I once drove my car two hours on back roads with 5.13 gears. It did 4,500 RPM at 60MPH. It got the same gas mileage on the trip as it did with 3.31 gears but it took 1/3 longer.
This is the exact answer from Dennis Manner when I asked him if I'll hurt my engine as I regularly cruise at 4K for hours at a time.
Well in terms of a motor that came out of Pontiac Engineering to go into production even with its cast iron Rods it had to run at 4500 rpm for 100 hours. I don’t see any issues with a street Buick motor doing the same other then fuel cost and not overheating.
