Motor/Rear axle/transmission combination question

[Taylors69]

Wondering what ring gear I should be using with our new motor/transmission combination. We installed a 351W/FMX combo from a 79 Thunderbird into my sons 69 F100. The Vin tag on the door indicates that the axle code is an 09. I am wondering what ring gear I should be using in the rear end. It seems like it is currently geared too low. We are looking for better highway speeds as well as gas mileage. The truck originally had 240 with a three on the tree and was used on the in-laws farm. Thanks for reading this post and sharing your expertise with me.

[forrestbump]

A rear axle code of 09 indicates a 3.70 gear ratio. Your rear axle is a 9" Ford, NON trac-lok, 'open' differential. I would consider changing to a 3.50 ratio or numerically lower (a higher ratio) for better highway performance.

Keep in mind that tire diameter will also effect your final ratio. By changing to a 31" diameter tire, for example, you'll have effectively changed your ratio to a taller gear.

[Taylors69]

Thanks for your help. Looks like we will be roadworthy by Wednesday or Thursday!

[mrtleavitt]

You probably won't see a big difference from 3.70 to 3.50 gears. You might wanna jump to a 3.25 gear.

[Whateverman]

if i was going to use the truck for hauling anything i'd go with 3.50 gears (and a posi if possible),i had 3.25's in one of my trucks - it was pretty sluggish down low and highway fuel mileage was not much better (if at all) than with the 3.50 rearend i ended up swapping into it