98 Eclipse 2.0L F-Inj. 5spd. I drive home every day up a 16 mile hill at a 7% grade (4000ft to 9000ft). Speed limit 45mph. I can maintain 45 mph in 5th gear if I keep it on the floor OR I can downshift to 4th and keep it at 45 mph at an additional 500 rpm but with pedal only halfway to floor. (1800 rpm vs 2300 rpm) Question 1: Am I doing any harm to engine by driving in 5th floorboarded vs 4th gear? Question 2: Which way uses less gas?
- downshift. The reason you can maintain speed better is that you’re in the engine’s powerband. Any time you’re attempting to accelerate or overcome gravity, that’s where you want to be. You only save fuel below the powerband when you’re cruising level at a steady speed.
Sweet car btw. One of the best-looking sport compacts ever made in my opinion.
And downshifting is safer when you have to react quickly to other drivers on the road.
downshift. Its easier on your engine and transmission. And at 45mph in that car, fourth gear should put you right in the low end of the power band, where the engine is efficient.
Downshift. And don’t worry about fuel consumption. Driving 16 miles uphill is going to be tough on your mpg statistics no matter which gear you are in. Consider only the factors that the other guys have mentioned.
I’d downshift. In fifth gear, you must be right at the edge of lugging, which wouldn’t be good for the engine. It’s also a bit safer to have some immediate acceleration ability without shifting.
I would downshift to 4th gear. Most 5 speed cars are designed to be in 4th gear at that speed.
I would also downshift.
While that 4 cylinder car can make it up that long grade by flooring the accelerator while in 5th gear, that does not mean that this is good for the engine. The OP would be doing the engine (and his wallet) a favor if he downshifted to 4th gear.
I’d leave it in fifth. I do something like this all the time in my Saturn, not quite as extreme, but I only downshift if the engine starts to lug. As long as it runs smoothly, you will use less fuel and cause less wear and tear on the engine.
I’d stay in 5th as long as doing so seems safe to you. The advantage of 4th is you can speed up quickly if – say – you see an avalanche coming down the mtn from the side, about to carry you off the road! Speeding up could prove useful in that event. Or you see a cutie in the car ahead, you might want to speed up! Excepting these kinds of unlikely events, staying in 5th is fine and you’ll probably get a little better gas mileage.
I would bet that if you had an automatic transmission car, it would downshift under these conditions. Therefore I would shift down to 4th gear. Even though the engine is turning at a lower rpm in 5th gear doesn’t mean that it is using less fuel. Think about riding up a grade on a multispeed bicycle. Let’s suppose you could maintain the same speed in a higher gear as you do in a lower gear. However, you wouldn’t have to put as much effort while pedaling in the lower gear even though you would be pedaling faster. Thus you would be burning fewer calories. I think with the pedal only halfway to the floorboard in 4th gear you are probably using less fuel even though the rpm of the engine is higher.
However, with fuel injection, and fly by wire, the computer will control how much fuel is dumped into the engine, regardless of where your foot is. Having said that…
Downshift. It’s just easier, both on you, and the engine. Plus, you have some leeway if you suddenly need power. You never know what that other idiot is going to do, and you need to always be prepared.
If you’ve got the pedal to the metal you’re either going very fast or in the wrong gear.
…or have a really, really gutless engine.
I see some tradeoffs here:
5th gear has lower pumping loss due to higher intake manifold absolute pressure, which increases MPG.
4th gear reduces engine torque, which puts less stress on engine bearings, piston skirts and cylinder walls.
I would go with 4th gear mainly to have the reserve power and torque immediately available.
I think circuitsmith intended to tell us that downshifting to 4th gear will increase available engine torque.
I’m putting in another vote for downshifting. Running 1,800 RPM @ full throttle for sustained periods can be pretty hard on the bearings, rings, and cylinder walls. It probably won’t cause a spectacular failure, but the wear will show after a few years, probably in the form of oil consumption and a bit of blue smoke. 2,300 rpm isn’t that fast, and is usually a nice engine speed for a long uphill pull in a 4-cylinder car.
Not only will more torque be available from the engine at 2300 vs 1800, the shorter gear ratio will make even more torque available at the wheels.
Why is this even a question?
Too all those who claim that downshifting will be easier on the engine, I say, show me the proof. I have been running engine at much lower RPMs and higher loads than most people I know and my engines have not suffered from it. That is one of the reasons that I prefer a manual transmission over an automatic, an automatic doesn’t give me a choice.
If anyone has some documentation of a scientific experiment that confirms that a heavy load wears out the engine sooner, please post it, otherwise you all are making assumptions.