I have a 1979 F250 4x4 with a 300 6, 435 4 speed, NP 205 transfer and 4.10 gears. I mainly drive highway and do not pull big loads. Thinking about changing to 3.73 gears to get better gas milage and highway drivability. Anyone out there done this on this setup? What were the results? Thanks for your feedback!

 

I went the other way. I had 33s with 3.73s and everybody told me that I would be better off with 4.10s. I did it and was disappointed. I ended up getting larger tires to reduce the overall effective ratio back to the 33s and 3.73s combo. The 300 just doesn't really like revving, so unless you are running some huge tires, you will be happier with the 3.73s

 

It would be too much expense for such a small change in overall gear ratio. I would jump down to 3.55s or keep the 4.10s, as I think you will be dissapointed if you are only dropping one ratio

 

I have a '79 with a 300 six and four speed as well. I had 4.10's with 31" tires. I bumped the tires up to 33's to to try and drop the ratio. It wasn't enough and I wound up putting 3.50's in it. MUCH nicer! I can cruise at 65 mph now and not feel like the engine is going to fly apart at the seems. I don't think 3.73's will be enough. I'd go higher. I'm very happy with the 3.50's in there now. (and I regularly pull a car trailer with my truck)

Stock 300 six cylinder, four speed trans, 33" tries and 3.50 gears....... no problem!

 

My '93 cummins had 3.54's when I bought it. I wanted better fuel mileage. After asking around, the consensus was that I would get better mileage with 3.73's or 4.10's. So I went with 3.07's and can now get 20 driving 80 and 23 driving 65.

 

the expense that you would have, it would take you a long time to make it up. I'd instead, go to a zf5 and be happier.

 

I have a 79 F350 Dually with a 351 motor and fmx tranny.

I changed from 4.56 to 3.73.

4.56 at 3200rpm gave me 61mph, which was crap slow at those revs.
4.10 would give 68mph, and 3.73 gives me 75mph.
So I reckon one ratio change would not be too impressive!

No real difference in feel on acceleration. ( I actually expected a more sluggish pull off)

No change in fuel consumption either, but that's coz I'm cruising at faster speeds.

 

a truck with a zf5/4.10s/33" tires at 60 mph will turn 1903 rpms
435/3.73s/33"s tires at 60 mph will turn 2279 rpms



EDIT: 3.55s are getting brought up. 435/3.55/33" tires at 60 is 2045 rpms

I just like the Idea of the ZF5 swap because you get all the benefits of your gearing now, but you'll gain the RPM advantage with 5th gear. the junk yards are full of these trans.

 

I would drop to at least 3.55s, what size tires do you run?. Buddy of mine has a 77 with a 300 and 4spd 4x4 flareside, all stock with 3.55s and 33s and gets 22 hwy at 70 religiously. The 300 has more than enough grunt down low.

 

Thanks all for the feedback, I run the stock tire size 235/85/16 which are about 32". I looked into the 5 speed = $$$$$, sounds like if I went to 3.54 gears I could increase gas milage and drive 70 mph without red lining and still pull small loads. My 300 is a fresh remanufactured engine with all pollution control removed including the cat and high flow exhaust installed and I average about 14 mpg, would love to see 20+ mpg!

 

I've seen zf5 2wds for my hi-boy for $200-$300 all day.

Can you do the gear swap yourself? or do you need to pay for labor?

 

For a 300 i dont think the zf will fit, have to use the mitsu tranny ? M5xx sumthin or other, sorry not much help.

 

ZF5 speed was used w/300 and 4x4 on F250's, 351 tranny will work too, do NOT use the mazda tranny, complete POS. BTW 4.10 axle x.76 OD on ZF=3.12 effective gear, way better than 3.55 or 3.73 w/4 spd, still pull stumps and spin slow on the highway. That combo yields 1960 @60 mph and 2300 @70 mph with 32" tires!

 

I looked into the 5 speed a bit, couldn't really find good info on fit without modifications, everyone says it can be done but its a serious pain in the ****, and the info is all over the map on how to do it. I live in the middle of nowhere Idaho so junk yards are few and far. I was quoted $2,500 by a transmission shop to install a new ZF5. I can do the gearing myself. If someone could really give me some factual info on installing a ZF5 on a 300 with an NP 205 that would be great. What vehicle(s) did this transmission come stock?, that might help me track one down.

 

Late 80s F250s had them. You can use the 300, 302, or 351 trans as they have the same bellhousing pattern. The 205 bolts to the back, but you need to notch the mount flange.

Where in ID are you?