Hi guys is it much of a job to change the axle ratios from 3.50 to 3.00 in my truck 1977 with 4-speed and 351M engine. Kind of puzzled once u get the pig out of the housing is where it gets confusing or is it better to just bring it to someone that knows what their doing been doing some research on the project I can get the pig out and from there I know things have to b in sync and right tolerences otherwise u could run into some trouble some suggestions would b nice and where is the best place to get the ring gear and pinion from. Tks in advance

 

Assuming your truck has a 9" axle in it, the easiest way to change ratios is by just swapping the "chunk" out for one with the ratio you want, otherwise yes to change the ratio of what you got requires some special tools and knowledge.

But have you looked at this from another angle, why not change the trans for one with an OD ratio like the ZF5.

 

I did this very swap, 3.50 to 3.00 in a 79 Bronco and love it. Much more comfortable at hwy speeds. I did swap entire front D44 with one that had 3.00. Truck not off roaded, but if it was the t-fer case low is there if needed. I recommend it. Have acquired two more complete 3.00 gearsets to do the other two trucks eventually.

 

I agree with Conaski about the overdrive transmission. It isn't as simple installing a different transmission as just swapping out the 3rd member in the rear end but, it gives you the best of both worlds in keeping your current ratio and to be able to cruise at highway speeds at greatly reduced engine RPMs.

Most modern Ford overdrives (from the '80s-later) are 2/3rds overdriven with overdrive ratios that are typically in the .68:1 to .70:1 range.

If you have a 3.50:1 ratio in the rear end and a .68:1 overdrive ratio in the transmission, your final drive equivalent would be 2.38:1 (.68 x 3.50 = 2.38). This is obviously much higher (lower numerically) than just swapping to a 3.00:1 ratio without an overdrive.

Swapping to 3.00:1 gears with a non-overdriven transmission means you're going to trade off acceleration, pulling and hauling ability just for the lowered highway RPMs. Overdrive would let you keep your ratio for these things but also greatly lower cruising RPMs when in overdrive so, you give up nothing but actually have a lot to gain by its addition.

Just as another example, if you had an overdrive in the .68:1 range, if you installed 4.30:1 rear end gears, the final drive equivalent would be 2.92:1 (still under the 3.00:1 ratio if you only changed the 3rd member out). With a set of 4.57:1 rear end gears and a .68:1 overdrive, you would be just over the 3.00:1 ratio in having a final drive equivalent of 3.11:1.

It's easy to see an overdrive has many advantages in the versatility your truck would have as opposed to the limitations you'd have by only changing the rear end gears.

 

I don't disagree with overdrive being better all around. That's a given. Much more difficult/expensive tho (for me) in a 79 Bronco. The only factory Ford overdrive trans that will bolt up to 351M/400/460 is the E4OD. Good strong trans, but computer controlled. Which means aftermarket (Baumann, etc) trans controller, or adapting Ford EEC IV somehow to control trans only. E4OD is also longer, and requires a different transfer case or modifications to 205 setup due to shifter interference/frt driveshaft clearance. Not impossible, but added complication. No AOD ever came with a big block bellhousing that I'm aware of. Adaptable probably (expensive), although the ability of a non-built AOD to survive long term to the torque of a 400/460 is questionable. Could probably adapt a GM 700R4, but not simple or cheap either. A 2WD truck would be simpler for sure.

 

I see I missed the fact OP has a manual. That probably simplifies it somewhat.

 

Hey there BLK79 I'm thinking about doing my truck with a gear ratio like you did. Do u know what RPM's u r running around 60 65 mph. I'm running 2600 right now at 65mph and want to lower it to around 2000 and was told not gonna have any power on the highway and gonna suck more gas than before how is you Bronco can u give me some details tks Yooperuno