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Another gear ratio question? With a twist

Nevadasmith

Full Member
Joined
Apr 30, 2016
Messages
182
Loc.
Fallon, Nv
Ok so 1971 bronco zf5 d20, d44/f9" 250 i6(Boss170)
35" tires(on there now), and or 32" tires...
I had a weak 250, and 4.11r/4.09f gears, and I couldn't really use 5th much, any hill would be down shifting to 4th or 3rd. I found out the 4.09 gears in the front end were roasted from a previous bad install when I put the tru-trac in...not wanting to run metal through the tru-trac, I needed new gears...I found a set of 5.38/5.42 gears at a swap meet for Penny's on the dollar! So that's what I'm running now, with the old motor they were working ok. Just ok..., Old motor ran out of go all together at 2600rpm. I could get to about 65 on flat ground.
I built and installed a new warmed up 250 that makes 200+ ft/lbs torque from idle to 2600 rpm. Actully pulls hard to 3000 rpm. Now I leave stop lights in 3rd high range and in 5th by 25-30 mph top out at about 72-73 on flat ground, ...I'm getting 16mpg regularly...but I have a goal of 18 mpg.
After A lot of math I have decided 4.88 would be great, but....and here is my problem, I love the way it pulls straight in mud and snow and ice with 5.38f 5.42r.....the only listings I can find is 4.88f 4.86r my fear is that the rear will be pushing the front....and I would have to drive my but off to keep it straight on low traction situations, so question 1 has any one found a better ratio combination????in that range?
Questions 2 is if not I'm looking at 4.55f 4.57r, or 5.12f 5.14r, Keeping in mind I want to run 32"(235-85r16) tall skinnys in the winter, and 35-12.50's summer, which would you choose??. Usage is 60% commuting, 40% hard to difficult trails, light to medium rock crawling, lots of rescue missions!!!!!
Thanks for the input!!!
 

DirtDonk

Contributor
Bronco Guru
Joined
Nov 3, 2003
Messages
47,355
I'd go with the 5.12 combination I think. It's closer to your existing 5.38 that you're happy with right now, other than the bit of breath it needs with the 32's on the road.
It's only slightly better for the small tires, but that's too big a spread in tire size for any single gear ratio with that engine.
The difference between the two ratio choices is just under 200 rpm if I'm not mistaken. Not a huge improvement in road rpm with the 32's, but not so much a difference as to lug the engine too much with the 35's.

At least that's my take on it. You won't have quite as big a spread between front and rear as you do now, but you won't lose as much rpm overall either.
And it sounded like you were finding the current rpm levels just about acceptable, correct? If so I would not go all the way down to 4.55's.
Great for the 32's maybe, but with the 35's (assuming about a 34" rolling diameter for now) you'd be down in the sub-2200 rpm range at 60mph.

You're more familiar with how it feels at the different levels. Do you think the engine would be happy at that rpm?

Paul
 
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Nevadasmith

Full Member
Joined
Apr 30, 2016
Messages
182
Loc.
Fallon, Nv
Thanks Paul, the last question is the one I've been struggling with, it feels like 2300 rpm is where it's the happiest, I get the most pull for the lease throttle, so I was shooting for that at 65, witch is like 4.77 so 4.88 's was the most logical choice......I'm afraid of being too high geared again, so I'm leaning towards 5.12 gears, but your right not much of a change.... Only 6% so like 150rpm at 65.
Well thanks, that gives me some more to think about
 

Broncobowsher

Total hack
Joined
Jun 4, 2002
Messages
34,835
As for the front/rear being slightly higher/lower, a non-issue. Broncos were built both ways and I have never heard anyone tell any push/pull feelings. The only time I have known of a split ratio is high HP mud drags. But those were a rather steep step. 4.27/5.12 type of things. Running a controlled hydroplane.
 

Boss Hugg

Contributor
Bronco Guru
Joined
Jun 8, 2010
Messages
2,140
I don't think a goal of two more MPG is worth the cost of regearing. Honestly, IMO, if you want 18, you'll need to lay the windshield back and that's more work than it's probably worth. My 99 explorer 4.0 (244 CI) gets 18.5 at 72-75. There's a LOT of aero differences to.

In the end, I think the best way to get better mileage is just to slow down, and believe me, I ALL against that! lol.

Just my thoughts. Someone made a joke (I think) the other day about how my comments were contradictory. All I had said was that (1) I was doing some work on my bronco and (2) all I wanted was complete perfection. I guess you can't have the latter?
 

patrickk1

Jr. Member
Joined
Mar 7, 2011
Messages
135
Loc.
Cambridge
I don't think a goal of two more MPG is worth the cost of regearing. Honestly, IMO, if you want 18, you'll need to lay the windshield back and that's more work than it's probably worth. My 99 explorer 4.0 (244 CI) gets 18.5 at 72-75. There's a LOT of aero differences to.

In the end, I think the best way to get better mileage is just to slow down, and believe me, I ALL against that! lol.

Just my thoughts. Someone made a joke (I think) the other day about how my comments were contradictory. All I had said was that (1) I was doing some work on my bronco and (2) all I wanted was complete perfection. I guess you can't have the latter?

I agree. Unless you already have the gears lying around and can swap them out yourself, the money you spend will buy lots of fuel.
 

sykanr0ng

Bronco Guru
Joined
Aug 11, 2014
Messages
5,363
Want better gas mileage in a Bronco?
Find secondary roads and drive slower like it was made for.
Take your time and enjoy driving your Bronco.

Want to drive fast and get good gas mileage?
Drive some other vehicle that is made for that.
 
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Nevadasmith

Full Member
Joined
Apr 30, 2016
Messages
182
Loc.
Fallon, Nv
Want better gas mileage in a Bronco?
Find secondary roads and drive slower like it was made for.
Take your time and enjoy driving your Bronco.

Want to drive fast and get good gas mileage?
Drive some other vehicle that is made for that.

I'm already doing this, but even on secondary roads hear you need to go 65 some times just because of traffic, I mostly drive 55 or less, but I have so much more torque the 5.42 gears are kinda ridiculous.
I can install the gears my self...but thanks for the input!!!
Maybe I will just leave it alone for a year and see how I feel in then?
 
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