Might as well get my two cents in. I have a 1974 1802 Touring, original drivetrain 240,000 km, 4:11 diff. I have to drive 4100 rpm just to get to 65 mph. It actually gets quieter around 4300 rpm. Couple of trips to Monterey this year 4 hrs at around 4300- 4500 each way in 90 degree weather. Intervals at 5k+, but the temp starts to go up a bit, and the frequency of speeding tickets also increases. Bottom line. The engine, even though old and tired, doesn't mind 4k+ driving. Actually seems quite happy.