Rear End Gear?

[mustang8]

My 48 ford f1 has some stump pulling gears in the rear end. Stock as far as I know. Truck has a 3 speed manual and at 35 mph it is spinning about 3000rpm. I was hoping to get the truck up and running so I could take to work a couple days a week but I don't see how I could ever think of going even 55 mph in it without sending a rod thru the hood! And thoughts on how to find out what gear I have in it? Tag somewhere?

[JSeery]

You can count the teeth on the ring gear through the fill plug. Mark one of the teeth and count the teeth through one full revolution of the gear. IF the gears have never been changed, the ratio numbers should be stamped on the housing, tooth count is stamped on the lower front part of center section .

I have never used this method, so no idea how well it works! But might give it a try:
(from Ford Flathead V8 http://www.btc-bci.com/~billben/reargear.htm)

JimC: 5/31/2000 - 8:16:04 AM
RE: ratio
Jacking one wheel up and turning the engine over by hand will let you determine the ratio. Make a chalk mark on the bottom of the tire and crank the engine over exactly 2 revolutions. Also, make sure all the slack is out of the system when turning the engine over. Then measure the rotation on the tire. It will be plus or minus one turn, but accuracy with these measurements is critical. Take this tire rotation value, divide it into 1 (in other words, find its reciprocal), then multiply it by 4. This will give you a pretty good ball park of the ratio, which can be compared with known ratios (4.11, 3.78, 3.54) to see which one comes closest. For example, I recently went through this with my car and found the tire rotated slightly less than one turn for 2 engine turns, so I used a tire rotation value of 0.95. Dividing this value into 1.0 gives you 1.05, and multiplying 1.05 by 4 gives you 4.20. So I figured the rear end ratio was 4.11. I then used this method on a friend's car before he pulled the rear end out. I got about 1.05 tire rotations for 2 engine rotations, which figures out to a rear end ratio of 3.8. When the gears came out, sure enough, they were 3.78.
1) Jack up one rear wheel and chalk-mark the 12 o'clock position on the tire. (Make sure the slack is out of the drive train)
2) Hand crank engine 2 complete turns.
3) See where chalk mark on tire ends up.
A) Between 11 and 12 o'clock, ratio is 4.11
B) Between 12 and 1 o'clock, ratio is 3.78
C) Between 1 and 2 o'clock, ratio is 3.54

Edit: corrected typo in original document, 4.54 to 3.54 per Marts catch.

[Mart]

Easier is to jack up and mark one rear tyre.

Jack up one rear tyre. In Neutral, turn the tyre two complete revolutions while counting how many times the driveshaft turns. If a closed driveline (early ford), in top gear turn the engine counting the revolutions until the tyre has done two full revolutions.

The number of times the engine or driveshaft turns equals the axle ratio.

Mart.

PS I like the method at the end of J Seery's post, it saves effort and you don't have to watch two things at once. Please note a typo, though, the last line should read 3.54 not 4.54. I might try that method next time. It also assumes Early ford ratio's too, not necessarily the case with an F1.

[MGG]

Would it give you the same answer if you counted the turns of the engine until the tire mark got half way around or, to the top, with the transmission in high gear? For example a little more than 4 turns would most likely be a 4.11 ratio?

[JSeery]

Originally Posted by MGG

Would it give you the same answer if you counted the turns of the engine until the tire mark got half way around or, to the top, with the transmission in high gear? For example a little more than 4 turns would most likely be a 4.11 ratio?

No, remember you are turn only one tire and there is a differential in the rear end. You have to account for the spider gears.

[MGG]

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I thought I did by specifying that the tire go only half way around.

[rotorwrench]

The rear cover comes off easily on a Dana 41 so it would only cost a gasket to count the teeth. It could probably use a cleaning in there anyway. The 1949 Mercury used the Dana 41 and normally had 3.91:1 gears most of the time. If you could find one of those coming out of some Merc update project, you could swap them pretty easy. The spring mounts are the only part that is different between the two so you could swap the whole thing if you swapped the spring mounts. Another option would be a later Dana 44. They are easier to get parts for.

[Mart]

MGG: You're compensating the wrong way, it needs to go twice round. (If we're talking about the rear wheel that's jacked up.)

[MGG]

Originally Posted by MGG

Would it give you the same answer if you counted the turns of the engine until the tire mark got half way around or, to the top, with the transmission in high gear? For example a little more than 4 turns would most likely be a 4.11 ratio?

Mart you are correct. - Me bad - Did the math backwards. Correction - The wheel needs to go around two times not one half.

[tanner]

Probably a 3.73 like my 49 had until I changed to a 9" with a 3.00. Got the 9"rear end from a 57 F100 and the gears from a 63 econoline, I think. Had same problem but now I can run 60 mph at 2000 rpm. Direct bolt in. Lots of info online about which years to look for.

[38 coupe]

You may have the 4.27 ratio that was optional in the F-1s. The 48-56 Ford truck forum has a lot of information about regearing your truck (one member even figured out how to put new Dana 44 gears in the original Dana 41). Forum here: http://www.ford-trucks.com/forums/forum41/

Also, have you put a tachometer on the engine while you are driving, or does it just sound all wound out at 35 mph? The stock truck fans and mufflers can make the engine sound like it is turning a lot tighter than it really is.

[Ross F-1]

Originally Posted by 38 coupe

You may have the 4.27 ratio that was optional in the F-1s. ....

Also, have you put a tachometer on the engine while you are driving, or does it just sound all wound out at 35 mph? The stock truck fans and mufflers can make the engine sound like it is turning a lot tighter than it really is.

I agree, 2500 in these old trucks sounds like they are really spinning.

Cheapest, quickest, easiest thing to do is get a 48-50 3.73 Dana 41 from someone who is putting a 9" in their truck. Totally a bolt-in and a very good overall ratio unless you are running short tires.

[scooder]

To be pulling 3000 rpm at 30 mph with 28" tyre the axle ratio would need to circa 8.3:1! A 4.27:1 would be 58 mph.
Martin.

[mustang8]

Yes it has a tach in it. It spun 3000 rpm at 35 mph.

[JSeery]

I get numbers similar to Martin for that combination.

Edit: Well, we may have both been using the same calculator! I think you may be getting the correct reading. I will do some more checking, but this chart seems to have the numbers you are seeing (hard to read, but has 28" tires with 3.73 comes out 2909 rpm).

I am going to calculate it by hand and not use the online calculators! and see what I get.

[mustang8]

Tach shows its idling at about 500 rpm.

[JSeery]

Well this is strange, not sure what is up, the math caluclations do not match the charts!!

mph X 336.1524 X final drive ratio ÷ tire diameter = RPM

35 x 336.1524 x 3.73 / 28 = 1567 RPM

[JSeery]

Formulas for Calculating Gear Ratios, RPM's, MPH and Tire Diameter
If you wish to do the math on your computer's calculator,
To multiply, use symbol *, to divide, use symbol /

GEAR RATIO = RPM *Tire Diameter / MPH *336

RPM = MPH * Gear Ratio * 336 / Tire Diameter

MPH = RPM * Tire Diameter / Gear Ratio * 336

TIRE DIAMETER = MPH * Gear Ratio * 336 / RPM

From, Carl/Roseville

[scooder]

Jseery, that chart makes no sence. There appears to be no speed given for the calculation.
I use the same calculator I always use, I've checked the math and it's correct. Our first calculations that suggest a circa 8.3:1 ratio to get 3500 rpm at 30 are right.
Something funky going on with the F1.
Clutch slipping? Tach telling lies? Is the tach an 8cyl tach?
The maths don't lie.
Martin.

[Ross F-1]

I might suggest the tach is set to the 4-cylinder setting.... usually a switch or dial on the back.