Gears, RPM's, Tire sizes and Trans

beernut · Aug 11, 2011

It is time for me to start looking for a new set of tires. I'm told I have a set of 4.11's on each end w/ Detroit lockers but I have not taken the pumpkins apart to check (They aren't leaking and I'd like to keep it that way). Currently running 33x12.50-15 and would like to keep this same size and they are being pushed by a ford 3 speed / 302. As far as I know it is a stock 3 speed.

There isn't any reason for me to believe that the tach is wrong but I'm running about 4200 RPM at 60mph. With this gear size and tire size does this sound right? I'm looking at moving to a 4 or 5 speed, leaning towards the NV3550.

Would the NV3550 be fine with the 4.11's and 33's?

Revelation · Aug 11, 2011

Here you go

kayakersteve · Aug 11, 2011

With my 302 into a C4 with 4.56 and 32 " tires, I run around 3200 at 60. Yours seems high to me.

Revelation · Aug 11, 2011

Either Tach is way off OR your driving around in low range.

70_Steve · Aug 11, 2011

beernut said:
There isn't any reason for me to believe that the tach is wrong but I'm running about 4200 RPM at 60mph. With this gear size and tire size does this sound right? Would the NV3550 be fine with the 4.11's and 33's?

I believe your tach is incorrect. Most likely set up for a 6 cyl instead of a 8 cyl.

If you've got Detroits, it should be easy enough to count the revolutions of the drive shaft for one revolution of the rear tire, to verify the differential ratio.

Here's a great calculator, to help decide on a transmission.
http://www.grimmjeeper.com/gears.html

Viperwolf1 · Aug 11, 2011

beernut said:
It is time for me to start looking for a new set of tires. I'm told I have a set of 4.11's on each end w/ Detroit lockers but I have not taken the pumpkins apart to check (They aren't leaking and I'd like to keep it that way). Currently running 33x12.50-15 and would like to keep this same size and they are being pushed by a ford 3 speed / 302. As far as I know it is a stock 3 speed.

There isn't any reason for me to believe that the tach is wrong but I'm running about 4200 RPM at 60mph. With this gear size and tire size does this sound right? I'm looking at moving to a 4 or 5 speed, leaning towards the NV3550.

Would the NV3550 be fine with the 4.11's and 33's?

With your tires/gears you should be at 2500 rpm @ 60 mph. Could be tach is setup for 6 cyl and speedo is wrong. 6 cyl tach will read 33% higher rpm.

Greg_B · Aug 11, 2011

kayakersteve said:
With my 302 into a C4 with 4.56 and 32 " tires, I run around 3200 at 60. Yours seems high to me.

Wow that is in interesting setup... I ran 32s with 3:50 gears for years... I bet 32s with 4:56 has some serious low end... I now have 35s with 4:56 gears which is really about like the stock gearing.

Greg

beernut · Aug 11, 2011

Yea my first thought was 4wd low but I have verified that I am in 2wd hi. I will check to see how the tach is configured. Believe I saw the manual in a box.

-bn

73stallion · Aug 11, 2011

with NV3550 33's and 4.11's i'm turning about 2150 at 60 mph.

rob wilson · Aug 11, 2011

Ignore the "HeeP" in on this page and plug in your numbers. Pretty handy.

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

Dirk · Aug 11, 2011

Revelation said:
Here you go

Assuming transmission is 1:1, like 4th gear. (damn mobile app didn't quote the pic)

The gear calculator on is damn nice, bookmarked.

fetorino · Aug 11, 2011

Here is my favorite on line tool

pretty handy and it doesn't say heep on the page.

http://web.archive.org/web/20050204061805/home.comcast.net/~jcmiley/bronco+gears.html

according to it with a stock 3 speed and 4.10 with 33s 4200 rpm will get you going 100 mph.

rob wilson · Aug 11, 2011

fetorino said:
pretty handy and it doesn't say heep on the page.

http://web.archive.org/web/20050204061805/home.comcast.net/~jcmiley/bronco+gears.html

according to it with a stock 3 speed and 4.10 with 33s 4200 rpm will get you going 100 mph.

Agree - Miley was pretty active here and w Broncos several yrs ago.

blubuckaroo · Aug 12, 2011

If you use these ratio charts and calculators, you need to have the dynamic diameter of the tire. To get the dynamic diameter you need to measure the radius to the ground with the vehicle weight on the tire and multiply that X2. Mine were 1.5" smaller than I thought they were. On my car, the RPM was increased by about 100.

kayakersteve · Aug 12, 2011

@gregB - Yes, mine has some git up & go with that gearing and tire combo - The tires like to squawk when I get on it. May go to 33's at next tire change to bring RPM down a little though.

dbevans2249 · Aug 12, 2011

I had a NV3550 with 33's and 4.11's but was not happy. I went to 4.56's and am having second thoughts about my choice. I have not driven it yet but think I may have been happier with 4.88's. If that wound up a little low 35's sure look nice. Just have to wait and see when it is finished.

BUCKNBRONK · Aug 12, 2011

4.88 was my choice with a AOD and 35's could have gone a little lower with the gears. With the od in, its doing 65mph at about 2100rpms. 5.29's would have been better when I go to 37's.

Gears, RPM's, Tire sizes and Trans

beernut

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Revelation

Bronco Guru

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kayakersteve

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Revelation

Bronco Guru

70_Steve

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Viperwolf1

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Greg_B

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beernut

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73stallion

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rob wilson

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Dirk

fetorino

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rob wilson

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blubuckaroo

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kayakersteve

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dbevans2249

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BUCKNBRONK

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