According to my 1975 US/Canada owners manual your rear axle ratio is a very tight 4.3 to 1 which translates to 16.9 mph per 1,000rpm or a little over 3500 rpm at 60 mph - which is just over half the 6,000 rpm limit of the B20 (shift to 3rd for passing). It sounds busy alright (I owned a '75 245 w/M40) but not harmful to the engine. My guess is that the OP is mistaken that his trans is an M45 - far more likely to be an M40 (but ratio wise it makes no difference). If the rpm racket is too much to live with the easiest path to lower road speed rpm would be to do a rear axle swap. A '75 244 would have a 4.1 ratio. Early 240's with the B21 have a 3.9 ratio and later 240's (with auto trans) a 3.73. Early V6 equipped 260's (with auto) have a 3.54 ratio. The later 240's with standard shift (M46) have a 3.31 but I think that would be far too long a ratio for the B20. - Dave
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