Regearing: How's my thinking?

AFBronco235

Crew Chief
My next big project on my 91' Bronco is to re-gear the axles to squeeze a bit more MPGs out of it. I'm currently getting 11-13 MPG, depending on my driving needs, highway vs. city. I believe it has the stock 3.56 gears in the 8.8 and the equivalent in the Dana 44 TTB, but I'll double check that in the morning. Anyways, assuming I'm right about that, I'm thinking I can get away with a swap to 3.31's without loosing much power and gaining some gas mileage. I also plan on throwing 33" tires on when my 31's wear out, so that will improve it even more.

My only real concern is gearing so high that I loose all the torque I've got. I do still want to take it off-roading sometime. Am I worrying too much or should I rethink my options?

E4OD transmission with BW 1356 t-case, 31" tires and 5.0 (302) engine.
http://www.grimmjeeper.com/gears.html
 

Lecoq

Explorer
I think you're going the wrong way. You should go up to 3.73 or 4.10. Especially if you plan on going with bigger tires. You'll lose a lot of mpg with longer gears and bigger tires.
 

AFBronco235

Crew Chief
I think you're going the wrong way. You should go up to 3.73 or 4.10. Especially if you plan on going with bigger tires. You'll lose a lot of mpg with longer gears and bigger tires.
I don't think so. I'm needing more speed at a set RPM. With the 3.55 gearing, I'd be getting 55 mph at 1500 rpm's in 4th gear. With the 3.31, I'm getting 59 mph @ 1500 rpm. That with 31" tires. Swap to 33" and it goes to 63 mph. Basically, I'm sacrificing some torque for higher speed, but I'm concerned about giving up too much torque to do what I want with it.
 

DaveNay

Adventurer
I don't think so. I'm needing more speed at a set RPM. With the 3.55 gearing, I'd be getting 55 mph at 1500 rpm's in 4th gear. With the 3.31, I'm getting 59 mph @ 1500 rpm. That with 31" tires. Swap to 33" and it goes to 63 mph. Basically, I'm sacrificing some torque for higher speed, but I'm concerned about giving up too much torque to do what I want with it.

Unfortunately, I don't think it's that simple when your target is maximizing fuel economy (or even increasing it.)

Ideally, you would need some sort or performance profile where you discover at what RPM your engine operates most efficiently at given myriad parameters (vehicle weight, rolling resistance, wind resistance, gearing, tire size, etc). You would then need to find which direction you need to go with the gearing to increase the overall efficiency.
 

dbandel

New member
I've re-geared several Jeeps and you ALWAYS go numerically higher gears with an increase in tire size to keep the engine within the designed power band. Also I don't think you are going to see some magical jump from your current mileage to anything that would challenge a Prius. :drool: I don't think you'd get anywhere near 15 mpg if you are currently in the 13's and going with bigger tires. Also, with fuel at it's current prices, I think you'd have to drive your truck to the moon and back several times to break even. :smiley_drive:
 

Lecoq

Explorer
I don't think so. I'm needing more speed at a set RPM. With the 3.55 gearing, I'd be getting 55 mph at 1500 rpm's in 4th gear. With the 3.31, I'm getting 59 mph @ 1500 rpm. That with 31" tires. Swap to 33" and it goes to 63 mph. Basically, I'm sacrificing some torque for higher speed, but I'm concerned about giving up too much torque to do what I want with it.
Your current setup will have your engine struggling to keep the set speed. It will take you out of your power band and therefore cost you mpg. Your Bronco was built and engineered for a certain tire size. The diff gears were meant to match the transmission and engine power band. If you go out of that comfort zone you lose mpg. Whether you're going up or down.

You need to find the balance and stay within the power band of the engine.
 

Riley

Observer
I run a 93 F150 with 3.31's, a 235/75 tire and a 302, I plan to go to a 3.73 with a tire upgrade to a 31 or so. I turn about 2K at 70 and 2.2 at 75.

You logic of minimizing cruise rpm's is correct but I think you may be overlooking the reality of acceleration of mass. Meaning getting these old things moving.

Force = mass times velocity. Going from 0 to 70 requires significantly more force or power than maintaining 70.

A few hundred rpm's at cruise where the engine load is the lightest is nothing compared to getting it to your cruise speed to begin with.

That's generally why, with heavy vehicles, torque is much better than horsepower and a deeper, numerically higher, gear set will allow you to get to your desired speed with less energy, or force, used. Which is the biggest user of energy, gas, in their operational range.

Hope that helps.
 
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Ivan

Lost in Space
You won't see much of a difference.

I had a 92 with a 302, 4 inch lift and 33's with the stock gears (3.55‘s IIRC), never really saw anything over 13's. Maybe 15's once or twice afrer a tune up.

Keep the stock gears or go up to 4.10's, and some tall 33's or 35's. Getting taller gears is pointless with a rig that big!
 

AFBronco235

Crew Chief
Okay. I'll just hold off on regearing until I get he 33's and see how my fuel economy looks then. Who knows, I may get the effect I want with just the change in tires.
 

underdrive

jackwagon
Your MPG seems pretty horrible... IMHO the combination of a 302 with an E4OD behind it is to blame. E4OD behind a 351 or larger is decent, AOD behind a 302 is also decent. But an E4OD behind a 302, yeah not so good. Tho I'd juts go for a 5-speed in that thing, the ZF5 not the Mazda one. I mean the Mazda is alright but it's very long, don't want that in a Bronco. The ZF also has lower 1st gear, good for offroading and stuff. easily doable with junkyard parts on the cheap, just pop the PTO covers off the ZF first to take a peel at what the fluid and gears look like inside.

Also keep in mind that regearing a Ford rear axle is a piece of cake, while regearing a Dana front axle is not for the faint of hear. So yes, get your new tires on first, drive it around, change the transmission if you want, drive it around some more, then figure out what you wanna do for gears.
 

AFBronco235

Crew Chief
Oh, trust me, a ZF5 swap is definitely in the plans. My problem is that in the junkyard, they either get snapped up too fast, or some a$$hole just grabs a few pieces off of it, that it becomes worthless to me, so I don't have any way to really swap it out.
 

underdrive

jackwagon
So like looking for a D60 in the junkyards, gotta be there at the right time. I does happen tho, persistence is key. We just picked up a junkyard ZF for a big-block project for a 1/4 of the regular asking prince on CL. Trans mount ears are busted off, but that's what we have TIG for :D Even factoring cost of that repair it was still well worth it... You'll find one, just be patient.
 

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