Ratio Calculations
    Harley-Davidson 45 3-speed transmission’s intermediate gears (1

st

 & 2

nd

) are the result of the transmission of power through two pairs of mating gears, the 1

st

 pair of which (high gear) is the clutch gear being turned by the countershaft high gear (on the cluster). The high gear internal ratio is the product of the high gear tooth counts.
    The 2

nd

 number is the product of the pair of gears (mainshaft + countershaft) for that ratio. The intermediate gear ratio is determined by multiplying the 1

st

 and 2

nd

 ratios together.
    In 3

rd

 (high) gear the clutch gear is locked to the mainshaft, so no power is transmitted through the gears, and the ratio is 1.000:1 (direct). Although the intermediate gears still turn, they’re only idling.
    Since 1935, there have been four different ratio gearsets used: two standard and two close-ratio. Please note that there are size differences between the components, and direct substitution of an alternate gear (early to late, close to standard, etc.) to provide additional an ratio selection is generally not possible. Factors Affecting Ratio Choice
    “Range” is the torque multiplication difference between 1st gear and high gear (3rd or 4th); wider-ratio gear-sets have more, typically between 2.6 and 3.1. This is the single most important determinant of low-speed acceleration.
    “Progression” is the next factor. This is the reduction or decay in the percentage drop in engine speed in the next gear (e.g. after shifting from 1st to 2nd). Most transmissions have some degree of progression in that the RPM drop on the 1-2 shift is larger than the RPM drop on the 2-3 shift (which would be in turn larger than the RPM drop on the 3-4 shift). The progression may not be linear (continuously reduced) or done in proportionate stages for various reasons, including a special need for a gear to reach a specific speed or RPM for passing, racing, etc. or simply economic necessity: the parts were available.
    The two factors are not mutually exclusive, but each limits the number of options for the other. A wide range, which gives a strong torque multiplication in 1st gear for excellent manners in low-speed traffic (especially with a heavy chassis or side-car) mean that the progression percentages must all be high. The amount of engine speed (and therefore power) that must be lost on each up-shift is higher than would be the case in a transmission with less range (but less power in 1st gear). A numerically low 1st gear (2.046:1, &c.) reduces available torque in 1st gear, but allows more choices of progression.
    Smaller motors, big cams, big ports or carburetors don’t pull well from low rpm, and when the 2-3 shift takes place at a speed where air resistance is high (70+ MPH), closer ratios are better and the factory (wide-ratio) gear-set is at a big disadvantage with its 36% RPM drop (5,000 RPM drops to 3,182). Close ratio boxes are an improvement under these conditions, but have the disadvantage of less torque multiplication in 1st.
    Below are some useful ratio tables, how they are generated internally, and their effects on performance. Click here to see the big twin tables: .

Standard Ratios
    For the 1935-40 standard (& 1933-34 reverse-gear) transmissions, the high gear tooth counts are: 21 for the main drive gear and 13 for the clutch gear, so the 1

st

 number is always 1.615385 for the 1935-40 standard ratio transmission.
    For the 1941-73 standard transmission, the high gear tooth counts are: 22 for the main drive gear and 14 for the clutch gear; the 1

st

 number is always 1.571429 for the 1941-73 standard ratio transmission.

Overdrive transmission
    If you already have a tall rear tire and tried a 34 tooth motor sprocket and either the engine is too "busy" or you want to go even faster (above 100), change the rear wheel to use a different sprocket that doesn't go outside the drum, like a Honda 350 or any disc brake.
    If you want to keep the original parts, and overdrive will help, but it's a PITA.
    The problem with overdrive is that high gear in most transmissions is direct (no gears transmit power, they're still touching but they "freewheel"), and the shift clutch (or dog) just locks the output gear (holding the sprocket) to the mainshaft (holding the clutch) so there is no ratio at all.
    An overdrive would need its high gear to be a "normal" gear (the result of power transmitted across the shafts twice), but with a final number less than 1.00, like .8 or .9. This leaves 3rd as direct (1.00:1), and 1st & 2nd can be where they are now. Of course, the shift drum positions are wrong...
    I don't know if the shaft center distance of a 45 matches anything else, which may make using Sportster or big twin gears possible.
    The easiest way to make an OD, using as many stock parts as possible, is to use the existing reverse position (since it has a drum slot, and shift clutch). The reverse idler and its shaft (which spins the output gear backward) come out.
    Instead, another, smaller gear is substituted for the 19 tooth reverse (35284-41) on the mainshaft (you can't use the original, because it doesn't mesh with the countershaft reverse gear, only with the reverse idler gear). The new mainshaft gear must have its hub machined to match the shift clutch recess on the reverse gear (or the tooth row attached to the existing hub).
    The stock 14 tooth reverse gear is part of the cluster, and it's the wrong size. To get an overdrive ratio, it's got to go the other way: bigger. This requires the existing gear to be ground off, and another (with the same tooth pattern) to be welded or key to the countershaft gear.
    A useful ratio, keeping the same 36 tooth gear total, can be made from a 13 tooth (but where to get one? a reverse idler is 13...) and a 23 tooth (Andrews close 3rd mainshaft gear?) substituted for the reverse gear, making the final drive .8882:1. This reduces engine speed by this amount, changing a 34 tooth into the equivalent of a 38 tooth, and increasing top speed by 12.6%. A 12/24 combination is too much change, a 14/22 does nothing.
    This also makes the shift into overdrive even closer than the WR shift into high gear, so it's actually useful for racing.
    It just takes a lot of work.

3-Speed + overdrive ratio calculation

Components

Teeth MS

Teeth CS

27/19 ×

Ratio

1

st

 gear

22

14

22/14

2.469:1

2

nd

 gear

18

18

18/18

1.571:1

3

rd

 gear

14 (clutch gear)

22

(locked)

1.000:1

Overdrive

13

23

.888:1

3-Speed + overdrive RPM loss on shift

Ratio

1

st

RPM drop to 2

nd

2

nd

RPM drop to 3

rd

3

rd

RPM drop to OD

4

th

Standard

2.469

36%

1.571

36%

1.000

11%

  .888