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I don't really understand the idea behind gear ratios. Does this just mean essentially that the gears (cogs) are all different sizes?

What specifically does the term "gear ratios" mean?

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    Great question. Was going to ask this. I know you can change the gears in your rear end for specific applications. Different teeth. Can't up vote for 4 hours. – Jonathan Musso Jan 15 '16 at 19:58
  • @JonathanMusso Thanks, me neither! Good work on the votes. What do you mean by you can change the gears in your rear end? – Max Goodridge Jan 15 '16 at 20:04
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    2quicknovas.com/rearcomplete.jpg You can remove the cover of the rear end, drain the oil, and begin the process of changing the gears. There are various options depending on your goals. I hear this is a good starting point to gain acceleration before modifying your engine. – Jonathan Musso Jan 15 '16 at 20:20
  • So is that a picture of the rear differential? – Max Goodridge Jan 15 '16 at 20:38
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    Yes, exactly. My mistake for not clarifying. – Jonathan Musso Jan 15 '16 at 20:44
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The generic term "gear ratios" refers to the ratio of the number of teeth on two meshed gears. Two gears that are meshed (teeth engaged with each other) must have the same sized teeth or they would not mesh. So, given the same sized teeth on both gears the term refers to the ratio of the number of teeth on each gear. For example, if the first gear has 50 teeth, and the second 100 teeth, the gear ratio is 1:2 (pronounced one to two).

This is important for things like the differential in your car because changing it will change the speed of your car for a given output shaft speed from the transmission. The transmission, your differential, and the diameter of the drive wheels combined determine vehicle speed for a given engine RPM.

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    The transmission, differential, and drive tire circumference determine vehicle speed ... right? – Pᴀᴜʟsᴛᴇʀ2 Jan 15 '16 at 21:38
  • Yes you're correct the diameter of the tires is the last piece of that but I was focused on just the gearing. You're right though, to be correct I should have included the diameter of the tires as the last factor in determining vehicle speed. I edited the answer to include this info. – cdunn Jan 15 '16 at 21:41
  • so ratios are referring to the number of teeth and not the number of rotations? – DucatiKiller Jan 16 '16 at 18:06
  • They end up being directly related. Two meshed gears the shaft speed of the second gear is related by the gear ratio but that happens because of the number of teeth and diameter of the second gear. – cdunn Jan 16 '16 at 21:56

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