Rear end ratio and tire size

Discussion in 'Transmission & Driveline Topics' started by Elliott Taylor, Jun 13, 2021.

  1. Elliott Taylor

    Elliott Taylor Member

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    Hey guys,

    I currently have a 3.73 posi 10 bolt rear end in my camaro, and am running 245/45/17 all the way around.
    I plan on changing to a 275/40/18 or a 275/35/18 rear.

    I’m getting confused as to what size tire this 3.73 ratio is based on? obviously changing tire height will affect the rear ratio, and Knowing this will help me determine how much of a change my new tires will make to my ring and pinion.

    Is there any easy way to figure this out ? I’m assuming the 3.73 ratio is based on some generic tire size?

    Any help is appreciated!
     
  2. biker

    biker Veteran Member

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    Nope, it's based on the relationship between the ring and pinion gears, that's it. The height of your tires makes a bit of a difference, but not a whole lot. Your wheel/tire is larger in diameter than your ring gear, so having a wheel/tire hanging off the end of the axle makes the final drive a bit taller, but 3.73 is the known constant for your car, so just realize you are impacting your mph a bit when you change tire sizes. For example, in my car with 3.73 gears and a 26.1 tall tire, I can go an inch either way and only make a 2 mph difference either way at 2500 rpm.
    Check out the tremec website and try their rpm/gearing/mph app. It's easy and includes tire height as a factor.
     
  3. Elliott Taylor

    Elliott Taylor Member

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    So to confirm, going from a 245/45/17 on the rear to a 275/40/18 won’t make much difference ?
     
  4. Elliott Taylor

    Elliott Taylor Member

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    Of the tire sizes I mentioned, do you guys have an opinion on which might look better considering I keep 245/45/17 up front? It seems like most say a 275 is about as wide as I can go...Maybe a 285?
     
  5. Elliott Taylor

    Elliott Taylor Member

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    FYI: I'm using American Racing VN2158961 18x9 w/ 5" of bs on the rear.
     
  6. berg2695

    berg2695 Veteran Member

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    The 60 mph cruise revs would be about 112 rpm less. That's a good thing, I think. :)
     
  7. Elliott Taylor

    Elliott Taylor Member

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    I agree! What's your opinion on the size ?
     
  8. berg2695

    berg2695 Veteran Member

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    I don't have any experience with really wide tires. Whatever fits and looks good to your eye is what you should do. I like the look of bigger diameter tires. That's why I have always stayed close to the stock late second gen Z28 size (27.4"). Others will have specific advise on the sizes you have mentioned. I imagine at 275-285 section width the rear seat belt hump would come into play as well as, possibly, the tail pipes. I had to dent the tail pipes where the tires come close when I installed a Flowmaster transverse exhaust.
     
  9. Elliott Taylor

    Elliott Taylor Member

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    I think a lot of guys on here have mentioned using 275/40/18 on the rear without mods, but I'm crossing my fingers that's the case for me. I believe that's what I will order, and it's the look I'm going for. You say a 27.4 is the stock tire size for late 2nd gen Z28s?
     
  10. berg2695

    berg2695 Veteran Member

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    Yup. 27.4" diameter. 225/70-15. Chevy had to restrict front suspension travel with spacers under the bump stops, however.
     
    Last edited: Jun 13, 2021

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