I am on the interstate all the time with my 240 manual. It maintains 70 with no problem, 75 the motor is kinda loud, and 80 it tells me is enough.
The mileage drops on the interstate compared to around town driving, but that is to be expected because of the extended high revs. I also have no problem maintaining 65 coming up Monteagle mountain on I-24.
I wish it had a 5th gear, but otherwise I love it.

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1985 300 TD 448K
1984 300 TD 278K

1983 240D euro 240k
1994 f-250 idi turbo 330K
1986 f-350 IDI
1987 F-350 IDI

1985 JD 1050 4wd
1965 IH 3660