Steve here are my gearing charts with the Hoosier TD 20.6 diameter tire. As your race A12 is making in around what my hotted up A15 is I think it would be close. the A12 is likely revving higher so the top speed is likely good. Note that I'm using overdrive transmissions so you need to look at the 4th gear column.

4.62 R&P 60 series 5 speed
20.6 diameter tire


RPM 1 2 3 4 5
4000 15 24 38 53 65
4500 17 27 43 60 73
5000 19 31 48 66 81
5500 21 34 53 73 89
6000 23 37 58 80 97
6500 25 40 63 86 105
7000 26 43 67 93 113
7100 27 43 68 94 115
7200 27 44 69 95 116
7300 28 45 70 97 118
7400 28 45 71 98 119
7500 28 46 72 99 121
7600 29 46 73 101 123
7700 29 47 74 102 124
7800 29 48 75 103 126
7900 30 48 76 105 128
8000 30 49 77 106 129
8100 31 49 78 107 131
8200 31 50 79 109 132

60 series 5 speed 185/60D13 4.38 R&P
20.6 Diameter tire
RPM 1 2 3 4 5
3500 14 23 36 49 60
3600 14 23 37 50 61
3700 15 24 38 52 63
3800 15 25 39 53 65
3900 16 25 40 55 66
4000 16 26 41 56 68
4500 18 29 46 63 77
5000 20 32 51 70 85
5500 22 35 56 77 94
6000 24 39 61 84 102
6500 26 42 66 91 111
7000 28 45 71 98 119
7100 28 46 72 99 121
7200 29 46 73 101 123
7300 29 47 74 102 124
7400 29 48 75 104 126
7500 30 48 76 105 128
7600 30 49 77 106 130
7700 31 50 78 108 131
7800 31 50 79 109 133
7900 31 51 80 111 135
8000 32 52 81 112 136
8100 32 52 82 113 138
8200 33 53 83 115 140