Proper gearing question

J

JonC

Well-known member
I have started to drive the 40 pickup quite a bite lately and am wondering about gearing. As some of you know I have built a stout motor (I think) with a L100 cam and shaved flywheel. it is a 48 engine with a .060 over bore and sharp heads running 3 holley 94's and runs very strong. I have a quick change in the rear and have been running a set of gears that give me a 3:48 or 4:11 final drive. I live in upstate NY so it is hilly but when running the 3:48's the truck pulls wonderfully. I put a tach in it with a 12V system but I get tach bounce. it ranges fron 2500 to 3500 going down the road at 55 (or so I think) so I really have no idea how fast the motor is turning. It sounds fine and is quiet but I don't know what gearing I should run, does this sound right or would you try a different set?
 
Having run many L-100 cams,I find the "sweet spot to be around 24-2600 RPM. With the proper ignition timing this will give the best economy. There are a number of programs on the internet that can calculate your engine rpms to MPH. I run a 3.73 rear with 38& OD which comes out to 2.8 (I thinK). I live in VT, we have a few hills too, but i'm running a stock cam in a 280 engine.
 
As O'Ron points out it is easy to calculate the engine RPM to see where your at. I have found that some of the online calculators do not provide the correct results, I would do the calculations by hand, it is not difficult math at all.
 
OK so if I did this right ?????? with my 28 inch rear tires (235/70R 15) and 3:48 ratio I am coming up with 2297 RPM at 55 MPH, sound right? Now I would like to find out why the tach is not steady, is that a problem because of the dual points? I am taking the signal from the negative side of the coil with a negative ground 12V system.
 
I wouldn't think the dual points would matter, one set opens the circuit and the other closes it.
 
If it is a modified ignition from FoMoCo stock, the dual point set up could have an effect. Does it have a stock distributor or is it an early aftermarket type?
 
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