351 zf 5 speed gear ratio help

[Broncopower96]

Hello,

I will be swapping a 351 EFI and zf 5 speed, and the plan is to run 33" tires. The engine will be mostly stock for now. Does anyone have advice on what the best gear ratio would be? I used the search, and it seems like 4.30 could be good. I'm just not sure how the 351 changes things. Any advice would be much appreciated!

 

[bronconut73]

it seems that when going overdrive nobody gears their axles down enough.....at least that is what I hear here a lot.

With 33's I think the boys here would say 4.56's.
With 35's I think the boys here would say 4.88's.

That seemed really low to me But they say it is necessary to make your overdrive useful at 65 to 80 mph. Otherwise you turn too low of an rpm.
I haven't gone overdrive yet but when I do I will likely go this axle ratio route.
I will probably do 35's and 4.56's or 4.88's....depending on the actual height of the 35's I may even go 5.13's.

 

[Broncobowsher]

A Bronco lacks the modern aerodynamics and has a bunch of rolling resistance. IT takes HP to push a Bronco down the highway. HP=TorquexRPM/5252. Simple equation. Compare it to a modern car, or even a modern truck. Get up to highway speed on a flat road and put it in neutral. The Bronco will slow down the most. That is the drag you are fighting.

There isn't much that can be done to reduce the drag and still keep a Bronco a Bronco. So you just have to push through it. For that you need RPM. A carbureted small block needs a little bit of RPM to be in the happy spot. Evan my 2010 daily driver with 260 HP and 280 torque turns a touch over 2000 RPM at highway speeds. That is what you need to aim for. Don't gear a small block for less than 2000 RPM at 60. A 351 will have a bit more torque and make it more drivable. If you are at a border between 2 different ratios, the lower gear will be the better with a 302 and the slightly higher with the 351. But there isn't a huge change in gearing needed between the two engines. Where a 351 is happy at 2000 RPM, a 302 would be better at maybe 2200 RPM.

 

[roundhouse]

A lot of variables
With a 351 , and 33's. 4:11 should be ok

What you running in the axles now ?

I'm running a efi 302 NV3550 4.11 and with 33s I can use OD on level stretches on interstate
If you hit much of a hill i drop to 4th

I just swapped to 35s so we will see how that affects things
Although I'm also preparing to swap in a351 also

I love the NV3550. The ratios are perfect even if you don't get to use 5th much
Sent from my iPhone using Tapatalk

 

[Broncopower96]

Thanks, this is great information! I have the stock gears right now for a 77, so it probably has a 3.50. I haven't checked the axle code, but I know it needs a rebuild anyways. The plan is to put in an air locker along with the new gear set.

 

[garberz]

4.88's would be my suggestion. You'll probably go to 35's eventually.

Mark

 

[sykanr0ng]

Don't fear the gear.

At least 4.56 and 4.88 will do you no harm.

 

[70_Steve]

http://www.grimmjeeper.com/gears.html

 

[Broncopower96]

Haha, I'll make sure not to fear the gear. I'm going to stick with 33's for the foreseeable future, since I am installing a 2.5 suspension, and 1 body lift. So no need to plan for 35's.

That gear ratio calculator is great! Using the Zf 5 speed, d20 transfer case, and 4.56 gears, I would be running 2,117 rpm's at 60 mph, and 2,646 rpm's at 75 mph. Not sure if I want to risk my life driving 75 in the bronco, but at least I'll know it's possible.

 

[Broncobowsher]

The 4.56s look to be a great choice.

Amazing to think that was a factory option with the I6 without overdrive and running little tires.

 

[SHX669]

A couple of things ; as most things " Bronco' - what do you intend to do with your Bronco ? Mostly off road with the occasional trip down the road for gas and cold beverages - or mostly highway ?
Overdrive was/is intended for use on the "highway/freeway speeds" not necessarily for a down the road to the store cruise .
Even my fairly new 360 hp , 6 speed tranny , computer controlled pickup shifts out of Od when pulling a hill.
See - no opinions - just some things to think about .
Ok - I cant resist - opinion - LOL - for me 4.10s are perfect for 33s and an EFI 351W used mainly on the highway .

 

[70_Steve]

With my 351 I run 4.56 gears with a NV4500 (virtually the same overdrive ratio) and 35's. I debated for quite a while between 4.33's and 4.56s. Now I think I could have gone 4.88's.

Play with the calculator and see the difference in RPM between 4.33 and 4.56 vs 33" tires and 35".

 

[AZ69EB]

I have the ZF, efi 302, 4.88, & 33" tires. 75 mph is about 3000 to 3100 rpm. I have to drop to 4th for hills. Also, the 3rd gear to 4th gear split sucks. The torque of the 351w would be perfect. Minimum 4.56 to 5.13 would work great. It all depends on how you will use it. I wish I had deeper gears for rocks and technical stuff. But, it's a great all around trail truck.

If you spend money on gears, put lockers in. Don't skimp. As it would cost more money in the long run.

Good luck.

 

[Broncopower96]

Thank you for all of the advice, this is great. I will use the bronco mostly for street driving, with occasional trips to the beach (sand) and out west (moab, rockies, etc.). It's not going to be an extreme off road rig, so I'll probably look for a gear ratio that can do both street and trails moderately well.

 

[Casey4wd]

This is really interesting post. I will be considering the same in the near future, but with a 302 EFI, ZF and 33" tires - Uncut, so 33's will be the max.

My use is very similar to yours. Mostly street around town, but I want to be able to drive it back and forth to the deer lease (2 hr drive) and to my parents/in-laws and some easy trails. I was thinking 4:10's.

 

[AZ69EB]

4.10 & 302 - not optimal. Not enough power with stock 302 for the overdrive. 302 needs more rpm's. Min 4.56. Look at a gear calculator. Just my opinion.

 

[Casey4wd]

4.10 & 302 - not optimal. Not enough power with stock 302 for the overdrive. 302 needs more rpm's. Min 4.56. Look at a gear calculator. Just my opinion.

I appreciate the advise. The calculator says for 70 mph in OD:
4.11 gears = 2,240
4.56 gears = 2,663

So, you think the engine will be better off pulling the higher rpms?

 

[sykanr0ng]

4.10 & 302 - not optimal. Not enough power with stock 302 for the overdrive. 302 needs more rpm's. Min 4.56. Look at a gear calculator. Just my opinion.

I appreciate the advise. The calculator says for 70 mph in OD:
4.11 gears = 2,240
4.56 gears = 2,663

So, you think the engine will be better off pulling the higher rpms?

In a word YES

For a 302/5.0L 2600 RPM's is much better than 2200

When I was planning on using 33 inch tires I was going to use 4.56 gears.

5.0L EFI (unless I stroke it), ZF5 D20

Now that I have a set of 315/70R17 tires and the wheels for them the gears will be 4.88

 

[Broncopower96]

Nice. You might want to consider a higher gear than 4.11, especially since the 302 has a little less torque. I'm no expert though!

 

[SHX669]

When you use those gear calculators remember that a 33" tire isn't 33 inches . You should use the actual height of the tire to be more accurate.
I don't remember ; what is the gear ratio of 2nd gear( the one you'll be starting off in most of the time) in the ZF?
I always find these " what gears " discusions interesting . I try not to interject my opinion but there are certainly a lot of guys who like to or have to cruise down the highway at higher rpms than i do.