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In any manual transmission, overall ratio of each gear = that gear ratio X final drive ratio. For example if 1st gear ratio = 3.4 and axle ratio is 4.5, then overall ratio in 1st (overall gear reduction in 1st) = 3.4 X 4.5 = 15.3

This is true if we assume we have 4 gears (2 sets of gears), first engaged gear set is 1st gear and the 2nd set is always pinion and crownwheel gears of the differential.

It is quite simple up to this point. However, I saw something like a 3rd gear set in some instructional pictures and animations about how a manual transmission works. One of them is the following here:

enter image description here

As you see above, 1st, 2nd, 3rd, 4th and 5th gears (gear sets) are identified clearly but it seems that there is another gear set which is engaged before any of the main gears. I mean the set I specified with a blue arrow. This gear set appears to have a ratio more than 1 : 1 because the green gear has a smaller diameter than the red gear. Therefore, it can affect the overall ratio noticeably. Imagine it has a 1.3 : 1 ratio, then the overall 1st gear would be 3.4 x 4.5 x 1.3 = 19.89!!**

(Please note that this is not a low range gear in 4x4 cars. I know what is the purpose of those gears and how they increase torque one step further for offroad climbing.)

If it was a 1 : 1 ratio, it would make sense (3.4 x 4.5 x 1.0 = 15.3) but it isn't. What confuses me here is that if there is such a gear set in all manual transmissions, why isn't it taken into consideration when it comes to calculating the overall ratio?

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  • I'm not sure I understand your question?
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Feb 28, 2022 at 18:50
  • What is that gear set I specified with blue arrow? If it is not 1st, 2nd, 3rd, 4th, 5th or reverse, then what gear is it?!
    – Nariman Asgharian
    Feb 28, 2022 at 18:54
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    That gear may or may not exist depending on the transmission design. In some cases the input shaft is directly driven by the engine (via the clutch of course). In other cases there needs to be an offset or the direction needs to be reversed. That ratio can be whatever it needs to be based on the design. If it's 1:1 then obviously it's a non-issue as far as ratio. If it's not, it's going to affect the overall ratios of the transmission gears. The input vs. output ratio, however, is going to be correct because the specs will factor that in.
    – jwh20
    Feb 28, 2022 at 18:59
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    It's a transfer gear, to move power from the input shaft (engine side) to the layshaft. It should be a 1:1 ratio. The gears on the layshaft are then meshed with the appropriate gears on the output shaft to provide power to the rest of the drivetrain. Don't let the look of red/green fool you. It's more or less a parallax effect you're seeing, because the red/green gears are offset and the green gear is behind the red one (in this drawing). It looks smaller because part of it is covered up.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Feb 28, 2022 at 19:01
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    While technically "yes" it is mainly 1:1, it doesn't have to be. It can be any ratio it needs to be to suit the engineer's wants/desires. For instance, if this was a transmission for a jet engine, it would have a very small gear on the input shaft and a larger gear on the layshaft, because a jet engine spins at such a high speed. Conversely, if this were a transmission for a large cargo ship, the input gear would be large and the layshaft would be small, so as to spin the prop faster. It's all in how its designed.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Feb 28, 2022 at 20:44

1 Answer 1

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As Paul says in the comments, that's a transfer gear, and for a car gearbox would usually be 1:1. If it's not, then it'll probably be already factored into the quoted ratios, so unless you dismantle the gearbox, you're unlikely to know about it. So if if were, say, 1.3, then the first gear ratio would be listed as 4.42 rather than 3.4 - the same way that only the overall ratio is quoted for reverse, even though there's an extra gear (the idler) there.

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  • Idler in and of itself will not change the gear ratio. It inputs at one and outputs at one. Gears on either side might do something funky ;-)
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Mar 2, 2022 at 22:49
  • Exactly, hence why it's ignored, rather than quoting two separate ratios, which would be meaningless
    – Nick C ♦
    Mar 3, 2022 at 20:48
  • @Nick C - Thanks for your answer. Yes, you exactly mentioned what I meant to say.
    – Nariman Asgharian
    Mar 4, 2022 at 7:31

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