How bad for your car are high RPM, still under the redline?

Factual Questions
1

I recently moved to a very mountainous region, and I still drive my tiny underpowered city slicker car (Honda Fit). It has basically no torque at low RPMs. Sometimes, to keep up speed uphill, I need to rev the engine fairly high for extended periods of time. One climb in particular has me at 5,000 RPM, at 40 km/h in first gear, for about a minute, but that’s pretty much the worst one, and I don’t drive it that often. More common is say 4,000 RPM in second gear, for about two minutes. Redline is at 6,500. There are no smells, no weird noises, no indicator lights activated; basically the car operates normally.

How bad is that for my car? One one hand, driving slower would mean the same number of revolutions, since it would take longer to complete the climb. On the other hand, logically, the redline should be a gradient from yellow to red, since it doesn’t seem likely that 6,400 RPM is completely fine but 6,600 will wreck the engine. Maybe that gradient would start at 5,000?

And how about fuel economy? Is that kind of driving disastrous for fuel economy or just somewhat bad?

2

For background: power output is basically torque * RPM, and for the most part, torque is about what you do with the accelerator pedal.

High RPM means more mechanical friction (oil films are experiencing higher rate of viscous shear, therefore higher shear stress, therefore more frictive force), which means less efficiency.

Light load (low torque) also means low efficiency, because A) you’re not making much power compared to all the RPM-related friction, and B) you’re consuming extra power to suck intake air past a mostly-closed throttle plate (the lack of any throttle plate at all is an important part of why diesel engines have good part-load efficiency).

So if you want good fuel economy, and you have a choice between light load+high RPM and high load+low RPM, choose the latter, but don’t take it to an extreme. Running an engine at very high load and very low RPM is probably worse for it than the opposite: now you’re dealing with very high cylinder pressures and temperatures, which puts high loads on the piston rings and skirts and really cooks things. If you have a car with an automatic transmission, then you know that this is exactly what the car does, i.e. it generally picks the highest cruising gear it can pick without lugging the engine, in order to maximize fuel economy.

It’s worth noting, too, that if you live in a mountainous region, you’re probably operating at significant altitude, and so (assuming no turbo) it’s not even possible to heavily load the engine. If you’re in Denver, your engine can only make about 85% of its sea-level rated torque; if you’re at the summit of Pikes Peak, it’s only good for 60%. (Want a slow ride? Take a Prius up Pikes Peak. The electric propulsion will be useless on such a long climb, leaving you with just 60 horsepower of internal combustion by the time you get to the summit. Don’t count on passing anyone.) So even if you’ve got your right foot down pretty far during your grade climbs, you’re probably still not taxing the engine as hard as you might think.

As far as rev-related beating on your engine is concerned, 4000-5000 RPM for a redline is 6500 isn’t a huge problem. It may cause a little extra wear, but really not much at all, and in the context of the entire life of the car, very little since you only spend a tiny % of the car’s life under these operating conditions. Change your oil regularly, avoid buying the cheapest/shittiest oil you can find, and try not to take a whole lot of very short trips (<5-10 miles), and your engine should provide you with a pretty normal service life.

3

As long as the engine is under load and you’re staying under the red line you’re good.

You’ll just wear the engine out faster.

4

Not even the latter, appreciably. Redlines are set so as to be well within the range at which lubrication starts to break down or any component is under strain to a degree likely to cause fatigue or deformation.