For my 1979 with a GM Crate Motor (249hp/350ci) what should my RPMs be at 70mph?  Right now, I'm about 3300rpm.  Is that normal for this engine?  I imagine the higher the RPMs the more gas is being used.  Is there anyway to lower the RPMs at highway speed (besides putting the trans in Netural)?

Adam, with my 3:36 rear end gears and a 5 speed TKO600 transmission, 395HP/350ci I'm turning 2800rpm at 85MPH in fifth. Maybe you could lean out the carb? Maybe lower the idle speed? Joel or Ken may know this but can you set the carb to run like a 2 barrel in stead of a 4 barrel? You would lose performance but save fuel? Just a thought..I just drive'em..

I think it's a question of feeling and hearing the rev's. With a overdrive or bigger end gear there will be less rev's but i think for the same power deliverd ( rolling and air resistance ) you will push the pedal a bit further ( more fuel ) : so you see less rev's but each rev needs more fuel ... no ?

I see it on my fuel meter ( Viper ) : constant speed in 5 or overdrive 6 doesn't reduce consumption that much but you see alot of difference in rev's.

The total resistance = power deliverd at constant speed so the rev's should be at the most efficient working point of the engine-curve. 

Ok, the lower the gear the more respons there is with pedal-play so the more fuel is used for acceleration.....

 

So Norbert, What is the answer? At 90MPH I'm at 3000rpm is that better efficiency than 85MPH at 2800rpm? That is almost half of the power curve on the dyno sheets for this engine? Well I'm not expecting to get good gas mileage on this trip? My hope is a constant RPM and speed will be the most efficient no matter what the cost of fuel? Ya got to pay if ya drive her, she's not free..

Adam, My '79 has a warmed up L48, with headers, big pipes, flowmaster mufflers, Edelbrock manifold and carb, aftermarket ignition. It goes thru a 350 turbo into a 3:55 rear. I also turn about the same number as you do. My fuel milage around town is about 12 and on highway about 16. The only way I know to change the situation is an overdrive trans. One the other hand, our new Dodge Dakota has a 4.7L (284 CI) with a overdrive trans goes down the road at 70 mph at about 1900 rpms. It's EPA milage est. is 15 city and 20 highway. I thought it would be a little better than that.

Dave

The engine rpm is determined by three things.  The transmission gear ratios, the final drive (diff) ratio, and tire size.

The tire size is a given on our cars.

If you change the final drive ratio to a lower number, the engine speed will decrease, and fuel mileage on the highway will improve, but accelleration will decrease.

If you change the final drive to a higher number, accelleration will improve, but rpm will rise on the highway, and fuel mileage will drop.

If you change the transmission and install an overdrive without changing the rear, the accelleration will remain the same, rpm and fuel mileage will both improve on the highway, but won't make a difference around town, unless you just coast along in overdrive.

Ken, you have a way with words..That makes sense to me..Thanks..Terry

[QUOTE=kstyer]

If you change the transmission and install an overdrive without changing the rear, the accelleration will remain the same...

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I agree with all of the above, except for one fine point...if you change the trans to a 700R4, 1st & 2nd gear ratios are lower than a 350. You would get better take-off "oomph", plus a lower cruise rpm.
My '74 has 3.36 gears, and runs 3000rpm @ 70mph, with 225/70/15 tires.


(btw..."oomph" is a highly technical term for "get-up-and-go".)

Talking about rpms...  Had tune and now at 700rpms at neutral and at 2800rpms at 70mph.    

I run about the same RPM's as Adam does.  Hate seeing the RPM's that high on the highway.  I think if there were another gear to shift into (instead of just 3) then the RPM's would be better, but I don't think my transmission and final gearing (3.55) are the only culprits for my pitiful MPG.

Yes, it's going to be expensive to drive the Pride and Joy this year!