how to determine ideal cruising RPM

[serialk11r]

Ideal cruising rpm is the lowest possible speed for the road you're travelling on. Unless you are going >80mph taller tires will probably help you on any car (some REALLY slow cars work their engines really hard at those speeds but there really isn't any car sold in the USA that has that little power).

[aerohead]

Originally Posted by 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.

Here is an example of an engine map Hucho published for a small-displacement engine from 1976.

*The brake mean effective pressure is seen at left.
*The horsepower per square-inch of piston area emerges from the top.
*The RPM/piston speed is on bottom.
*The road load power (Aero and Rolling Resistance) is evolving from the lower left as a function of RPM.
*The islands of brake specific fuel consumption lay across the road load pathway.
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For a number of thousands of dollars and a dynamometer,you could construct a BSFC map for your car and then you'd have a chance at predicting what would actually happen for any given modification.
Without it,you're kind of shooting in the dark.
*It could just be possible that Ford has already 'optimized' your powertrain for 'real-world' driving transient conditions,and that what you have is already worth keeping.
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When you start comparing 'rules of thumb' to published BSFC maps,you discover that there are no hard and fast,generic,one-size-fits-all solutions for gearing.
Mod at your own peril.

[UltArc]

I haven't been able to find one on my Mustang since my last search, a few months back.

My technique to find it was monitor fuel use (easier with OBDii), and drive as fast I can before it starts plummeting. 54-59 is all the same, then it starts dropping off. So 59 MPH is the ideal speed for me, pre current underbody/diffuser build.

[fusion210]

I'd do it. You'll also increase the load on the engine, so it'll bump you upward when it comes to bsfc.

I don't think you'll gain much from it, certainly not as much as getting a nice set of lrr tires.

[thomason2wheels]

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.

This seems like the best practical test I have seenposted for determining ahead of time what a gearing change would give you. I remember on an old 83 honda civic I had once with the 1200cc motor I cnanged the 12" wheels for 13s with fatter and taller tires and the fe dropped several mpg. If I had known the vacuum guage trick I could have saved a ton of money and trouble.

[serialk11r]

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.

My goodness, 15 in Hg is high vacuum? My car runs at...21 inches