low speed high gear
 

I am wondering if there is any concern with driving in your highest gear at low speeds. For me it's 5th gear at 30-35mph. Let me be clear this is just to maintain speed not accelerate. I wait for downhill to do that then coast for as long as possible. When my minimum speed is reached generally 30mph in town it's back into 5th to maintain.

I've been told on performance minded forums that this stresses the engine way to much and I should be in 4th for my highest gear at that speed. Thing is I tried that and it raised my RPM less than 250rpm but also drops my instant mileage between 7-10mpg depending on speed.

So what's the correct way to do it to ensure as little harm as possible is being put on the engine?

Also I searched around and found various answers from it's fine to you're going to blow your engine to carbon will destroy your valves...

Now I could maybe see the carbon build up.But, I will have a look to verify, I dont believe that would be an issue if fuel is washing the valves proir to entering the chamber. Please correct where I am wrong.

Sorry for the long winded post.

Mods if this is in the wrong spot please move.

If you're lugging the engine you could be doing damage. However, if its not lugging or vibrating abnormally you're fine.

it's above 1000rpm and it's not vibrating or lugging. If I drop below 30mph it will yes. But above 30mph in 5th it's almost like she's idling.

It's only lugging if you demand high output from it. What rpm does it idle at? I'm sure that's well below 1000. As long as you're gentle, anything from idle on up is fine.

the engine will let you know if you're in too high of a gear for the output you request(throttle) of it.

foot to floor at 30MPH in 5th? probably a bad idea.
more or less closed throttle at 30MPH in 5th? not a whole lot more load on the engine than idling out of gear.

I regularly run my truck down to 20 MPH (650 RPM) in fifth gear, and even accelerate from that speed. No engine problems so far, but I only have 86,000 miles on it.

Although I have to replace the kill switch again this weekend. The shaking of the shift lever destroys the kill switch.

Right I only open the throttle enough to maintian speed to the next down hill. Never floor in those conditions that obviously wouldn't be smart. LOL

and Yea the idle is below 1000rpm I'll look tonight and see but I think it's like 750ish IIRC.


Thanks for the input guys :D

The engine will say it's unhappy when it bucks, protests, or fails to accelerate. Other than that, it's OK.

maintaining speed at low low rpm is fine.
although i do try to avoid really high load at sub 1.5k rpm. I worry that high cylinder pressures and low oil pressure will squeeze the oil out of the bearings causing wear.

I use 5th @ 1000rpm for 30mph in my car , much lower and it stumbles. I find the need to drive slower then 30 is far and few between , I about never use 4th for more then a quick gear change.

I routinely drive in 5th gear on city streets in the range of 600-1000 RPM and have had no issues. The problem comes in high load situations at < 1000 RPM's. In this situation the oil pump may not provide enough volume to maintain the film that protects the engine from excessive wear. At higher RPMs under high load the lubrication is not really a problem.

The other part of it is pinging/detonation issues. Back in the days before computer controlled ignition you would have to worry about "lugging" but in modern engines with knock sensors and O2 sensors you really never have to worry about ignition/fuel related problems.

Same here. I shift to 5th at 30 & then maintain unless I'm climbing a steepish hill and/or trying to accelerate more to better blend with traffic (like a freeway on-ramp). 30 mph in 5th is about 1350 rpm in my car. In previous tests, that showed as my best mpg speed too (70 to 72 mpg in 2-way tests)

I use 5th gear in the S10 down to about 30mph or so to maintain speed or cruise. This is also my highest mpg window for this vehicle as well (55ish mpg in testing). No problem as long as you are not lugging it or putting a heavy load on the engine. When I encounter a hill or need to accelerate, I just shift into 4th (or 3rd if the incline warrants it) until I can shift back into 5th.

Engine is fine (including oil pressure) at 157,000 miles. I did torch a transmission last year, but I'm sure towing a trailer, at or slightly beyond the recommended tow weight for thousands of miles, and my wife learning to drive a manual were the culprit. Actually I'm 95% sure my wife was the culprit, but I would never tell her that.

Short answer, use 5th gear whenever possible...get better mileage.

I sometimes run 6th gear down to 20 mph. If I'm in stop and go traffic, and on flat road, I might slip into 6th and just let the car idle along. It might dip to 900 rpm briefly, but I maintain 1000 rpm or above generally.

As you said, you aren't accelerating in top gear at low speeds, so all is fine. The stress on the engine is very low at those slight throttle openings.

If you're in top gear below 1000 rpm, you're basically not using the engine for much of anything. That's a good time for it to be off, or at least in neutral down at 700 rpm.

^^ what PaleMelanesian said. Maybe my engine is old and grumpy or maybe my car just has way less noise insulation than usual, but it will make unhappy noises below 1300rpm in 5th even if I'm just rolling along. I used to chug along at 25mph, 1400rpm, trying to keep the speed just right, and then I realized my mpg would be better if I just pressed in the clutch and let it idle at 700rpm instead, using 4th gear to give it a boost occasionally.

If anything, pulse and glide at lower speeds is better since you don't have massive air resistance slowing you down quickly.

I need the engine on to maintain speed or coast down/accel down a hill to get up another. I really don't have a good place for engine off coasting as far as safety is concerned.

30mph puts me right at about 1000rpm anything below that and ill shift to a lower gear. I do alot of engine on coasting.