Decided to install a set of 4.56 gears in my 1960 merc ragtop. I have a Lincoln motor and od transmission to put in it, so figured it needs more rpm to get it moving. It's the 4cam motor, not hamb approved but a neat looking engine
Easy to the math first. Final drive, top gear ratio, tire height. With those you can figure your optimum cruise RPM. I run 4:56 in my 34. Have .63 overdrive and 30" tall tires which gives me 62 mph at 2000 RPM.
even with OD deep gears can be a nuisance, as driveline vibration can be a problem at highway speed regardless of the engine RPM. you might want to check you shaft diameter to make sure you are not going to exceed the shaft's critical speed if you wind it out. i had one let go at over 100 because i upgraded everything else and didn't give the shaft a second thought because it worked. that cost me a transmission.
You might want to play with this put in any three and find the fourth speed, rpm tire size , final gear ratio calculator. Tire Size, RPM, Speed, and Differential Ratio Calculator (advanced-ev.com) It doesn't have a built in setup for overdrive so if the output of the trans is anythign except 1 to 1 you have to multiply the overdrive ratio by the rear gear ratio to get your gear ratio. Meaning that a 4.56 with a .80 overdrive would come out as 4.56 x .80 = 3.648 or a ralistic 3.65. With a 28 inch tire that is 3088 rpm at 70. The calculator is something to play around with to see what a couple of changes make as far as speed at a certain rpm or rpm at a certain speed.
The Jalopy Journal
