I'm starting to look at a gear ratio change on my '70 4X4 to accomodate larger tires. My truck has a Dana 44 in the rear and what looks like the Dana 30 in the front (no casting mark in the usual place). The tranny is the 4 speed NP-435 and the transfer case is the Dana 21. The funny thing I'm running into is when I lift the front end and lock the hubs, I only see about 1.75:1 ratio at the diff when I make 1 revolution at the wheel. I thought the Dana 21 was a straight 1:1 gear driven case (1 speed). Shouldn't I see a standard 3.52:1 at the diff when turning the wheels 1 revolution?

 

Pull the diff cover and count the teeth on the ring and the teeth on the pinion, that should be more accurate.

Since you're counting tire revolutions to driveshaft, it doesn't really matter what ratio your t-case is as long as your tranny is in neutral.

 

Thanks Comegetsome, I ended up pulling the cover anyway, just to be sure. Then I realized that my helper (turning the other wheel) didn't make sure the hub locked, so that axle didn't turn. Unfortunately I found out that my project truck has mismatched front to back gearing.

 

How far off are they mismatched?

The 4xs usually came with an open 4:09 in the front and a clutch locked (Trac-Loc) 4:10 in the rear.

 

While we're on 4x4 gear ratio's, what's the reason for the mismatch?

Thanks!

Barry

 

Manufacturers. I've found early Ford's to have 4:09's front and 4:10's rear. It won't make a difference, and in fact tire pressure will affect gearing more than that.

Dana/Spicer offered 4:10's in their axles and not 4:09's. Ford was the exact opposite offering 4:09's and not 4:10's.

 

I was told it was for traction purposes. The 4:09 in the front pulls just a touch faster and keeps the truck going in the right direction in slippery situations.

 

Interesting! And I was told it was to keep the driveline taunt or wound-up and take out the slack.

Barry

 

Shoot, my front drive line is so wore out it might be a bad idea to keep it taunt.

I've gased mine around iced up corners and it seems to pull itself out of the sideways slide and back straight.

 

DesertFE, the 1.75 you came up with would be about right for 3:52 gears. Since it is an open diff and only one wheel is turning, you would only see half the actual amount of revs the ratio would indicate. If you had a posi and both wheels were turning the same then you would see the actual 3.52 turns.

 

Well, When I pulled off the covers on both diffs they turned out to be 3.92 rear and 3.54 front.... I think I'm gonna do the gear change myself. I hope it doesn't explode!!! All of the charts I have come across indicate that running 4.56 gears will put me in the good torque range with 35" tires. Any inputs?

Thanks
Brad

 

Whoa, you do need to fix those gears. 4:56s would probably be ok with 35s. Some one will probably jump in here with the exact formula but you can figure your engine rpm pretty easily. Take the circumfirance of the tire in inches ( 35 x pi or 3.14) and divide it into 1 mile ( converted to inches 5280x12=63360 ) That should give you 576 tire revolutions per mile. Multiply that times 4.56 and you come up with 2626 engine revolutions per mile. So at 1 mile per minute (60 MPH) your rpm would be 2626

 

Hey there,

I was also under the impression the 4.09 front and 4.10 rear was for pulling the truck through slippery stuff rather than pushing it. As for ratios for 35" tires- what are you doing with the truck? My '70 f250 weighs a good 7000 pounds has a $100 dollar special 390 (new 390 to be back from machine shop within the week!!!!!!) has a 435, 4.10 gears and 35" tires. Every once in a while I still try to grab 5th gear at around 50 MPH. In other words, I'm saying I have a gutless turd of a 390 and with 4.10 gears and 35" tires the gears feel too low. Takeoff is nice and easy not requiring much from the clutch at all. A 390 is a very torquey engine, or atleast it should be unless you're running a bracket racing cam in your tow rig. My new 390 is balanced, torque plate bored, has hypereutectic pistons, 9.5 cr , later small port heads w/oiling mods, performer intake, crane rv cam, windage tray and a 625 carter. This engine should make an easy 425 ft/lbs @ 2000 rpm and a good 300 horse. the next addition is going to be the nv4500 5 speed which will drop the rpm's to around 2100 @ 70 and maybe, just maybe put me over 15 miles per gallon on the freeway. If you're gonna drive your truck on the freeway, 4.56's are rediculous. Also if your gonna use your truck off road, the more numerically higher ratio you use the more stress you put on axles and knuckle joints because the gears multiply engine torque. Plan on replacing shortside dana 44 axleshafts and u-joints quite a bit with a 390 35" tires, and 4.56's.

Dustin

 

If all you want to do is run down the highway efficiently then a 4x4 with 35's would be out anyway you look at it. As far as any serious off roading goes the 44 front is way too wimpy and you can break one with stock tires and gears. I think the lower gears would be easier on spiders and the cross pin which I have broken a few of with stock 360s, 4:10s and 7.50x16 tires. The stub shafts are weak too but usually only break if you are on the gas while the wheels are turned and start to wheel hop. This condition is going to destroy something and you should learn to drive without flooring the gas while turning sharp unless it is absolutly a must.
Those old pickups with the .01 mismatch were a fright to get shifted back to 2wd if you had to come out onto a dry hard surface. The theory of keeping them going straight sounded good but wasn't necessary as the 4x4s of today prove.

 

> Those old pickups with the .01 mismatch were a fright to
> get shifted back to 2wd

Only way I have been able to get the Dana 21 back into 2wd is to reverse, give it the gas forward, and try to ease it into 2wd while coming off the gas. With a load of wood it can be a real pleasure

DesertFE, looks like a Dana 30, but, it is a Dana 44. I thought I had a Dana 30 too, people here set me straight, it is a Dana 44.