how to determine ideal cruising RPM

[paintme205]

I have seen some people mention their is an ideal RPM to cruise on the highway based on the torque curve of the engine.

Others seem to say the lower the RPM, typically the better.

I'm cruising around 2800-3000 RPM's now in my festiva at 65-75 MPH.

Peak torque is at 3000 RPM.

I'm considering Taller tires to get RPM's closer to 2k while cruising.

Good idea or bad idea?

It looks like changing to 155/80r13 tires in front would lower rpms 7% and also reduce tire width by .4 inches compared to my factory festiva wheels. If I lower the car a 1/2 inch, wind resistance / aero should remain close to the same.

If I got super motivated, I could try and find some LRR tires.

[ksa8907]

that is a good idea, but remember that you will be changing the ratio for all gears, not just cruising gear.

[Frank Lee]

I think the best way is to find a BSFC chart for your engine and note the sweet spot.

Failing that, I've been using a 1000-1200 ft/mn piston speed theory that postulates best or near best efficiency is found in that rpm range. I am still not sure about the validity of it but I've run the numbers on many vehicles and even if it isn't perfect it isn't going to be too far off.

[JRMichler]

Or put in a vacuum gauge and practice DWL. If you run consistently at 12 to 15 In Hg vacuum or higher, a taller final drive ratio will help. If you are consistently less than 8 to 10 In Hg vacuum, your existing final drive ratio is good.

[3-Wheeler]

<<how to determine ideal cruising RPM >>

In a way, that is a loaded question!

This depends on so many factors:

Are you considering this question on a flat section of road?
What is the outside temperature?
Is there snow on the ground?
Going up hill? How steep? What gear and engine load?

This is the ART of eco-driving.

For example I use a different technique to crest the same hill in winter as the road/tire friction is different, and thicker air. In the summer I can coast down a one mile section of road with the engine off, but need to keep it running in the winter.

There are times in the summer when I can easily get more MPG's on flat sections in fifth gear, but keep it fourth in the winter, because *that's* better. And this varies with different speeds. It really depends.

The Insight has an Instant MPG display, and if I really want to maximize MPG, I watch that pretty closely. If that is not an option, then certainly get a ScanGage II, which gives very good readings. I use that to keep an eye on radiator temps, air temps, and so on. Very useful.

I've had the Insight for over five years, and *still* refining the high MPG technique !

Jim.

[cosmick]

Originally Posted by Frank Lee

I think the best way is to find a BSFC chart for your engine and note the sweet spot.

Failing that, I've been using a 1000-1200 ft/mn piston speed theory that postulates best or near best efficiency is found in that rpm range. I am still not sure about the validity of it but I've run the numbers on many vehicles and even if it isn't perfect it isn't going to be too far off.

Uh, NO! Going off BSFC is as wrong as cruising at the RPM of peak TQ at WOT. Try that in a '98-'02 Z28, your MPG will drop from ~31 to upper teens.
Separate from that, I've had at least 4 different vehicles where a bit more axle ratio actually helped my MPG. Ya gotta try it and see, there is no substitute.
Like with this RX-7, I'm starting with a 2.29:1 axle gear until I figure out the aero, then I'll try a 2.14:1 axle gear.

[user removed]

Any engine in any car made, with very few if any exceptions, will get better mileage at lower RPMs, same speed and conditions. If a hill is too steep then downshift, assuming only top gear for comparison.

Best torque is always too much power to maintain any reasonable speed, unless you're talking 75 plus MPH. A 28 hp early type 1 vw would probably be most efficient floored at 75 MPH (if it would do that on flat ground ).

You should see a decent improvement in MPG with the larger diameter tires, but it may take a few miles to break the new ones in so it might be awash until then.

regards
mech

[Frank Lee]

Was just driving a 2014 F150 5.0. Noted 1400 rpm @ 55 mph. That is 852 ft/mn piston speed- 15% slower than "guideline". IIRC my '94 has similar piston speed at 55. Don't know what to make of that. Cars, motorcycles, whatever, it seems everything of mine but the Ford 5.0 sticks closer to 1000-1200.

cosmick: Yes, thanks for the reminder. There are BSFC charts that display non-WOT values; use those.

[paintme205]

Originally Posted by JRMichler

Or put in a vacuum gauge and practice DWL. If you run consistently at 12 to 15 In Hg vacuum or higher, a taller final drive ratio will help. If you are consistently less than 8 to 10 In Hg vacuum, your existing final drive ratio is good.

Thanks for the tip. I'll get a vacumm gauge and see what I come up with.

What DWL?

[Xusn96]

Driving With Load- as noted in the Glossary at top of the page.