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Thread: rpm compared to gear rastios

  1. #1
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    rpm compared to gear rastios

    what the difference is in rpm and response between the ring /pinon gears of 3.73 and 3.90 at 70mph with a 700r4 and 25 inch tires? Which do you think would be best to use?
    this is for a driver[non race]with a small cam/headers/intake/650cfm carb.used for car shows and cruises but like to have a little more balls for burnouts without missing with engine.
    friends car just trying to find rpm for him and i don't know myself how to figure it..help

  2. #2
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    Rpm with 3.73 =3550 approx.

    Rpm with 3.90 =3660 approx.
    25" tire @ 70mph

  3. #3
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    Quote Originally Posted by rob41willys
    Rpm with 3.73 =3550 approx.

    Rpm with 3.90 =3660 approx.
    25" tire @ 70mph
    that seems very high for a car that has a overdrive trans in it..sounds right for a turbo 350 or 400 trans

  4. #4
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    700R4 is basiclly the same as a turbo 400 but usually has a lock-up type converter. I'm assuming it's not a lock-up type converter?

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    Quote Originally Posted by rob41willys
    700R4 is basiclly the same as a turbo 400 but usually has a lock-up type converter. I'm assuming it's not a lock-up type converter?
    the transman that built the 700r4 told us that when in 4th gear it was 60% overdrive..turbo 400 is a 3 speed trans and a 700r4 is a 4 speed..there is a big differance between the 2 trans.

  6. #6
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    need to reword this.
    what would rpm be with gears of 3.73 or 3.90 at 70mph with a 700r4 and 25 inch tires? car now has 2;3? gears and just idles going up highway and no balls in town so wanting to go lower to help town performance still have descent cruising rpm on highway
    this is for a driver[non race]350sbc with a small cam/headers/intake/650cfm carb.used for car shows and cruises but like to have a little more balls for burnouts without messing with engine.
    friends car just trying to find rpm for him and i don't know myself how to figure it..helphotrodharley

  7. #7
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    just incase anyone else was interested in the rpm running these gears then i found answer in another fourm.want to share answer with everyone on here.

    gearhead1011 wrote:
    I think the 700r4 has .7 overdirve. If that's right with a 2.38 rear it would be turning about 1700 at 70 mph with a 25" tire. That would go to about 2,650 with a 3.73 and 2,775 with a 3.90.


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