This is written for US spec cars:
M46 gas 260's were 3.73. Bw55 were 3.54, as were Diesel M46 260's and 240's. Diesel autotragic was 3.31.
1976 (possibly 1977 as well) 240's, M45/46: 4.10:1 or 3.91:1; Bw55: probably 3.91, could be 3.73
1978-80 240 M45/46: 3.91:1, Bw/Aw55: 3.73 for 1980, probably 3.91 for 1978-79
1981 240: M46 B21F/FT with Bosch ignition, as well as Aw55 B21F with Bosch: 3.73; B21F-MPG: 3.54 if M46, 3.91 if Aw55.
1982 240: B21F-MPG and B21F-LH, same as 1981 MPG, except that Aw70 became available. B21FT actually was supposedly 3.73 with the Aw71 in 1982, at least, according to the greenbooks.
1983-on cars: B23/230F: M46/47, 3.31:1; Aw7x, 3.73:1. B21FT: M46, 3.73:1, Aw71, 3.91:1.
This also assumes that the cars were either built at Torslanda or in Belgium. If Halifax, anything goes.
Re: a 3.54:1 rear axle with a B21F, I could see that, if the head had an L cam in it. The B cammed B21's have enough oomph to deal with a 3.54 and a .79 gear ratio. After all, swedishiron.com drove a '79 245DL that he switched over to a 3.54 from a '76 264GL Bw55 car for many years. As long as you kept the speed above 45 mph in OD, it would pull uphill grades without losing steam. Otherwise, yes, it can be as slow as a M-cammed B23F or B230F/M46 car with the stock 3.31 rear axle. Great for mileage and top speed, not great for acceleration. You know it's bad when you're doing 4500 rpm in a B230/M46 245 up a hill in 2nd gear and an Aw70 equipped B23F 245 is keeping up with you and actually trying to push you harder going up said hill.