What gear ratio should I run?

[Snackslinger]

I have a 347 with a 4r70w and 35” tires. What ratio should I go with? This bronco is mainly used on hwy/interstate. The engine has about 425hp.

 

[distended]

4:56

I don't really know...4:11?

 

[Yeller]

My choice would be 4.56 or 4.88.

 

[Broncobowsher]

4.88 will make for a really nice highway gear. The high HP 347 will need the little extra RPM to run happy. Don't try to gear this like it makes stock power or you will just be lugging the engine down. If you were running a stock 351 you could get away with less RPM.

If you were not after highway driving and wanted a fun street performer, I would call out something in the 5s.

 

[m_m70]

4.88 will make for a really nice highway gear

yup...........this^^^^^^^^

 

[MonsterBIlly]

I had a 408 with 4r70w, 4:88s and 37s. I know its only a small difference but i wish i had done 5:13s. Even with 480hp.

The difference is only about 5 percent but that makes a big diff in the highway. .

I recommend you go 4:88s

 

[BUCKWILD]

My vote is 4:88 unless you see a 37" tire in the future then a 5:13.

 

[Apogee]

Most folks would go 4.56 to 5.13 with a build like yours, with 4.88 being the logical choice in the middle. If you ever think you might go bigger on the tires, then 5.13's. If you had thoughts of going 351W stroker in the future, then maybe 4.56's.

I've got a 351W/NV4500/4.88's, but she's not on the road yet, so I can't give any real world feedback...looked good on the calculators with respect to RPM's and shift points though. While 5% one way or the other can be significant, it's not typically make or break IMHO. The additional torque multiplication you're afforded with an automatic and torque converter combined with a slightly bigger OD (.70 versus .73) should both work to your advantage.

 

[DirtDonk]

I have a 347 with a 4r70w and 35” tires. What ratio should I go with? This bronco is mainly used on hwy/interstate. The engine has about 425hp.

What ratios are in there now? Have you ever run it with the 35's and the current gearing? Or is this all from scratch?
If the axles are not already blown apart, have you thought about just running the existing gearing to see what it feels like with the new transmission?
You know you're going to have more power than any old stock engine, but the first gear in the 4R70 is lower than a C4 and way easier to get rolling than a stock manual trans, so even 3.50's would be drivable to a certain extent I would think.
So my thinking is that even if you have the lowly 3.50's, but especially if you have the optional 4.11's instead, just run 'em as is for not and make your decision based on some real world experience.
None of the recommendations above would be bad choices. But they might not all be the right one for you personally.

My '68 can go down the highway any day of the week with no struggle. But it's at the ragged edge of too low, what with 4.56 gears and only 31" (29.5 actual) tires.
I can go 75mph no problem, but it's revving up more than it needs for the engine/gear/trans/tire combination I have.
The engine revs fine, but would be more efficient if I dropped 500, or even 1,000 rpm on the highway. And I'm sure it would still pull just fine from lower rpm.
I'd run 3.50's with these tires no problem, but would rather go up in tire size than down in ration numbers.
Being geared a tiny bit too tall (lower number) would not kill your around town, but you'd lose a little of the fun at stoplights. Being geared a tiny bit too low (higher number) would pull strong but maybe be shorter than you like out on the road.
So, what highway speed are you contemplating?

Bottom line though, is don't go by the sound of the engine and outside world in your tin-can called a Bronco. Use a tachometer!

Paul

 

[ba123]

Yeah I put 4.56 in mine and been running some scenarios (not driving yet) but starting to wonder why I didn’t go one more step up
with 35’s.

How it is, I’ll be going 102mph at 3000 rpm. Might be a little too fast in a Bronco.

 

[rocknhorse76]

Another vote for 4.88

 

[bigmuddy]

I ran a ZF, with 4.56 and 35" tires being pushed by a stock 5.0, really wished I had gone 4.88
I am in the middle of a new engine 5.8 and I am still thinking about a 4.88 swap...

 

[bax]

I run 4.88 with 37's and no overdrive. If I had overdrive 5.13 would of been my choice. Why don't you set up a 4.88 or a 5.13 in the rear and drive it around. Find out what you like. When you're happy you can do the front. My choice would be 5.13

 

[pcf_mark]

I can say this - under geared feels a LOT slower and heavy driving around town. I have 31" and 3.5:1 gear with a 4R70W and is idling at 55 MPH but feels really sluggish in anything but first. Good gas mileage however

 

[spap]

I would measure your 35 s and then go to a gear calculator and see where you want to be at. My semi new bfg at 35/12.5/15 measure in at 34 to 34.5 inches
I run a 4r70w with the 35s and 4.56 gears and really like the gearing. My motor has it s torque (280lbs)down low at 2000/2200 rpm . It’s very comfortable on the fwy s of cal where you have to cruise at least 70 not to run over and I have gotten 15 mpg.
rpm wise I think it’s just a couple hundred rpm difference from 4.56 to 4.88. I do more mountain trails and desert nothing very serious.

 

[rjrobin2002]

I ran 4.56 with 35s and the c4 and it ran great at 60mph.

If I had OD I definitely would of wanted 4.88.

 

[Colo77Bronco]

I am doing Super Duty axle swap and was going to run 5.38 with 38s but nobody currently makes a ring and pinion gear beyond 4.88 for 2011+ Sterling 10.5 which is my rear axle. I was sad I was limited to 4.88 until I looked at the gearing in my 10R80 and I have two additional low gears vs my frame of reference which was 1st gear in a C4. I think it'll be awesome.

 

[JSBX]

I have 2 Broncos with 4R70Ws. one has a mild built 351W and the other a mild built 302. Both with 35s. The one with the 351 has 4.56 gears that work good but I think 4.88s would be better. The 302 has 5.13s that work great. My vote for your set up would be 4.88s.

 

[markw]

OP if you want to try 4:88 I have a new ring and pinion set as well as an overhaul kit I’d sell cheap. I don’t need it and it might be just the ticket.
Mark


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