Quetion about gear ratios

Discussion in 'General Chevy & GM Tech Questions' started by ajarman, Aug 31, 2010.

  1. ajarman

    ajarman Epic Member 5+ Years ROTM Winner 1000 Posts

    I am trying to understand the whole rear gear concept. I think I have pretty good idea, but maybe some of y'all can clear a quesiton up for me. I have a '08 Silverado with 3.42's and my stock wheel size is 265 70 R17. My buddy has s '10 Silverado with 3.42's and his stock tires are 245 70 R17. I have read if I go to a 285 70 R17 that would be like having 3.23's. So to get to my question; if my tires from the factory are 265's and my buddy's are 245's was my gearing a bit lower (higher number) from the factory or am I really more like 3.30's. I am hoping this is not a noob question, but I just can't wrap my little brain around it.


    DAREDEVIL Rockstar 4 Years 1000 Posts

    NO WORRIES , put them on.....its only a differents of 14mm ( 9/16 ) overall diameter...if it would change your gear ratio, worn tires would do the same ( just oposit ) !!!!! LOL
  3. Greg84

    Greg84 Rockstar 100 Posts

    The gearing number is set by the ring gear and pinion gear ratio the rear axle housing. Whether you're running a 265/70/17 or 315/70/17, your truck will still have 3.42 gears in it. The numeric ratio is the number of times the pinion gear (the smaller one connected to your driveshaft) has to turn in order to make the ring gear (the larger gear connected to the rear axles) turn one time.

    With everything else equal, a numerically lower gear ratio (examples - 3.23, 3.08, 2.73) is going to have less acceleration than a a numerically higher gear ratio (3.73, 4.10, 4.56). When people say "285's make the truck feel like it has (insert numerically lower gear ratio here) gears", they mean it feels slower and they're trying to use a comparison that other people can relate with. Nothing with the actual ratio has changed though.

    Now you're probably wondering "why does my truck have 265 tires from the factory and his have 245?" Could be for lots of reasons. Some trim levels just have smaller tires. Could be his truck is a 2500 and yours is a 1500. Your truck probably has a 4 speed transmission, his could have a 4 speed or 6 speed.

    And daredevil - 265/70/17 to 285/70/17 is over an inch diameter change, I think you figured out the sidewall height difference but you'd need to double that for the overall diameter..

    DAREDEVIL Rockstar 4 Years 1000 Posts

    OOPS, sorry, your right ..lol

    and nice explanation on the gear ratio's.....!!!!!!
  5. ajarman

    ajarman Epic Member 5+ Years ROTM Winner 1000 Posts

    Thanks Greg84! Both trucks are 1500's and mine does have the 4spd and his has the 6spd.

    So if you took both of our rear ends apart it would have the same pinion and ring gear, but the fact mine has larger tires makes it "feel" I have *smaller number* gears. Right?

    I had this thought that because my truck came with 265's that the gearing was *higher number* to compensate for the tires.
  6. silveradotrailblazer

    silveradotrailblazer Epic Member 5+ Years ROTM Winner 5000 Posts

    Nice explaination Greg84.
  7. Greg84

    Greg84 Rockstar 100 Posts

    Yes. And no. :happy:

    Yes on the first part, if you pulled apart the rear ends on both your truck they'd be identical inside. Same parts, same gears, same housing.

    The tires may be a part of why your truck seems (or is!) slower than your buddy's truck, but the transmission is the main reason. The gears inside the transmission have different ratios between your trucks. I can't remember the exact numbers, but first gear in your 4 speed transmission is around 3:1. That means the engine is turning over 3 times to make the driveshaft go around once. With the 6 speed transmission, the first gear in the transmission is numerically higher, around 4:1. That numerically higher gear in the transmission is whats making his truck accelerate faster.
  8. ajarman

    ajarman Epic Member 5+ Years ROTM Winner 1000 Posts

    I am slowly getting it. Thanks for taking the time to explain Greg84. If I can think of anything else I will post em up!

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