Hello All!

I am trying to find a solution to a complex problem. I am trying to find a combination of rear end gears and Transmission that will allow me to get my old ford truck (1959 F-100) up to freeway speeds (70-75) without blowing up?

Current rear end gears are 389 in a 9-inch rear end.
Transmission is a manual 4 on the floor. Designation "F" on the vin tag.
Engine is a 223 6 cylinder with 4,000 rpm.

Any help or insight into solving this is GREATLY appreciated!

 

Easiest thing would be to switch to a 5-speed. I think the T-18 is a pretty common swap for the 1/2 ton trucks.

 

Thanks Byrd.dog. So swapping the 4 spd with the t18 5spd will allow the rear end gears and transmission to reach freeway speeds?

 

Need more info such as tire size you are using on the truck

 

@byrd.dog I currently have different sized tires for front and back. Can we suppose I use a stock tire size? Or, if there is a tire size I can switch to that will allow the truck to freeways lead then I can switch. The tires that are on now are crap.

 

So we assume a stock tire diameter of about 28" . So at 75MPH with a 3.89 rear end you are looking at about 3500 RPM at 75 MPH

Here are the following Engines RPM's with varying rear gears.

Rear gear Engine RPM @75 MPH
3.70 3330
3.50 3150
3.25 3000
3.00 2700
2.75 2475


Ideally a swap to a trans with and OD would be the best a solution or increasing tire size with a rear gear swap.

 

F - 1957/64 = Borg-Warner T-98 4 Speed // F - 1965/79 = Borg Warner T-18 4 Speed.

No such thing as a T-18 5 speed.

 

Get a "new" 9" pumpkin with 3.0 gears. Serving suggestion.

 

I have the 292 V-8, not the 223. However the concept should work fine. I changed my rear gear ratio from 3.70:1 to a 3.00:1 and now use my Granny gear as a real 1st gear and 4th gear is comfortable for highway speeds. The 223 doesn't have the same hp so maybe some else here has done similar with that engine and can comment. If not you may want to try a gear set closer to 3.25:1 instead of the 3.00:1. The 3.89 you have would make you truck quick to start out, but top speed will certainly be limited. Good luck.

 

My 223 with 3.89 gears, 3spd OD and 10.5x15 tires 31 diameter will run at 70 with no problem. But unless you live in the middle of nowhere you'll need to figure out how you are going to STOP it.

 

Thank you all for the help with this problem. I must admit this problem is at the upper limit of my knowledge. If I swap my rear end gears from 389 to 300 will I be able to use my same transmission (4sp manual type F)? I have swapped the front drum brakes for disc brakes in anticipation of getting the truck to higher speeds.

 

Only the gears change. Your transmission and drive shaft stay the same.

You can get an entire 3 member assembly from various suppliers, or have the gears changed in your existing one. Gears are about $200. Labor varies, expect $500. It's a bit of a tedious job, requiring some special tools. Some folks DIY, and some of the are back wonder why it whines.

While your at it, new bearings all around and your rear axle will be good for another 50 60 years.

 

Yes it will allow you to use your same transmission. Just beware it will play hell with your acceleration but with the low granny gear in first it will help off set it.

 

Sometimes you can find ford 9" differentials in your local junk yard and just swat the entire 9" differential. I used my original yoke and installed a new seal. You need to know if it is a 28 spline or a 31 spline (number of splines on the axles). The 28 spline are more common. I found mine on ebay. It is not that big of a job to swap differentials. Remove tires, brake drums and pull the axles. Drain the gear oil, unbolt the driveshaft, unbolt the differential and remove. On the salvaged differential, replace the differential seal and swap the yokes if necessary, new gasket. reassemble and fill with gear oil.

When driving you will need to start out in granny gear instead of 2nd gear for normal driving.

 

In 4th gear you transmission is 1:1. This means that the driveshaft is turning the exact same speed as the engine. One revolution of the engine equals one revolution of the transmission output shaft and thus the drive shaft.

The rear differential ratio determines how far you rear tires travel with each rotation of the drive shaft. In simple terms, the lower the number, the farther the rear tire travels. So if you change from a 4.89 to a 3.25 or 3.00, since both numbers are lower, thus the rear tire travels farther with each revolution of the drive shaft, thus the truck goes faster as a given engine RPM. In more detail, today your engine has to rotate 4.89 times for your rear tires to rotate once (4.89:1) and if you change the to say a 3.00:1, the engine only has to rotate 3 times to make the tires rotate once.

This will however slow you acceleration quite a bit, so that is why you will have to start out in granny gear to help offset the acceleration loss. You will also have to down shift to third more often when doing things like climbing a hill.