whats the slowest RPM you can hold/top gear?
 

Assumedly this involves slower cruise speeds as well, but basically whats the slowest speed and engine RPM you are capable of holding in top gear WITHOUT "lugging" (torsional vibrational resonance causing a shuddering and similar) along with what type of vehicle/other relevant information?

My 1999 Saturn 1.9L will hold 1200rpm in 5th fine (which I think is around 25mph), below that it shakes, shudders, and acts ugly. I can accelerate from 25mph but not 20mph, even 25mph is really iffy, that's the absolute bottom floor possible. 30mph is far more comfortable without shimmying and such.



A related question if anyone has a comment - i'm wondering whether top gear is always the best possible efficiency for a vehicle at lower speeds, could going even slower in 4th achieve better mileage?

If you look at BSFC charts, your engine's most efficient operating speed is likely not 1200rpm. Usually it's more around 1500-2000rpm at about 80% load. I can't say whether going 30mph in fifth gear is good for your engine (even if it's not lugging), but if you're going that speed you should probably be either accelerating through your best BSFC, or coasting in neutral. That is, if you're looking for the most fuel efficient way of driving.

As for being in 5th or 4th, the higher gear is almost always better if you can pull it off. The question is whether it's prudent, considering you make more hp for each drop of gas when you're in the BSFC sweet spot. You'd have to find a chart and compare it to your driving habits to find out for sure.

My Rabbit doesn't like to go under 40mph in 5th, but it doesn't necessarily shudder. The Probe will happily go down to about 38mph in 5th. I don't feel that driving slower than that is safe on the highway, anyway. In town you should be coasting at those speeds when possible.

thats pretty surprising you can hold that low of a speed in 5th without lugging, i can hold around 37 in my escort in 5th gear and if i was still constantly driving it i wouldn't be constantly on the throttle like i was

Without any shaking or grumpiness? 790ish in 6th
As low as I go? She can handle 600. But that is rare to do that to her. She is more well behaved when she warms up a little.

I am usually at 39 mph in 6 around 900 rpms.

I usually shift by 1200, try to cruise at 900 ish, or just top gear.

My celica wont go below 45 mph, 1200 rpms without lugging. My old maxima would do 25mph 800 rpms in 5th no problem, drove 300mi in 5th even going through several towns.

Wish I had a manual, but 41 is as low as mine will go before unlocking the converter, about 1400 rpm.

I am so jealous, my 5th gear is 2300rpm at 45mph (radar speed detectors tell me I'm always going slower than indicated) :/ Perfect for going 30-45mph, anything higher it starts making a huge racket.

I can give the throttle the tiniest amount of pressure in 5th and hold 25mph like that, but if I move my foot just a tiny bit more it starts buzzing and shaking. I'm not totally sure whether this is better than running it in 4th gear, I guess it's something that I'll have to check when my OBD scanner comes in the mail.

At 1000rpm my 1ZZ will vibrate at any throttle input level, at 1200 it accepts the tiniest amount of throttle input without making noise, at 1400 it will buzz a little at high throttle openings, and a little past 1500 it doesn't complain audibly, but the torque is really crappy. At 2000rpm I can feel the available torque increasing by a noticable amount.

1200 RPM is 38 mph in my Saturn sc1. I can drive at a constant speed at 20 mph in fifth gear. It bucks a lot at 15. Check that your rubber motor mounts are not cracked.

I cruise through town at around 1100 rpm @ 35 mph in 5th gear. I wouldn't try to accelerate that low without downshifting though. When the speed drops to 25 I have to shift down to 4th at about the same rpm. I have no idea if what I'm doing is helping my FE, just ordered my MPGuino last week.

My Elantra can hold 30km/h in 5th gear @ 750-800rpm ... though that is on flat ground!

I use this mainly when "crawling" in heavy traffic, I just shift to higher gears and let the car move on it's own (somehow there is no lugging at around 600rmp and up, so I just play with the clutch to keep it higher than 600 and the car very slowly accelerate on it's own).