Kind of a silly question but worth asking. I'm rebuilding a '68 9" and putting in a 3.70 gear. Will that jive with the front Dana with a 3.73? My next best is 4.11 and 4.​​​​​​10 and I wasn't really wanting to go that high. Thank you for taking the time to read this.

 

I'm not a gearhead and really just wanted comment and not just be a viewer. I've heard of being a point off in the past but not 3.
I'm sure someone will chime in shortly.

 

It is no problem as the EB's/F-100/150's/Big Broncos had ratios of 3.54 (Dana 44)/ 3.50 (9 inch).

 

We're talking about the difference of 3/100ths of a rotation. You'll never be able to tell any difference. ESPECIALLY since 4wd is usually used on loose surfaces anyway.

 

It leads to all kinds of destruction of the drivetrain. Ford used similar different ratios front and back from the factory.

 

Dana 44 front/Ford 9" rear:

3.54/3.50 // 3.73/3.70 / 4.09/4.11

 

Wrong. In theory yes. In actuality no. Traction is the buffer. And in fact, in competition pulling, it is common practice to put one, two or sometimes even three ratios higher in the front differential, to increase the front wheel speed over the rear.

With so many different axle designs, it is IMPOSSIBLE to get everything to have the exact same ratio, unless the ring gear/pinion tooth count can be the same.

 

Hoo, boy. No, absolutely correct. Pullers can use different axle ratios and even front/back tire combinations because the pulling trucks are almost never on dry pavement PLUS the front vs. back tires are broken loose from the loose surface and spinning, not all four locked firmly to a hard surface as would be the case driving on the highway. Watch some videos on youtube to readily confirm this if you've never actually been to a truck pulling contest in person.

 

Farmall, I'm not here to get in an arguement with you. The fact of the matter is that a .03 difference in gear ratio is perfectly acceptable, even on dry pavement. But why are you in 4wd on dry pavement anyway?

 

2X. Back in the day of the Bumps and Dents Ford said the front axle should only be engaged on loose pavement. I had a brand new 79 Bronco. 3.54/3.50. And it's Owner's manual.


And the difference in the gear ratios was the reason why.

 

For whatever reason the drivers did it, I'm not sure but I have seen driveshafts twisted off like a pretzel from going on the pavement with the 4x4 engaged, on factory stock ratios, and also other worse damage. I'm not sure why the factory made them different, no practical reason since some mismatches have the front going slower and some faster, even as ND's example shows. Just my personal opinion but I think it would be best for a street driven daily driver to have them as close to the same as you can get.

And amusingly enough, the owners manual for my 59 F250 4x4 tells you to jack up a rear tire, if you have to, in order to shift back to 2wd on dry or hard packed surfaces. And even that truck, one gear option has the front faster and the other option has the front slower. 3.89r/3.92f and 4.56r/4.55f. No rhyme or reason to why they do this, should not have been that hard to keep tooth counts the same on both gearsets.

In conclusion, your ratios are within the acceptable range by Ford's standards.

 

Easy to figure out, WB, with the difference in ratios, as it called $MONEY. As Dana Spicer was about the only axle manufacturer until GM got with Chrysler to start American Axle Manufacturing (AAM), they tried to match ratios as close as possible with their designs to the Big 3's use of their own rear ends. The Big 3 bought the Dana's at price $X. If the Big 3 wanted Dana to match their rear axles ratios, it would have cost $X plus $Z development cost + $A engineering cost + $B machining cost +,+... You get the picture? The Big 3 found it acceptable to have a minute difference in ratios. Now a days, many transfer cases have a center differential which evens out the axle wrap-up when in AWD and 4wd.

 

A 35" diameter tire has a 110" circumference. It makes 576 revolutions in a mile.
3% of 110" means that a front wheel slips 3.3 inches every revolution or 1900" a mile with respect to the rear if the 4WD is engaged.
That is 158 feet of slippage between the front and rear tires every mile on a hard surface.
Isn't that the equivalent of skidding that distance or laying down a patch of rubber that length every mile?

Methinks you should have dead-on matching ratios or disengage the 4WD before asphalt. Or buy a lot of tires.

All in good fun.

Semper Fi

 

At least one I know of that twisted off a driveshaft had to come off a muddy logging road onto the pavement so not easy to switch to 2wd until off the mud and on the asphalt. He was young and didn't know better, it was bound up and he wasn't savy enough to get it out so drove 8 miles to town. The front driveling twisted off in about 3 miles. My buddy and I were driving 70s highboys and knew the drill, get on the pavement, put it reverse and back straight up, occasionally gun the gas and reef on the shift lever until it came out. And those were the 4.10/4.09, can't imagine the bindup with 3-4 points difference

 

Respectfully, I believe these Numbers are flawed. I'll try my best to explain.

The real difference between, say, 3.70 and 3.73 gear ratios is not 3%. It's actually less than 1%.

Broken down into hundredths, we have 370 and 373. Turning this into a fraction to find the difference would result in us dividing 370 by 373. The result of which, is 0.991957. That's over 99% of being "equivalent".

So, the before mentioned 35" tires would only be slipping "roughly" 7/8ths of an inch every revolution. Which would add up to "roughly" 506" of slippage in a mile. Or roughly 42 feet 3 inches. Not 158 feet.


Granted, this is still an issue. But what happens when you turn in 4wd? The front tires travel in a longer arch anyway. So unless you're going perfectly straight all the time, you're dragging tires anyway. And this all leads back to, why are you running 4wd on dry pavement???