Most Fuel Efficient RPM at Highway Speeds (65-70mph)

I have a few other threads going right now that have provided me with tons of information regarding transmission overdrive, rear end gearing, and generally helpful drivetrain data to consider. The guys who responded to those threads helped out a lot.

I have a stock 292 y-block (2bbl) with headers and Pertronix ignition, and would just like some advice as to what would be the most fuel efficient RPM to cruise at around 70mph? My plan is to do the math and set up the ring & pinion gear accordingly...

Some 292 data to consider:

160 hp @ 4000 RPM
270 lbs-ft @ 18-2000 RPM

Thanks --Matt

 

if your max torque is between 1800 would be perfect. i'd try and shoot for 1900 so you can still cruise at 60 comfortably.

 

Cruise speed with the highest vacuum and least throttle. My guess...1750 RPM

 

Good luck calculating it....it's probably a trial and error thing, and will depend on lots of variables such as cam profile, vehicle air resistance, and other stuff that is kind of hard to quantify the effects of.

Also I would not try for maximum vacuum, I'd try to get the throttle open more and rpm down, so the engine is using more of the power it makes to move the vehicle, rather than to overcome internal engine friction. That's the whole idea behind overdrive or highway gearing, eh?

 

'77 Jaguar, 6 cylinder, twin carburetors, 700R-4 transmission, 3:31.1 rear end, 4,200lbs, at 70mph got 23mpg at 2,100 RPM from Ft.Pierce to Syracuse...

 

1800 rpm would give you the best econ. but i would aim for 2000 rpm to give you some poke to overtake .... IMHO.

 

one of the few things I remember from doing my mechanical engineering training....
"best fuel efficiency at peak torque.."
Obviously there are other elements involved, and generally ones peak torque is reletaively well spread, so, depending on how 'worked' your engine is, if it's relatively 'normal', then as suggest, 1800-2000 rpm is your number....

Cheers,

Drewfus

 

1800-2000,tops! Over 2000 and it will start to drop. That's on most smallblocks. But there's so many factors involved. Timing,mixture,carb size,air cleaner set up,gear ratio,just to name a few...

 

You might try the calculators at www.currieenterprises.com - won't give you mpg but will give you just about everything else you need to calculate.

 

Best way to do it is with a overdrive. You want the last gear to be a small jump. That way when it bogs down you don't have such a big drop into a lower gear. The best gear won't allways work because there are to many conditions do deal with- wind, hills, pulling trailers, etc.....

 

you can buy a gas saver gauge, its really just a vaccum gauge but it tells you when your getting the best gas mileage, my mother bought a 1977 z28 with one on, an old guy had the car it for 3 months from new and sold it to her, i guesss he didnt like the mileage, the gauge lasted one day and was removed from the car.