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Gear change for milage

Discussion in 'The Hokey Ass Message Board' started by 1949 caddyman, Jul 6, 2012.

  1. 1949 caddyman
    Joined: Jun 30, 2010
    Posts: 225

    1949 caddyman
    Member
    from arizona

    I have 350 chevy engine in 1965 Chevelle, 4.11 rear, 200R4 trans. Tire is 26D. At 75 MPH it gets 15-17 MPG at 2700 RPM. I am thinking about gear change to help milage. With a 3.08 gear it would turn 2000 RPM. Anybody have ideas on posible milage increase?
     
  2. squirrel
    Joined: Sep 23, 2004
    Posts: 50,261

    squirrel
    Member

    It probably depends a lot on how your engine is set up...cam, intake, exhaust, etc.

    Sometimes mileage goes up, sometimes it goes down, when you drop rpm that much.
     
  3. 1949 caddyman
    Joined: Jun 30, 2010
    Posts: 225

    1949 caddyman
    Member
    from arizona

    The cam is one step above stock cam. Idles good in gear with AC on at 600 RPM. Has good vaccum at idle. It has dual plane intake, stock exhaust manifolds, dual exhaust.
     
  4. You can't just change one thing and expect magic....unless something is broken. Aerodynamics, engine design and subsequent modifications,transmissions, rear end gears, driving style, tire air pressure, bearing rolling resistance...etc. all play a important part in determining your fuel mileage. I bet the gear change would get you a few at most. but only if it puts the engine into the power range were it is most efficient. Also don't forget to adust the speedo gear or all calculations are junk.
     

  5. seb fontana
    Joined: Sep 1, 2005
    Posts: 7,211

    seb fontana
    Member
    from ct

    Uh, I figure with what you got [26d tire, 4:11, 75mph] is closer to 3900 rpm!! 3.08 would be around 2900..I'd say you'd be ok..
     
  6. you forgot something in that equation......

    200r4's have a .67 overdrive 4th gear.......
     
  7. 1949 caddyman
    Joined: Jun 30, 2010
    Posts: 225

    1949 caddyman
    Member
    from arizona

    The car has a 200R4 OD trans with a .67 OD. 2700 Rpm at 75 MPH is correct.
     
  8. 200r4 is an overdrive transmission
     
  9. Is elevation an issue where you are? If you go to high with the ratio you may not reach the HP and torque curves and milage could get worse.
     
  10. George
    Joined: Jan 1, 2005
    Posts: 7,479

    George
    Member

    Used to have a Crown Vic P.I. that had 351W/AOD/3.08 & got around 20 & was fast enough for the cops to use.
     
  11. 1949 caddyman
    Joined: Jun 30, 2010
    Posts: 225

    1949 caddyman
    Member
    from arizona

    Im at 1300 feet altitude. The engine runs like a stock set up would. It has low end torque, I think it would pull ok at 2000 rpm just dont know what milage to expect.
     
  12. 1949 caddyman
    Joined: Jun 30, 2010
    Posts: 225

    1949 caddyman
    Member
    from arizona

    Yes speed limit on I-10 is 75, At that speed most cars pass me.
     
  13. Man I would LOVE that. :)
     
  14. Cerberus
    Joined: May 24, 2010
    Posts: 1,390

    Cerberus
    Member

    Lowest numerical axle ratio 3.42 with 350/200 4r. Do a search here. Plenty of posts on this topic. Check out my album. I run od's in all my rides. I made the mistake of installing a 3.42 ratio behind a 350 with a 200 4R....should have installed 3.73 ratio for multi-purpose driving.
     
  15. seb fontana
    Joined: Sep 1, 2005
    Posts: 7,211

    seb fontana
    Member
    from ct

    Yep, forgot the od!! If its the only thing I forget today I'll be doing good!! Sorry..
     
  16. Ole don
    Joined: Dec 16, 2005
    Posts: 2,915

    Ole don
    Member

    Most newer cars run about 1700 to 1900 at 70. They also shift down often. Your 200 R should be OK with shifting down, are you?
     
  17. Where is the motor's torque peak RPM? Gear it to cruise at or a tad under that RPM.
     
  18. railroad
    Joined: Feb 3, 2010
    Posts: 242

    railroad
    Member

    this is what you want to work from. If you can find your cam card or get the info, it will tell you what rpm your tq peak is. I would try to run the rpm at the beginning of the tq fast rise. Good cruising rpm.
     
  19. squirrel
    Joined: Sep 23, 2004
    Posts: 50,261

    squirrel
    Member

    Most newer cars also have efi and really mild cams, and are designed to operate well at much lower rpm than we usually make "hot rod" engines endure.

    Good point about the downshifting...it can get annoying
     
  20. Deuce Roadster
    Joined: Sep 8, 2002
    Posts: 9,519

    Deuce Roadster
    Member Emeritus

    [​IMG]

    Get yourself a vaccuum gauge and install it. Drive until you find the best RPM range for mileage. ( highest vacuum reading ) then change the gear ratio so that the engine will be at that RPM at the speed you want to drive :D

    Problem SOLVED !!
     
  21. fiveohnick2932
    Joined: Mar 29, 2006
    Posts: 891

    fiveohnick2932
    Member
    from Napa, Ca.

    I run 2.75 in my truck with a big block chrysler and it get good mileage for what it is (270 cam 750 holley and performer intake). I gets way better milage than the 3.73s I used to have in there but I think it got the best milage when it had 3.23s in it. Why? because it let it rev a little higher on the freeway and was a little easyier at taking off so it didnt have to work as hard (probably better gears matched for my 270 cam). The 2.75 gears are great and its still fast but you can tell it wants a little more rpm when driving at 65 or lower.
     
  22. 3:08s might be too much of a change from 4:11s....the performance will really drop off around town....maybe you could split the difference and use 3:50s or 3:42s...jmho.




    :
     
  23. AnimalAin
    Joined: Jul 20, 2002
    Posts: 3,417

    AnimalAin
    Member

    With a 3.08 rear gear and 0.67 OD, your top gear ratio will be about 2.06. I doubt a slightly-warmed-over carbureted V8 will be happy at the combination of load and rpm resulting from this combination. The first gear ratio in the trans is plenty low enough to get you away from a stoplight, but I really doubt you'll be happy with the performance on the highway with that gear combination. I would guess a 3.42 or thereabouts would be substantially better. Of course, if you really need to get the mileage better....
     
  24. Chevy Gasser
    Joined: Jan 23, 2007
    Posts: 703

    Chevy Gasser
    Member

    3.08's are perfect! You now have the lower 1-2 gears so around town no problem. What most people forget is 2.70's were a standard rear gear for the automatic overdrives. Your Chevelle is a lighter car. 3.08's are what I've been running for 15+ years. Still love'em. I have a mild 350/700r4 in my '55 Chevy 235 60 15's, 1800 rpm at 70mph. 20mpg
     
  25. mustang6147
    Joined: Feb 26, 2010
    Posts: 1,847

    mustang6147
    Member
    from Kent, Ohio

    I wouldnt go below 3:55. I would think you might consider 3:73... Remember its about 300 rpms per gear drop....
     
  26. Thing is with a torquey motor and a decent 1st gear the rear ratio doesn't matter much. I had a truck with a 700R4 and a 2.73 and towed stuff with it. It actually pulled harder than an identical one with a 3.73. If the thing goes, it will still go with the taller gear.
     
  27. 1949 caddyman
    Joined: Jun 30, 2010
    Posts: 225

    1949 caddyman
    Member
    from arizona

    Thats what I thought. The engine makes 335 # torque @ 2000 rpm, max out at 375 # @ 4000rpm. With the 4.11 First gear shifts in 20-30 feet on slow accel. or smokes the tires on full throttle.
     
  28. outlaw256
    Joined: Jun 26, 2008
    Posts: 2,023

    outlaw256
    Member

    ive got no advice for ya, as i wouldnt change much. as far as gas mil. i wish just one of mine got that good. my worst is just a tad over 3 mpg and my best is about 12.
     
  29. squirrel
    Joined: Sep 23, 2004
    Posts: 50,261

    squirrel
    Member

    heh....I have 3 on the road right now, they are all getting 10-11.
     

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