Could be the speedo cable. When they start to fray, the reading foes erratic. Happened to me also.

Here's the link. Should work with you speedo too, since the earlier 244s had the same one.

http://www.odometergears.com/

It looks like your 74 144 is quite similar to mine. Mine originally was also Glacier White, 1.03 speedo and 4.10 rear end, with the M40. Except that being a Euro edition, mine came with a single ZS, yours is FI.

I replaced my speedo from a 244 with the 0.98 ratio. Works well, but if you are going 50mph, it will register 45, and so on. As expected, the odometer quit few months ago, the prime suspect being that pesky plastic gear.

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...and the bricks keep on rolling