, four speed and 2.78 gears. But the motor and box are tired. I'd much rather a 283 in there just because I like those motors and it suits the car.

    The T5's commonly found in Au have either a 3.25:1 or 2.95:1 ratio in first V6 and V8 boxes respectively. To reach 12:1 first gear for a v6 box you'd need up around 3.70 diff ratio. Seems a little short.


    Sent from my iPhone using The H.A.M.B. mobile app
     
  15. Jalopy Joker
    Joined: Sep 3, 2006
    Posts: 33,994

    Jalopy Joker
    Member

    should also check sites that have calculators to help figure most all of this out like project33.com
     
  16. Aerodynamics DOES play a large part at highway speeds as to what HP you'll need to push a box vs. a jellybean through the air.

    Sadly, until you get it on the road, you'll just be guessing.

    Cosmo

    P.S. Just a couple extra thoughts on this: Cadillac used, in 1970, a ratio 'round 2.20:1, in a boxy sedan with 500 ft/lbs of torque at like 600 rpm (slight exageration). In 1982, with a 368ci, somewhat less torque (400??), they used about a 2.70:1. In 1983, with the new HT4100cc (250ci, and a DAWG!!), they upped the GR to 3.50:1 due to the utter lack of torque (or power in general, but that's my opinion...). All else was the same, as the body did not change.
     
    Last edited: Apr 19, 2017
  17. Too many variables not listed. If it were me I would not gear it any higher than a 3.08. But I drive in town some. And I normally use considerably more torque than your mildly built 283 will put out.

    Airplane gears are for long miles on the highway with a motor that makes a boat load of torque down low. The 283 even in stock trim is not going to make a boatload of torque down low, you need more stroke for that type of performance.

    Anyway too many variables here, what the car weight is, cam choice, induction and etc.
     
  18. Rusty O'Toole
    Joined: Sep 17, 2006
    Posts: 9,756

    Rusty O'Toole
    Member

    The rule is, for maximum efficiency you want the engine to run at a speed right around its torque peak or maybe a tad lower. Since the 70s American cars have been using much higher (lower numerical) rear gears, this is why.

    The next question is, will your car pull all right with that gear and not have to shift down all the time? With a low torque engine and heavy car you may need to let it rev a bit higher. You see every choice is a compromise.

    A stock 1960 283 2 barrel produces max torque at 2200 RPM so either of your choices is in the ballpark.
     
  19. I have a 327/300hp in my 50 merc. which had a Standard Shift
    now I have a Auto Power-glide trans.
    the Standard rear is 3:73
    I don't have a Automatic rear 3:23 so I will change the Rear end ratio
    with Tire size & hope that works better than what I have.
    so why not use Different Tires Sizes to get what you Want.!
    Just my 3.5 cents

    Live Learn & Die a Fool
     
  20. els
    Joined: Sep 11, 2016
    Posts: 359

    els
    Member

    So many variables. engine size, torque, cam, transmission, tire size. Depends on what you want to do with your car. Cruise or go light to light. Drive it and see what works best. If you want fuel mileage you may have to drive it for a while to see where you are at on fuel consumption . I know your budget wont let you test gears. BUT if you can find junk yard gears that would help the wallet. I am a light to light kind of guy. Get out of the whole fast. My corvette has 6:50 gears, YES, 6:50 gears. fun to drive, but never on highway.
     
    Last edited: Apr 19, 2017
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  21. gimpyshotrods
    Joined: May 20, 2009
    Posts: 24,476

    gimpyshotrods
    ALLIANCE MEMBER

    My Falcon has 5.13:1 gears, and is on the freeway, every time that I drive it.

    Of course, the 0.50:1 2nd. overdrive helps.

    As has been mentioned, way too many variables to make an accurate judgement. Every single part needs to work together.
     
  22. els
    Joined: Sep 11, 2016
    Posts: 359

    els
    Member

    gimpyshotrods, please tell me you have overdrive or gear splitter. THAT IS SO COOL YOU DRIVE IT ON HIGHWAY.
     
  23. gimpyshotrods
    Joined: May 20, 2009
    Posts: 24,476

    gimpyshotrods
    ALLIANCE MEMBER

    T56 6-speed, from a Camaro. TWO overdrives. It is like a close-ratio 4-speed, but with two more gears on top, to make it practical.

    1st: 2.66:1 (combined final drive: 13.65)
    2nd: 1.78:1 (9.13)
    3rd: 1.30:1 (6.67)
    4th: 1.00:1 (5.13)
    5th: 0.74:1 (3.77)
    6th: 0.50:1 (2.55)
    Reverse: 2.90:1 (14.88)
     
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  24. Baumi
    Joined: Jan 28, 2003
    Posts: 3,342

    Baumi
    ALLIANCE MEMBER

    I have 3.36 gears in my 56 Chevy with a 327, 11:1 cr and a 234/244° cam @ .050 lift with 110° lsr. No OD trans.It really likes to rev and is getting very good mpg between 2500 and 3200 rpm. It starts to wake up at 3500 rpm. That´s were it feels at home.Most likely a mild 283 will be very fuel efficient at around 2500-3000 rpm. Anything lower the vacuum signal is getting too weak and that kills your torque.I´d try to bump up the compression to about 10:1 and use a cam around 216-220° @ .050 lift with 110° lobe separation.A dual plane intake with small intake runners will help, or maybe even stay with the 2 bbl manifold and carb. Depending on your tire size I´d say 3.08 to 3.36 is the lowest gearing you may want to end up with...
    The 327 loves long distance cruising at 4000 rpm. hahaha fotos 027.jpg
     
  25. gimpyshotrods
    Joined: May 20, 2009
    Posts: 24,476

    gimpyshotrods
    ALLIANCE MEMBER

    Keep that up for long over here, and you will be doing a little prison time.
     
    Baumi likes this.
  26. Baumi
    Joined: Jan 28, 2003
    Posts: 3,342

    Baumi
    ALLIANCE MEMBER

    Hahaha,I know. I never really dared to try that when I was driving in the states, I only had underpowered rental cars, so Iguess I was pretty save...but free stretches of Autobahn without speed limits are getting rare here aswell. Plus as I´m getting older I find myself taking it easier lately,... but once in a while, when someone happens to tailgate me in his late model family sedan or newfangled BMW I allow myself to shift back and show what a warmed over 365 hp 327 can do, hahaha...
    ( sorry for hijacking this thread)
     
  27. I like no OD and gears in the 3.2-3.5 range with a 28" tire. I ran 3.08s with a 28" tire and was happy but @ my sweet spot that put me way over the speed limit.
     
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  28. gimpyshotrods
    Joined: May 20, 2009
    Posts: 24,476

    gimpyshotrods
    ALLIANCE MEMBER

    This is an important point. Most old-school engines with carburetors don't like being lugged (run at low rpm), as the vacuum signal is too weak, causing fuel metering to become poor.

    Combine that with the American "bigger is better" approach to carburetion, and you have the false notion that all old engines are inefficient, and that power = waste.

    Anything that can be done that keeps the velocity through the venturi high will increase both power and economy.
     
    Baumi likes this.
  29. My 86 C10 has a 700r4 trans (.7 o/d) and 2.73 rear. Its a stock smog 305 with 28 inch tires. No tach but cruises fine on the highway but needs to down shift to pass or climb inclines.

    I had a 78 caprice with a 330hp 350, th350, 2.41 rear and 30inch tires. That car cruised sub 2000rpm at 65. But i could also go 80mph in 1st gear.

    Both got good fuel mileage but were dogs. The caprice had a 2.4second 60ft time with no tire spin. Overall i think both cars would be happier with a 3.23 or similar gear. I find that to be a good compromise between performance and economy
     
  30. carbking
    Joined: Dec 20, 2008
    Posts: 3,939

    carbking
    Member

    As others have mentioned, WAY too many variables to make any kind of recommendation other than homework.

    Some food for thought that has mentioned by several: too low an RPM will COST you fuel economy in a big way!

    One example: in 1973, before I was bitten by the "buy American" bug, I essentially did what you are doing. I wanted fuel economy. I evaluated fuel efficiency curves, bmep curves, static load (the work necessary to move the vehicle), dynamic load at a certain speed (air friction) etc. I then special ordered an economy car, specifying within the manufacturers options, engine size, rear end ratio, tire size (diameter), a 5-speed manual, etc.

    Once the vehicle was broken in, did several mileage runs at different constant speeds. Peak economy was obtained in 5th gear at 74 MPH, and was 39.4 MPG.

    Then some bonehead got the idea less speed saves fuel, and dropped the national limit to 55! The following weekend we made a 1000 mile trip at 55 MPH. Could NOT maintain 55 MPH in 5th gear (engine no longer in torque curve)! Best mileage now obtained in 4th gear and dropped to 32.8 MPG. By the way, I kept a record of every mile driven, and every drop of fuel, and had previously calibrated both odometer and speedometer for rallying purposes. Mileage figures are correct!

    The transmission had a 15 percent overdrive. If one compares the mileage at 55 and adds 15 percent, one gets almost the mileage at 74.

    So suggestion: look at the AVERAGE highway speed you wish to drive. Now figure the gearing, tire diameter, etc. to keep the engine very close to the peak torque for your engine, maybe a 100 or so RPM lower, in the highest gear in your transmission.

    Remember, other than being independently wealthy, math is your best friend. ;) Do your homework.

    Jon.
     

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