AX-15 QUESTIONS about gearing and tire size ,drivability

[danny]

Hello Bronco folks,

I have a 68 Bronco, owned for 20 years and had two others before this one. 289 mildly built with 1970 351 heads , RV cam ,roller rockers, .060 over pistons ,runs very good, stock 3 speed RAT transmission , stock 20 Transfer case , stock 4.11 gears and 35" tires/ lifted .I do not four wheel much any more as all the land is now sitting with houses everywhere. I mostly use it for driving in town and pulling boats around the yard. I am wanting a more modern transmission . The AX-15 has my interest but I am concerned about drivability of the different gearing . Does anyone have experience with this or a similar set up ? I do not want to change my 4.11differential gears .

 

[904Bronco]

I had a 67 with Injected Explorer motor, the NV3550 (pretty much the same as the AX), 4:11's and 33" I found it to be a great trans for everything I did.
65-70 mph was my comfortable freeway speed and it had good power for lugging, 1st gear, well I tended to start off in 2nd as being in first didn't last too long... I am sure others with the taller tires will chime in, I'm guessing others might suggest 4:56 with the 35's...

 

[1969miller]

The AX15 is one of the best things you can do to modernize the feel of your bronco. The 4.11 gears you have will be just fine for what you do with your bronco. You will love the overdrive and be able to do freeway speeds.

 

[DirtDonk]

If you are fine with your current set up, you should be 90% fine with the new transmission.
The first gear in the AX 15 is lower than your three speed, so moving things around the yard, and getting rolling on the street is going to be better even without going into low range in the transfer case.
The only question is how strong is your motor at lower RPM, which will dictate whether you get into the fifth speed overdrive very often or not.
But since it’s a manual transmission, and you get to choose the gear you’re in, I don’t see any single downside to the transmission swap at this point.

 

[ssray]

Plugged in the ratios and your tire size using a spreadsheet I’ve created and here‘s the speeds in different gears and rpm’s.

I have a Rubicon with 35’s, 4.11 and the 3550 (which is close), but it just has the 4.0L. Decent torque but really needs more HP in overdrive. It’s fine on flatter roads without wind. Fighting wind or hills that are past mild and 4th gear is needed. You may be fine with that V8 HP.

600	3.9	6.4	10.4	15.0	19.0
1000	6.5	10.7	17.4	25.0	31.7
1500	9.8	16.1	26.1	37.5	47.5
2000	13.1	21.5	34.8	50.1	63.4
2500	16.3	26.9	43.5	62.6	79.2
3000	19.6	32.2	52.2	75.1	95.1
3500	22.9	37.6	60.8	87.6	110.9
5000	32.7	53.7	86.9	125.2	158.4

 

[jrussells]

I’ve got the same set up with you minus the 35” tires. I’m running 31s.

From what I’ve driven since doing the swap, mine seems like the revs are high trying to do highway speeds. Like 2500rpm to pull 65/70, but what I’ve read says the 302 can stand the revs. I don’t really have to downshift on hills either with my setup either.

I think everybody else is right, you’ll be fine running low speeds but you might have to downshift on hills or under heavy loads in 5th.

Either way it’ll be an improvement over the 3 speed

 

[76BroncoPete]

Any glaring differences between the 3550 and ax15 or even a minor one that seems substantial? Shifter location basically in the same location? Crossmember issues? Interested in this swap as well.

 

[danny]

Any glaring differences between the 3550 and ax15 or even a minor one that seems substantial? Shifter location basically in the same location? Crossmember issues? Interested in this swap as we

Any glaring differences between the 3550 and ax15 or even a minor one that seems substantial? Shifter location basically in the same location? Crossmember issues? Interested in this swap as well.

I Just ordered hopefully everything to complete the swap from Wild Horses. I have a lot of work to do before I’m ready to reassemble.I have 20 years on the frame off body restoration I did and the ole 3 speed has been a champ . Cleaning, a small rust hole in floor I’ll have to weld repair and paint work. The exhaust is looking like it’s 20 years old too . I have to give a huge thumbs up to Hooker headers . I put on headers and their full exhaust system and still not a leak or holes . Prolly time to replace while I’m in there and not look back for another 20 years!

 

[danny]

Plugged in the ratios and your tire size using a spreadsheet I’d created and here‘s the speeds in different gears and rpm’s.

I have a Rubicon with 35’s, 4.11 and the 3550 (which is close) except for the 4.0L. Decent torque but really needs more HP in overdrive. It’s fine on flatter roads without wind. Fighting wind or hills that are past mild and 4th gear is needed. You may be fine with that V8 HP.

600	3.9	6.4	10.4	15.0	19.0
1000	6.5	10.7	17.4	25.0	31.7
1500	9.8	16.1	26.1	37.5	47.5
2000	13.1	21.5	34.8	50.1	63.4
2500	16.3	26.9	43.5	62.6	79.2
3000	19.6	32.2	52.2	75.1	95.1
3500	22.9	37.6	60.8	87.6	110.9
5000	32.7	53.7	86.9	125.2	158.4

This is interesting, is this with the AX gears or 3550 ?

 

[ssray]

This is interesting, is this with the AX gears or 3550 ?

This is with the AX. The 3550 1st is 4.01 vs 3.83. That’s the biggest difference. 3rd is .04 different and overdrive is just .01. I also used 34.6 as most tires are undersized. If they stated rolling radius I would use that but you don’t find it listed very often so I didn’t set it up that way

 

[ssray]

I’ve got the same set up with you minus the 35” tires. I’m running 31s.

From what I’ve driven since doing the swap, mine seems like the revs are high trying to do highway speeds. Like 2500rpm to pull 65/70, but what I’ve read says the 302 can stand the revs. I don’t really have to downshift on hills either with my setup either.

Here‘s what the spreadsheet shows for yours using 30.6 for tire size. You can check sometime and see how close it comes. You’ll only be able to hit 140 in 5th at 5k rpm. compared to Danny’s 158.

600	3.5	5.7	9.2	13.3	16.8
1000	5.8	9.5	15.4	22.1	28.0
1500	8.7	14.3	23.1	33.2	42.0
2000	11.6	19.0	30.7	44.3	56.0
2500	14.5	23.8	38.4	55.3	70.1
3000	17.3	28.5	46.1	66.4	84.1
3500	20.2	33.3	53.8	77.5	98.1
5000	28.9	47.5	76.9	110.7	140.1

 

[1969]

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

Here is a great calculator that you can play with.

 

[ssray]

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

Here is a great calculator that you can play with.

Pretty amazing job putting that together. Wonder where he got the specs for all those transmissions? Even Eaton Fuller 10 sp truck transmissions and looks like every version of any transmission ever made. It would take hours and hours of research to find all that unless there’s a database available somewhere.

 

[BOBS 2 68S]

AX-15 here and highly recommend it. Have not noticed a big MPG with it. I just like the extra gears for my use.
302, 4:11's, 31 inch tires

 

[stout22]

I have a 70 Bronco with 250 6cy, 4.11 gears, 32" tires and NV3550. The 5 speed gives the Bronco a much more sporty feel.
As stated previously I sometimes use 2nd gear to start from.
Next set of tires will be 33s

 

[danny]

Thanks for all the replies folks! I am in process of Cleaning up the underside of body while waiting on the Transmission kit to arrive. Do any of you AX-15 drivers know what exhaust system works best with this setup? I do have duffs long arms up with 2" body lift and 3.5 springs.

 

[BOBS 2 68S]

I am running stock manifolds and duel exhaust

 

[661buster1963]

I would think with 35’s you would want 4.56 or 4.88 to be able to use OD, unless you have gobs of torque around 2800 rpm

But same could be said for Ax15 or the stock 3 speed.

I have 351w with about 390 ft# tq, at as low rpm as cam and iron heads can get it. I run 4.88 now but had 4.11 for awhile. I COULD use OD with 4.11, but liked it better with 4.88

You can tell everything is spinning quickly with 4.88, but I don’t like to drive it over 65, maybe 70.

Not that it matters but I use steers PS and teamed pitman arm for stock
Linkage

If your three is still on a tree that’s rocks solid theft deterrent right there