I do about 30k miles a year in in my '89 110 with disco 200tdi. I run the original lt77 gearbox and a 1.2 discovery transfer box and 265/75 tyres. this will sit at 70 all day with no issues. A 300tdi has the same power and revs as the 200tdi, if you do not like the high revs it is the gearing...