Gear swaps.....should I put the 3.73's back in?

Discussion in 'Transmission & Driveline Topics' started by Millwright, May 15, 2019.

  1. Millwright

    Millwright New Member

    Messages:
    24
    Likes Received:
    4
    Joined:
    Mar 30, 2019
    Location:
    Ontario Canada
    I just bought a 73 LT Z28. The last owner did the restoration and overall its a nice car. one thing he did was replace the 3.73's with 3.42 gears to make it cruise better on the highway. Took the car out tonight and it goes but I can see that the bottom end could be improved. I live about 30 minutes out of town so the taller gears is a bonus running country highways pretty much everywhere I go.

    How much of a difference in bottom end performance would I see going back to the original gears (came with the car)?

    How much higher rpm am i going to see cruising on the highway?

    Engine is supposed to be around 350hp and has an M21 4 speed.
     
    Last edited: May 15, 2019
  2. badazz81z28

    badazz81z28 Veteran Member

    Messages:
    15,864
    Likes Received:
    292
    Joined:
    May 4, 2001
    Location:
    Las Vegas, NV
    You’ll feel some difference with the 3.73s. It comes down to if the cost is worth the slight change. With a 2.20 first gear...I wouldn’t shy away from 4.10s if you’re looking for the 1st gear go.
     
  3. ULTM8Z

    ULTM8Z Veteran Member

    Messages:
    8,984
    Likes Received:
    275
    Joined:
    May 19, 2000
    Location:
    Los Angeles
    What ever rpm you're cruising at, simply multiply it by the ratio of 3.73/3.42 and it'll tell you the new rpm.
     
  4. big gear head

    big gear head Veteran Member

    Messages:
    6,103
    Likes Received:
    57
    Joined:
    Jul 30, 2001
    Location:
    Central City, Ky.
    The M21 is a bigger problem than the 3.42 gears. The M21 has a 2.20 first gear. Swapping to a M20 would give you a 2.52 first gear and would be more noticeable than the 3.73 swap, and still work pretty well on the highway.

    In the way of originality, keeping the original transmission and 3.73 gears would be the way to go.
     
  5. CorkyE

    CorkyE Veteran Member Lifetime Gold Member

    Messages:
    9,975
    Likes Received:
    340
    Joined:
    Nov 4, 2004
    Location:
    Ringgold, GA
    I have 3.73 gears in mine and with a non overdrive transmission, keeping up with interstate traffic wasn't a lot of fun. I kept the gears and swapped in a 700-R4 transmission. FWIW - 3.73 was factory stock in later model 4 speed 2nd gen's. Here's a good set of calculators that can help. http://wallaceracing.com/Calculators.htm
     
  6. ZEN357

    ZEN357 Veteran Member

    Messages:
    300
    Likes Received:
    56
    Joined:
    Feb 12, 2018
  7. Gary S

    Gary S Administrator Lifetime Gold Member

    Messages:
    21,866
    Likes Received:
    559
    Joined:
    Apr 14, 1999
    Location:
    Bismarck, North Dakota
    The gear difference between 3.42 and 3.73 is about 9%. So, you can expect about a 9% better launch with the 3.73 and 9% more rpms.
     
  8. badazz81z28

    badazz81z28 Veteran Member

    Messages:
    15,864
    Likes Received:
    292
    Joined:
    May 4, 2001
    Location:
    Las Vegas, NV

    Exactly why many of us have 5 and 6 speeds. If you’re going to drive the car, with today’s speeds you gotta have OD.
     
  9. big gear head

    big gear head Veteran Member

    Messages:
    6,103
    Likes Received:
    57
    Joined:
    Jul 30, 2001
    Location:
    Central City, Ky.
    Na, you just have to take your panties off and put on your big boy britches. I've been driving mine for 35 years with a 4 speed and 3.73s, 4.11s, and 3.70s at 70 and 80 MPH for hours at a time. That's the way they were back then and nobody cared. People are spoiled today with overdrive and low RPMs.
     
    Last edited: May 17, 2019
  10. Gary S

    Gary S Administrator Lifetime Gold Member

    Messages:
    21,866
    Likes Received:
    559
    Joined:
    Apr 14, 1999
    Location:
    Bismarck, North Dakota
    big gear head is right. When these cars were new, we thought nothing of driving them all the way across the country at 4500 rpms. After all, they were built for it so why not enjoy them?
    The car can do it, can you? :)
     

Share This Page



  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.