Classic/Antique Car Repair: rearend gear ratio

Question
Hi Brad,

I have a 1940 Hudson Super six. I am thinking of changing out the rearend to a higher gear ratio. As I understand by doing this I will be able to drive my car at higher speeds without the hearing the whining noise from the lower geared rearend.

I have heard that I can switch out a Ford Explorer rearend because the width is the same. My question is how do I measure that? from hub to hub? What do you recommend that I do?

Thanks,
Marvin

Answer
There are several dimensions that need to be considered. The first is from spring saddle to spring saddle. These are the mounting points where the rear springs rest and the spring U bolts hold the axle housing in place. If the dimension is not correct it is no big deal to have a welding shop cut them off and re-weld them in the correct position. The overall dimension from hub to hub is more important as you don't want the wheels sticking out or in too far. Some variation can be overcome using a different wheel with a different hub offset. Now in changing the ale ration to give a slower engine speed at high speed will also change the low speed characteristics of the car. Low end torque, off the line performance will be significantly reduced. You will have to shift from high gear to a lower gear at a higher road speed. You will have to run a bit higher road speed before up shifting from first to second and second to third. So what you will gain on high end you pay for on low end performance.