Thread: Dummy has a question

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Generally speaking, the lower the rpm's, the better the fuel mileage at cruise. Of course, like anything else, you can go too far to the extreme and the curve goes the other way. For instance, if you had the car geared to rpm the motor at, say, 1,200 rpm at cruise, then the motor wouldn't be making enough horsepower to pull the load and you'd be down on the loud pedal more to get it to pull, decreasing the mileage.

In my opinion, an engine speed of maybe 1,800 to 2,200 rpm's would be optimal for mileage/power if used with a short duration cam that would support making enough horsepower to pull the load at that rpm. Here's an example of such a cam, the very first one listed in Crane's lineup for a 350. Read the description in the upper right hand corner....
http://www.cranecams.com/?show=brows...tType=camshaft

That's the neat part of having an overdrive trans like the 700R4, you can have a very low first gear for launching the car and a nice, tall fourth gear for cruising. Here's a chart comparing a TH350 and 700R4, showing the effective final drive ratios.....
http://www.enjoythedrive.com/content/?id=7088
Note that the 3.06 first gear of the 700R4 combined with a 3.73 rear gear will be the same final drive as a TH350 2.52 first gear used with a 4.56 rear gear!!!!!!!

So, using the information given you by Mike, you can plug in some numbers and determine what you want to do. For instance, at 65 mph with a 28" tire, 3.73 ring and pinion and 700R4 with its .7 fourth gear, engine rpm's would be 2,037. If you allow 4% for converter slip, engine rpm's would be 2,118.

This information supports what I've been saying over and over on this board:
THE CAMSHAFT SHOULD BE THE LAST ITEM CHOSEN IN A BUILDUP.