Hello all,
I used to have a formula many years ago. It was usedto figure out the best operating engine rpm for a given vehicle. If you knew the engine size, rear end ratio, transmission, and tire size, you could factor all of these together. The resulting nuber tells you what engine rpm is the most efficient for that vehicle setup.
I took a highway trip last week and it FELT that the best engine rpm was about 2000 to 2200 rpm. That is, of course, based upon nothing but feel.
Has anyone heard of this before? Anyone have that formula?
Thanks

 

I don't have a formula, but if you have a trip computer in your truck you can figure it out easily enough? I used mine to determine what RPM to run by setting the cruise at a given speed, resetting the MPG indicator and running a few miles. I did this for several speeds and found that mine likes 1800 RPM at an indicated 68 MPH. This will get me 21-23 MPG, depending if it's cold outside or if I'm running the A/C. By the way, I get less MPG on the highway with the windows down, than with the A/C on! [sm=confused06.gif]

 

I have not heard of that, butI would like to know if it works.

 

This link to Mitsubishi-Fusco Trucks has the background information to find this operating rpm...but keep in mind this really only works if you are above
70% open throttle (gasoline) or 60% rack (diesel)

http://www.mitsubishi-fuso.com/en/te.../05_02.html?b5

The graphs at the Mitsu-Fusco link above
like this one for the engine:



and this one for the transmission:



can be used to either find:

best fuel efficiency operating rpm
best hill-climbing rpm for maximum weight load
best acceleration range of rpm

However, most of the time on a gasoline engine you are operating at
'part throttle'
where the factory setting are quite a bit less than 70% open throttle. Manufacturers like Dodge do this to keep the typical customer happy, because the typical customer wants to be able to speed up, pass, or hill climb without a gear downshift.

To the typical customer,
having to downshift is a sign of a 'weak engine'.

To a drivetrain engineer,
having to downshift to speed up or hill climb is just PROOF that before the downshift your vehicle was at the best fuel economy setting for that road condition, wind, and grade.

So the 'best operating rpm' for daily driving with an automatic transmission is simple:

It is just the slowest rpm that you personally are willing to accept for the number of gear downshifts that will occur as you approach hills. As you get closer to the 'best' fuel economy operating rpm on level ground, your trans will downshift on less and less steep hills.

Many customers 'hate' this,
but for best fuel economy
the transmission would be shifting from 5th gear on flats
to 4th gear for nearly any hill,
and then 6th gear for downhill sections.

CVT transmissions like on the Dodge Nitro or Nissan Murano already do this in a smooth continuous manner without a 'gear jerk'.

Diesel engine powered vehicles are similar,
but the broad torque curve and lack of a throttle on a diesel
allows a slightly wider rpm range where the fuel economy is little changed.

However, for Cummins powered Dodge Rams in particular, operating at low rpms means the 5 speed manual 3500 transmission got hit with such high torque during top gear acceleration and hill climbing that inside the trans the fifth gear retaining nut kept coming loose. Similarly the 47RE auto trans internal clutches had trouble with slippage and seal wear.

 

If you are really interested in finding best operating rpms,
in the way that all auto companies and engineers have agreed is the correct way,
The Bosch Automotive Handbook:



has about 4 pages at the front of the Drivetrain chapter that explains it even better than the Mitsu-Fusco link above.

For those that don't know the Bosch Automotive Handbook is a 75 year old voluntary effort where automotive engineers are 'invited' to write up the state of the art on a subject. It is quite an honor to be invited to write something. Bosch gives money to the book project as a public service to encourage education. In this way this handbook is like wikipedia.

If you have a free Google Mail account
the Bosch Automotive Handbook is one those books that has already been digitally scanned and you can read and search through for free.

http://www.google.com/books