All,


Contemplating changing gear ratio and seeking opinions, both pro and con. Current configuration: 1977 F-150 4 x 4, 351M, 4 speed with 3:50 gear ratio. Running LT 235/75x15 tires. Objective is to lower RPM at highway speeds and extend engine service life. Do not tow with the truck.


Can go down to either 3:25 or 3:00, understand this will lower RPMs by about 200 RPM increments. Currently driving at 55 MPH, RPMs is at 2,450. Recognize the cost for changing gear ratios, well is an expensive that may never be recovered and I'm okay with that. Again, asking for pros and cons that I should consider.


Research hasn't provided a turn key solution getting an overdrive (5th gear) with the RB 435 4 x4 configuration yet. Thanks for your time

 

My $0.02..and now I have add more words.

 

Put taller tires on, will give you the same end result. Less $$ too.

 

Gear Ratio Calculator

I don't know how much you've played around with this calculator, but it's pretty slick. It can help you figure out a few things beforehand....

One big thing to remember, is that the gear ratio change from granny to direct a NP435 is a LLOONNGG split, just as far or farther than the common overdrives guys use - really the only thing you gain is another gear (which is valid, depending on your use, powerband of the engine, etc.). But, if it's just a driver, Tall gears and your direct tranny will get you the same road speed, and a similar first gear to what you'd have with, say, a ZF5.

Personally, I've ridden in a 150 that had 3.23/3.25's and 235/85R16's, and it seemed about right - that would be roughly the same as your tires with 3.00 gears.....ballpark. I just really like 16's, lots of gravel out here, I like 10 plys.

If you're looking for 3.00's, there were a couple loaded 3rd members on ebay for 9" rears, not sure on the axle shaft splines, though. They were pretty cheap, might be an idea, save some install/setup expense.

 

First off, good luck finding taller gears ( smaller numerically) than 3.50 for your d44. Larger tires are the cheaper option but either way you go (bigger tires or taller gears) you have to consider the power loss that will be caused by either one. If you don't do very much city driving then the low end power loss won't be much of an issue.

In my opinion, either way you go the gains are so minimal that I dont think its worth it. It would be a different story if you didn't already have the 3.50 gears though.

 

You certainly will be hard pressed to recoup your money from fuel savings unless you do a hell of a lot of driving and live in Flatlandia. Running even at 3000rpm for hours on end on the highway will not harm your engine in any way, so there certainly is not an engine service life issue.

Going from 3.50 to 3.00 is only a 14% reduction. At 2500 rpm you gain 350 rpm.

On the other hand you also lose 14% of your torque. You may find out that you cannot climb some highway grades like you used to be able to in 4th. You may have to shift into 3rd and with the huge gap in ratios, you could be much worse off.

My friend has a Chevy half ton with a 400 in it. He has something like 3.08 gears with 37 inch tires. His 4th gear is about worthless unless you are on a straight stretch.

Just a thought, if it is fuel mileage you are after what about putting in an EFI 302 with a ZF5 and the transfer case that comes with it out of a later truck. Can probably find on craigslist fairly cheap. Not sure how much fab work and electrical would be needed.

 

My 2 cents. I did this very thing in one of my 79 Broncos. Had 3.50 gears, found a complete 78 truck with 3.00. Did axle swap both ends. 33" tires, couldn't be happier. Much more comfortable at hwy speeds 65+. Still plenty of torque. Have since sourced another frt 44 gearset off ebay, already had a 9" 3.00 (those are easy to find). Bronco #2 is next.

 

I like the 350 ratio, but as always taller is nicer. less rpm, lower rpm at cruising speed, quieter engine, and possibly less gas, but at some point the gas starts back again. You are so low rpm, the engine is below it's effectiveness and not able to build the power to push the car and always laboring.

 

All thanks for your input and in particular BLK 79, your first hand experience is convincing.
As indicated in my first Post, purchased the truck June 2016 with the intent of pleasure use along W/an annual trip to Eastern Sierras trout fishing (~ 750 round trip). I've since been using the truck once every other week for work and it's all highway. I'm not concerned about MPG improvement, just want to lower RPM for highway driving.
Did something similar with our 95 F150 Lighting, changed the stock 4:10 gear set to 3:73 and was forewarned of the implied torques sacrifice, but the results have been very positive. Moreover, highway mileage went from 17.8 to 20.4 MPG carrying about 500 LBS of camping and fishing gear in the bed W/AC on.
Although I'm relatively new to the forum becoming a member, I've been reading it several years and I've found it not only to be a excellent technical source, a good sounding board and even sometimes I get a good genuine laugh from a fellow member's post. Imagine most of us come from similar fabric, share similar values and is essentially why this is truly the best forum out there today, speaking from my humble opinion.
Again thanks for all of you that weighed in, much appreciated. I do believe I'll plan and schedule this endeavor for March / April time frame barring no unforeseen circumstances prevents accomplishment.