I easily have gotten up to 90 mph on level ground with my 261 and 3.55 rear-end and 31" inch tires. But I try not to cruise at more than 65 mph (the gas mileage decreases significantly above 62/63 mph).
That (90 mph) would be a smooth/quiet 3500 rpm - no big deal.
My Suburban is quiet - we can talk at normal noise volume - saying something like "that's pretty cool".
It sure does impress my friends, though (as I immediately settle down to an even quieter 60-65 mph).
