Overdrive

[Chuck W.]

I have been thinking about installing an overdrive unit on my bone stock1964 CJ3B. This jeep has the stock 4:27 axles and cruses easily at 55 mph, which is about all I want to do. I just wonder if it's worth the extra expense. I don't make long trips, just driving for pleasure several times a week around town. What do you guys think?

[Mark W.]

a 25% OD with those gears would be the same as having 3.20 rear gears. I would be willing to bet you would have a had time using it on anything but a flat level or down hill road.

[Chuck W.]

Thanks. I don't think there would be any advantage, either.

[oldtime]

I have experimented with exactly this same scenario.
The ideal final drive gearing for a strong running F-134 is right near 4-1 ratio.
Geared any faster and the engine hp cannot keep up and your mpg will decrease.

[SteveKfl]

I think tire size run also plays a big part of the effectiveness.  Oversized tires, especially wide ones, can create more rolling resistance and gas consumption while reducing the running RPM/speed ratio (and speedometer) full time (like a 4:1 gear).  An OD is also another "moving part" to maintain for consideration.  Every modification usually comes with a "compromise" to fit the "owner's specific need/desire" for the modification.
 
HP was mentioned with the "ideal 4:1 ratio", which made me wonder... how does the approximate "2200 RPM Torque Max" rating fit in to the equation, if it does at all?  Thanks.

[oldtime]

Well I prefer to run about 30” tall but narrow tires with any of the 134 engines.
Mainly to gain additional ground clearance and not intended for better economy.
You can go to “Grover’s Willy’s Speed Calculator” to see the road speeds at various rpm’s with various height tires.
But yes according to Willys dynamometer specs, the unmodified  F-134 puts out maximum torque at about 2200 rpm.
In practice maximum torque yields an rpm that produces the greatest amount of gravitational force (“G” forces).
So naturally 2200 is the ideal rpm to use when passing
I believe this maximum torque output happens at near 50 mph with an overdrive engaged. This particular speed should provide near the best gas mileage possible for an old Jeep on the backroads.

[SteveKfl]

Thanks for the response and info and insight.  50 MPH on back roads sounds like "plenty" to me.  I agree, the "tall narrow tire" is the way to go for increased ground clearance as well as slowing down the RPM saving the engine some stress of high RPMs.  Happy Motoring is the "key".