I was thinking that ICRPG could be converted to just d6s. Some rolls would be a bit complicated, but here are my ideas:
Attempt roll 1d20 => 3d6.
Not a 1 to 1 conversion but should be a good replacement. Higher targets (14+) may need to be shifted down to account for the bell curve. Make 3, 4 or 5 on the 3d6 have the same result as a “natural 1” on a d20 and a 16, 17 or 18 the same result as a “natural 20” on a d20.
1d4 => 1d3 (on a d6 1-2 = 1, 3-4 = 2, 5-6 = 3, average = 2, 1d4 average = 2.5)
1d6 => 1d6
1d8 => 1d6+1 (average = 4.5, 1d8 average = 4.5)
1d10 => 1d6+2 (average = 5.5, 1d10 average = 5.5)
1d12 => 1d6+3 (average = 6.5, 1d12 average = 6.5)
Having the effort just be a d6+x makes the rolling simpler and quicker. If you need a full range of 1d4, 1d8, 1d10, 1d12 for the tables and such then you can use the following. Each result should have an equal probability:
Equal Probability Rolls:
1d4 => Roll a d6, re-roll 5,6
1d8 => Roll 2d6. First d6 result, keep 1-4, re-roll 5 or 6. Second d6 result: 1-3 add 0 to the first result, 4-6 add 4 to first result.
1d10 => Roll 2d6. First d6 result, keep 1-5, re-roll 6. Second d6 result: 1-3 add 0 to the first result, 4-6 add 5 to first result.
1d12 => Roll 2d6. First d6 result, keep 1-6. Second d6 result: 1-3 add 0 to the first result, 4-6 add 6 to first result.
d100: Using the “d10” above, roll the 10s digit, then roll the 1’s digit.
This is a little more complicated than I originally thought it would be. Just thought I’d throw it out there.