Engine RPM vs Cruising Speeds Part 2

Preamble
The last week’s article on the above subject evoked some interesting responses from some of my ‘regulars’.

Here’s one from a power systems engineer by ‘training’ (like me!) and this is how the QnA’s went with him…

Question
SKG, what is the rpm that is recommended for the best mileage? I tend to ‘cruise’ at 2K rpm in my Wagon-R - not that one can really ‘cruise’ in a city like Bangalore but I do because I leave home at 5.15/5.20 am for the golf course for teeing off at 5.45 am and so I can actually ‘cruise’ at that hour. I choose the 4th or 5th gear to be able to drive at 2K rpm. Is this OK or should I do a higher or lower rpm while cruising? – NJR/B’lore

Answer
It’s somewhat difficult to answer your question with respect to today’s Mpfi’s, where the engine ‘performance’ can be manipulated at the design stage to a very large extent, by programming its ‘ECU’ accordingly. In the pre-EU-II/carbureted cars of yore, it was believed that an engine was most fuel-efficient at its ‘peak-torque’ rpm - which was generally around 2500-3000 rpm and max/bhp rpm’s were of the order of 5-5,500.

However, to me it appears that this is no longer true. Take the example of a 1600cc car like a Baleno, where its peak torque occurs at 3500 rpm and in its Vth gear at this rpm, it’d do close to 140 kph! Whereas its ‘red-line’ happens to be around 6000 rpm - with a top speed ‘capability’ of 185 kph. The corresponding figs for a W-R, peak-torque and max-bhp rpm-wise, are quite similar to a Baleno’s. I remember my one such ‘92/M800 with an ingeniously designed ‘on-board’ Tacho used to do 80kph in IVth (no Vth gear then) at 3000 rpm.

It may be interesting to know that the present day tendencies of car designers are to have higher and higher peak torque rpm’s, to boost the accelerating capabilities of a car. Whereas for cars basically meant for pottering around town with minimal gear changes, such as a W-R, it’s desirable to have them around 3000 rpm - clearly a clash of interests.

To sum-up, I feel one can get the best out of a present day/petrol car - small or big - by limiting the highway cruise rpm’s to 3000-3500 from engine health/longevity points of view and to get the best ‘overall’ mileage, cruise between 80-90 kph in the Vth. The latter will correspond to 2000-2500 rpm for most petrol cars. For diesels, depending on their max torque and Bhp, these figs could be lower by, say, 500 rpm for the same road speeds.