Another way to look at this is to compare what your equivalent ratios would be between the M21 and the TKX. With your M21's 2.2 1st, the 2.87 TKX would be like having a 4.32 rear with the M21 (or like a M20 with a 3.77 rear). The TKX in OD (.81) would be like a M21 with a 2.68 rear. 2.68 is a pretty respectable highway gear. But again, if you're cruising for long periods at > 75 mph then maybe the .68 which would be like a 2.25 rear in the M21.
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70 TA, 467 cid IAII, Edelbrock D-port heads, 9.94:1, Butler HR 236/242 @ .050, 520/540 lift, 112 LSA, Ray Klemm calibrated Q-jet, TKX (2.87 1st/.81 OD), 3.31 rear
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