after putting about 1500 miles on my recently revived 49 f-1 (revived with help from ross and several fte regulars) i would like to be able to go about 55 on the highway. with my stock set up 45 is about cruising speed and i'm ready to shift into a non existent 4th gear. so i'm looking to zero in on the best 9" specs for an easy swap. from what i've read on fte, 68-72 f-100 is a bolt in and will fit stock wheel config (with new u-bolts). some questions: do all the 68-72 f-100's have the correct spline count (if not, which do i look for?) also, what would be a couple good choices of gear ratios? i have a good running eab in it now and may have my merc rebuilt to put in it. if there is a tag on a rearend, is it easy to figure out the ratio? would be a shame to put a new diff in and have the same ratio. what is the ratio on the stock set up?? i do have the motor manual, i'll see if i can find that myself, but i'm sure someone here can tell me quicker. i'm adjusting my travel patterns to travel on smooth roads with 45mph speed limits for now. dick r.

 

https://www.ford-trucks.com/article/...ial_Swaps.html

This link may give you the info you need

 

Well, my 51 half ton with a V8 flathead has 3.92 gears stock and 3spd on column. I think if you go to a higher gear like 3.00 or 3.25 it might not perform like you want ( read that some where in this forum lol). I think 3.50 would be the lowest I would go but keep searching around on here. The stock flathead needs that low gearing to move around good.

 

It depends a great deal on th esize of your back tires and wheels.

Here is a link to a gear ratio calculator - remember that 99% of all transmissions (that are not overdrive) have a top gear of 1:1 - ie, direct drive.

Measure your role out distance (the exact circumference of your back tire) and plug the data into the calculator.

As an initial guess, I would venture to say that with 16 inch back wheels you will be looking at a 3.0 to 3.25 back rear ratio to cruise 55 at 2300-2500 rpm.

Try it:

http://www.et-studios.com/motorsports/gears/gears.html

 

thanks to all who replied. i did read the sticky and didn't see much about gear ratios. i'm assuming since the sticky says bolt in for the 68-72 that the spline count is right and driveshaft length should be fine. i am looking to cruise at about 50 to 55 so i will follow julies suggestion and use the calculator. just out of curiosity does anyone know the roll out distance for a stock 6.00 16 tire? i have 225x75x15 on mine and the speedo calibrates exactly to our other car going 45mph on cruise control. thanks again (if this shows up twice, sorry, i thought i had sent it, but it did not seem to appear), thanks again. dick r

 

The roll out distance is very easy to measure - you need a carpenters square, tape measure and piece of chalk.

Take the carpenters square and set it next to your tire - one side on the ground the other vertical going through the center of your wheel.

Make a chalk line on the lower part of the tire at the center of the wheel. Extend that line down and mark it on the ground too.

Now roll the truck backwards until the chalk mark on the tire has gone around exactly once and is again at bottom running vertically. Take the carpenters square and check that the chalk line is again centered vertically with the wheel then make a second chalk mark on the ground.

Measure the distance (within 1/2 inch) of the two chalk lines on the ground. That is you roll out distance.

Plug that into the calculator at the link and select "roll out distance" then 1.00 for third gear, and your different options for rear end gearing. I'd plug in 3.50, 3.25, 3.00 and 2.80 for rear end gearing. The speeds and RPMs will be right on!

 

"Drop in" doesn't exist, but the 9-inch is as close as it gets. You need to lengthen the driveshaft a little (or some have used a longer slip-yoke at the front), you need to use a different lower shock mount (I think F-2 stock plates work?), and you should correct pinion angle with 3-deg wedges on the perches. I can't remember for sure but it seems like a different U-joint with two caps that match the driveshaft and two that match the yoke may be required (they are a common piece, no big deal).

There's more to the RPM/speed calculation than cruising RPM. Consider that with the stock 3-sp, you still need to pull out of corners in 2nd;, too steep of gears and you'll have to slip the clutch. 3.50 seems to be the consensus for overall "best fit" with stock engines, with a wide range of tire sizes.

 

Using a little math, the roll out should be close to 98 inches. Tires are sized by tread width and aspect ratio. Tread width of 275mm, sidewall height is 75% of tread width, 15 inch rim. (25.4 mm per inch.)

Formula is: 75% of treadwidth x 2 (or 1.5 x tread width) plus rim for height (2 sidewalls being measured).

Then PI times the diameter (height) gives you the circumference (roll out).

Tread width of 275mm = 10.83 inches
75% = 8.12 inches

16.24 + 15 = 31.24
31.24 * PI = 98.1438...

The 6.0X16 should be about 28 or 29 inches tall for passenger tires. Can't find info on light truck tires.

Hope this helps.

Should be close enough to calculate rear end ratios.

 

OOPS!! I need my glasses when I read these. 225x75x15 rolls out at about 89 inches.

 

Isn't it against the law to do that much math in Oklahoma on a Wednesday?

 

thanks for the add't info. i'm on vacationing now, wifi from Lake Placid NY. i got the roll out on my 225x75 by doing the C=pi D. (also, julie's ref chart has an option for diameter in inches). i have plenty of time for research. i was asking about roll out for the stock wheel/tire because i don't have mine anymore, am curious to see if they are approx the same rollout as my 225's. when i replaced my speedo gear, the one i found (for HD 3speed) had 22 versus 21 teeth), my calibr run with our other car confirmed that the speedo/tire combo was giving me accurate speedo readings at 45mph. i do know that i have a heavy duty 3 speed, i'm now wondering if i have the 4.27 rearend. if that's the case, i would happy switching to the 3.92. i don't highway much, superhighway, never. my diff needs new seals anyway. when i get home i'll check out my existing rear end and look for any id info. if i have the 4.27 gears, can the 3.92 be subbed in without changing the ring gear? thanks, dick r

 

Short answer: No. Long answer: It's the ring and pinion that give the ratio. 4.29 turns of the pinion to 1 of the ring, etc... The only way to change the ratio is to change the number of teeth, and subsequently the gears. Fortunately, it's not hard to do on a 9".