whats the slowest RPM you can hold/top gear?

[bobo333]

my F100 will idle along all day at around 12mph in top gear... i really need new gearing lol!

[MetroMPG]

Originally Posted by stillsearching

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?

It is possible on some vehicles that the 2nd highest gear at a lower speed (than top gear's lowest non-lugging speed) will be more efficient.

It will depend on gearing & aerodynamics of the particular vehicle.

EG: from Honda Ridgeline MPG vs speed graph: fuel economy with & without cargo trailer ...




The leftmost 3 points on the graph are in the 2nd highest gear. Probably a function of the truck's relatively high aerodynamic drag (and the fact that it's an automatic transmission with a programmed speed-based shift point) that makes cruising in 4th at a lower speed slightly more efficient overall than top gear.

See more speed vs. fuel economy graphs: http://ecomodder.com/forum/showthrea...you-15182.html

[Sporty Modder]

My car is more efficient in 4th for sure. The sweet spot seems to be between 1500 &2000 rpms 50&60 mph in 5th, 40&50 in 4th. When I accelerate its typicaly between1500 &2500.

[Big Dave]

750 RPM and just under 40 MPH in double overdrive.

[AndrzejM]

700rpm 5th gear around 40km/h it's less than 25mph

[stillsearching]

Originally Posted by Sven7

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.

My understanding is lugging is torsional vibration resonance, if you feel it its bad, if you dont feel it it's probably fine... if anyone knows that speeds shortly above lugging that you dont feel are damaging please tell me. :P


The original EPA figures were 29city for my car, the new ones something like 24 or 25 I think. I consistently get 29-30 around town. The tight traffic and angry drivers dont allow me to P&G around town without getting shot in the face. I don't know if the BSFC drop at 1200rpm is worse than running 2000rpm in 4th. Does anyone know where one could find BSFC maps of engines offhand? (if anyone had a repository i'd think ecomodders would :P)

You guys who are under 1000rpm, could you post engine details? I was trying to find engines without natural torsional resonances at those speeds in another thread and decided to try this different angle.. it seems to be a function of the engine itself and unchangeable.

[Sven7]

Went down to 30mph in the Rabbit yesterday. It was pretty smooth but I didn't feel comfortable keeping it there. There are no problems using low RPM's because like most VW's it's pretty torquey- 1.7L 8V gas. I regularly accelerate in 2nd from idle speed, and 3rd at probably 500rpm or so.

[jalmir]

Originally Posted by stillsearching

You guys who are under 1000rpm, could you post engine details? I was trying to find engines without natural torsional resonances at those speeds in another thread and decided to try this different angle.. it seems to be a function of the engine itself and unchangeable.

My engine specs (accurate from other sources I can't find as fast) ;

Hyundai Beta engine - Wikipedia, the free encyclopedia

[UltArc]

I realized today at 29mph I use 6th that low, at 6xx rpm. No lugging, bit if I go uphill at to high of a grade it will just start lugging and not performing. This is also when 5th is more efficient, 24-33 uphill.

[JRMichler]

Down to 18 MPH in 5th gear at 600 RPM without bucking. Level ground only.

The alternator does not keep up at that speed.