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Help me understand my gear ratio change

WSM29

Sr. Member
Joined
Mar 29, 2010
Messages
371
So while I am waiting on my topend kit from edelbrock, I went ahead and finished my work bench mock up of my NV 3550. I know the gear ratio is lower in the NV3550 than the standard 3 speed, but how does this effect my over all drive ratio of my 3.50 gears? Will this put me at like a 3.73 or does it work that way? I know I will be happier with the first gear in the NV3550 than I will with the 3 speed, but I wasnt sure how it effected everything else. thanks again in advance!
 

Ranchtruck

Sr. Member
Joined
Feb 7, 2008
Messages
766
Depending on your tire size vs engine torque curve, the overdrive may be useless with 3.50 gears become something like 2.70's in 5th. 4th gear will be the same as 3rd was in the 3spd.
 
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WSM29

WSM29

Sr. Member
Joined
Mar 29, 2010
Messages
371
I am running 33x12.50 Cepek Mud Countrys now... I have room for 35s but no cash.
 

Viperwolf1

Contributor
electron whisperer
Joined
Aug 23, 2007
Messages
24,143
If you were to change the axle gearing to something in the 4.30-4.56 range you would get several benefits. 5th gear would be more usable at highway speeds. 2nd gear would just like 1st was with the old 3 spd and 3.50 gears. That would make the new 1st gear more like a synchronized crawler gear. You could drive normally by starting in 2nd (1st for steep hills/heavy loads) and would still have plenty of overdrive effect in 5th gear. Plenty of gear for future 35's too.
 

Ranchtruck

Sr. Member
Joined
Feb 7, 2008
Messages
766
Here's a handy link: http://www.grimmjeeper.com/gears.html
Put in nv3550 wide ratio, dana 20 2.46 conversion 3.50 gears and your tire size and it tells you crawl ratio, crawl speeds, and highway speeds

At 65mph in 5th, you'll be turning 1,700 rpms, which is too low unless you live in a really flat place.

With your planned 35" tires it get worse at 1,600 rpms at 65. You won't be able to fight a mild headwind at those rpms unless you've got a big block in the bronco that makes all it's torque right off idle.

An overdrive transmission doesn't make sense unless you regear the axles to take advantage of it and match your tire size. The nv3550's 4:1 first only gives you a 35:1 crawl ratio with the gears you have, so you won't have much added low speed capability and you won't be able to use overdrive.

A np435 would make more sense if you're not regearing. It's 6.7:1 first gets you a 58:1 crawl ratio which is reasonable for a mild wheeler and you don't need the overdrive. You start in 2n'd, which has a bit better reduction than the 1st in your 3 speed so getting it moving off the line will be easier. You'll also be turning 2,300 rpms at 65mph (2,200 with the 35" tires), which is a more tolerable cruise rpm.

What it boils down to is you need to either match your transmission to your axle gears, or you need to regear your axles to match the transmission you want to put in.
 

360 4V

Sr. Member
Joined
Feb 3, 2011
Messages
519
You want to be cruising at the lower end of your torque curve, 80% of the rpm at which peak torque occurs.

Do you cruise at 65 mph or more like 75 mph? I am defaulting to 70 mph.

Hypothetical condition:
your overdrive is .75:1
your running 33" tires
your making peak torque at 2,500 rpm
(.8) 2,600 = 2,080 rpm
you would be revving 2,000 rpm at 70 mph

70mph (.75)(3.73:1)(336)
-------------------------------- =1,994 rpm
33"



your making peak torque at 3,900 rpm
(.8) 3,900 rpm= 3,120 rpm

70(.75)(5.89)(336)
----------------------=3,149 rpm
33
 

360 4V

Sr. Member
Joined
Feb 3, 2011
Messages
519
I like the NV 3550. The gear spread is much nicer than the super-wide gear speads of truck gear boxes.
 

Madgyver

Bronco Madman
Joined
Jul 30, 2001
Messages
14,234
3.50 gears and 33"s that you have now and you don't notice it?

proper gear ratio to tire size set-up and you'll feel the difference regardless of what trans you're running..
 
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WSM29

WSM29

Sr. Member
Joined
Mar 29, 2010
Messages
371
I haven't installed the new transmission yet, so I am just thinking ahead. Once I get my parts in for my motor rebuild I will be installing everything at once. I planned on running 4.11s, but this again is why I ask! I am now going to look into getting 4.56s., it may not be until the end of the year once my savings is back up, but its my next big spend for the bronco. Will I be OK to drive the bronco as it is with 3.50 gears, mild 302 and the NV3550 or will it cause issues? This is exactly the info I was looking for. Thanks!
 

Ranchtruck

Sr. Member
Joined
Feb 7, 2008
Messages
766
Yeah, you can drive it, just don't expect to ever use 5th until you regear the axles.
 

Viperwolf1

Contributor
electron whisperer
Joined
Aug 23, 2007
Messages
24,143
I haven't installed the new transmission yet, so I am just thinking ahead. Once I get my parts in for my motor rebuild I will be installing everything at once. I planned on running 4.11s, but this again is why I ask! I am now going to look into getting 4.56s., it may not be until the end of the year once my savings is back up, but its my next big spend for the bronco. Will I be OK to drive the bronco as it is with 3.50 gears, mild 302 and the NV3550 or will it cause issues? This is exactly the info I was looking for. Thanks!

It'll be ok. Just have to start in 1st and might not get to use 5th much.
 
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