Is using 5th gear at 40mph messes up my engine?

[Loren]

You'll hear this sort of thing from old school guys... or guys who have learned things from old school guys. The thing is, they're trying to compare the sophisticated 1.5 liter twin-cam, 16-valve, fuel-injected, variable valve timed Yaris engine to... some similar displacement 8-valve, carburetted, 70's era emissions choked engine. The Yaris engine makes at least 75% more power and torque than the low-tech equivalent, and it can handle running at low RPM just fine.

If you're NOT climbing a hill or carrying a lot of weight, and you don't mind accelerating slowly (or you're cruising), you're not going hurt the Yaris engine going even as low as 35 mph in 5th gear. Of course, if you're carrying a load or going uphill, use a lower gear.