Excelent question
Firstly, I am no expert on this in real world changes as I am considering the same options, but the calc are quite straight forward.
The two gear sets are independent of each other, so you can change one or both, you need to do some calculations to make sure you don't invest in new spurs and obtain the same gear ratio.
I am running a beetle and the 18/46 on the clutch and 16/48on the lay shaft.
I have a 30.5 Oneil engine, stufffed and stroaked crank, samba pipe and I find the low end power spins the wheels way too easily, also my max revs will be reduced.
My idea has been to increase the gearing to give more control lower speed, this will give me a better top speed (not that its slow at the moment)
Your easiest and most dramatic change is the lay shaft to 15 or even 16, but do you have the torque to pull that high a gearing.
:beta:
I have assumed a diff ratio of 1.7, excel sheet attached, comments or corrections welcome.
Clutch Spur Lay Diff Gear Final KMH MPH
24 38 17 48 4.47 2.63 104.68 65.43
24 38 16 48 4.75 2.79 98.53 61.58
24 38 16 50 4.95 2.91 94.59 59.12
24 38 15 48 5.07 2.98 92.37 57.73
22 40 17 48 5.13 3.02 91.16 56.98
24 38 15 50 5.28 3.10 88.67 55.42
22 42 17 48 5.39 3.17 86.82 54.26
24 38 14 48 5.43 3.19 86.21 53.88
22 40 16 48 5.45 3.21 85.80 53.63
24 38 14 50 5.65 3.33 82.76 51.73
22 40 16 50 5.68 3.34 82.37 51.48
22 42 16 48 5.73 3.37 81.71 51.07
22 40 15 48 5.82 3.42 80.44 50.27
22 42 16 50 5.97 3.51 78.45 49.03
22 40 15 50 6.06 3.57 77.22 48.26
22 42 15 48 6.11 3.59 76.61 47.88
20 44 17 48 6.21 3.65 75.34 47.09
22 40 14 48 6.23 3.67 75.08 46.92
22 42 15 50 6.36 3.74 73.54 45.96
22 40 14 50 6.49 3.82 72.07 45.05
22 42 14 48 6.55 3.85 71.50 44.69
20 44 16 48 6.60 3.88 70.91 44.32
22 42 14 50 6.82 4.01 68.64 42.90
20 44 16 50 6.88 4.04 68.07 42.55
20 44 15 48 7.04 4.14 66.48 41.55
18 46 17 48 7.22 4.24 64.86 40.54
20 44 15 50 7.33 4.31 63.82 39.89
20 44 14 48 7.54 4.44 62.05 38.78
18 46 16 48 7.67 4.51 61.04 38.15
20 44 14 50 7.86 4.62 59.56 37.23
18 46 16 50 7.99 4.70 58.60 36.63
18 46 15 48 8.18 4.81 57.23 35.77
16 48 17 48 8.47 4.98 55.25 34.53
18 46 15 50 8.52 5.01 54.94 34.34
18 46 14 48 8.76 5.15 53.41 33.38
16 48 16 48 9.00 5.29 52.00 32.50
18 46 14 50 9.13 5.37 51.28 32.05
16 48 16 50 9.38 5.51 49.92 31.20
16 48 15 48 9.60 5.65 48.75 30.47
16 48 15 50 10.00 5.88 46.80 29.25
16 48 14 48 10.29 6.05 45.50 28.44
16 48 14 50 10.71 6.30 43.68 27.30
:beta: