Transmission Regearing when changing to bigger tires ???

DjSoundScan

DjSoundScan

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Montreal, QC, Canada
I have a M/T Nv3550. whats the gear ratio on that?
what would happen when i put the 33"s on? will i be reving higher?
Let us compare:
1500rpm in 5th gear going 50mph with 235/75r15;
what will happen when installed 33x12.50r15?
what will I need to do?
 
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First, let me start by saying don't drive your Jeep in 5th at 50 miles an hour. 1500 RPM is lugging the crap out of it, personally with 3.07 gears on stock tires, I wouldn't ever use 5th gear. 4th gear on stock tires will keep it right where it needs to be.

The overdrive on the NV3550 is 0.78, which means that your engine turns 0.78 times for each time that the driveshaft turns. So, your driveshaft is turning 1,923 RPM when the engine is turning 1500 in 5th.

Second, 33's are going to ruin performance compared to your current tires. You want to regear to 4.56 for that NV3550.

If you install 33's, that 1500 rpm will become 1300 rpm or so, making it even worse for the engine. Learn to drive in 3rd and 4th at lower speeds and regear your Jeep to 4.56 when the time comes to regear. Still, with 4.56, I wouldn't put it in 5th until 60-65 or so.
 
I second the suggestion for 4.56 gears. That would be the ideal ratio given all of the variables.
 
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www.grimmjeeper.com/gears.html

This will give you the gear ratio of your trans as well as rpm's with X tire and X gear ratio. 4.56 would work well as would 4.88 if it is more for off road than on. Remember, get the actual height measurement of your tires to obtain the most accurate numbers.
 
DjSoundScan said:
I have a M/T Nv3550. whats the gear ratio on that?
what would happen when i put the 33"s on? will i be reving higher?
Let us compare:
1500rpm in 5th gear going 50mph with 235/75r15;
what will happen when installed 33x12.50r15?
what will I need to do?
Click to expand...
Going with bigger tires lowers the engine RPM at the same speed so it can't develop as much power. That means you'll have to downshift more to maintain speed.

For 33" tires, you'd want to regear the axles to 4.56 to bring the engine rpms back up to where they need to be, close to where they were with the smaller tires. Also, you don't regear the transmission, it is the axles that get regeared for bigger tires.
 
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I saw this post and it got me thinking about my gearing. I bought the Jeep with 33’s already on it (no idea what was done by PO). I checked the speedo versus a GPS and it is spot on so obviously the speedo gear was changed. The shift light on the dash is always bugging me to shift way before it's time so that is obviously set for the original factory tires. Is there a way to find out if the diffs have been re geared without pulling the housings? At 45 in 4th I am turning about 1800 rpms and if I shift to 5th it drops to 1500 with adequate power. Just wondering.


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A typical 33" tire measures out to around 31-32" height. I ran a 31.5" tire in the chart and came up with 3.73.

The shift light is set for like 2000 rpm. Definitely don't ever listen to it, it was useless in stock form coming off the factory floor.

Sounds like you have stock 3.73 gears and the PO just happened to change the speedo gear for 33's.

You could pull the speedo gear and look at the tooth count on it and based on a chart, we could figure out the ratio that way too. Only takes 5 minutes.

What rear axle do you have?
 
You definitely still have 3.73 gearing. The chart lines up perfectly with your rpm numbers. If you ever plan on regearing, go with 4.56 unless you have plans for 35's in which case you should go with 4.88.
 
It hasn't been a problem, I was just curious. I can shift into 5th at 45 or better and it seems to be happy and doesn't bog down like a previous comment had said. I have driven a clutch almost all my life so I ignored the shift light already. I thought since maybe it wasn't lugging in 5th maybe the gearing had been changed. Thanks for the info. Can you post a link for the chart you referred to earlier?


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Bird Dog said:
It hasn't been a problem, I was just curious. I can shift into 5th at 45 or better and it seems to be happy and doesn't bog down like a previous comment had said. I have driven a clutch almost all my life so I ignored the shift light already. I thought since maybe it wasn't lugging in 5th maybe the gearing had been changed. Thanks for the info. Can you post a link for the chart you referred to earlier?
Click to expand...
If you regeared from your present 3.73 to 4.56 with your 33" tires, you'd slap yourself for not doing it sooner. Having the right gearing is better in all ways... less strain on the drivetrain, clutch, and engine. And you wouldn't have to be constantly downshifting for even minor grades. And my bet IS that your engine is lugging in 5th gear. The 4.0L can't loaf at low RPMs like a V8 can in a lighter car.
 
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Yeah...you'd wonder how you ever got around on 3.73. It really is that big of a difference.

As for how I determined that you have 3.73 from guessing, I used grimmjeeper.com and used his RPM calculators. Put in the transmission, and 31.5" for the tire size and then entered 3.73 to see if the numbers lined up. They did, so I was pretty sure you have 3.73. Then you told me your Jeep came with 3.73 so I know for sure that's what you have.

Back to regearing, you may think it's fine how it is but once you experience a better setup, you'll wonder why you spent money on every other modification when you could have bought gears instead. That's how it went for me, anyways.
 
Brantley said:
you may think it's fine how it is but once you experience a better setup, you'll wonder why you spent money on every other modification when you could have bought gears instead.
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I used to think my mother's overcooked hamburger was good until I tasted my first medium-rare Ribeye steak. If you've never experienced anything better, then what you have is acceptable. :D
 
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Yep, that's what I realized when I regeared. I drove 31's on 3.55 for 2.5 years without knowing any better. For the first year I didn't even know what regearing was or did. The other year and a half I just chose to spend money on other stuff. At the end I finally got tired of it and regeared and wondered why I waited so long to pull the trigger on it.

Brings me to another point. I've been looking at TJs for sale lately and it bewilders me the prices on some of them. I found a 2004 4.0 5-speed with Smittybilt bumpers (I HATE Smittybilt), 35x12.5R17 on a Pro Comp lift and a bunch of other crap. It's sitting on a Dana 35 with 3.07 gears. Asking price? $16,500. That's the definition of insanity if you ask me. I can't imagine how poorly a test drive would feel with those 3.07 gears and 35's. Geez.
 
I know lugging. My last vehicle was an F150 5 speed Quad cab with the 4.2 V6! That thing used to lug up the hills. I guess this TJ feels so much better by comparison since it doesn't seem to strain except on really steep grades. I may step down to 31’s with my next set of tires since I don't really do any serious wheeling currently except traversing my hunting trails. My son says it maneuvers like a bumper car!


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Jerry Bransford said:
Going with bigger tires lowers the engine RPM at the same speed so it can't develop as much power. That means you'll have to downshift more to maintain speed.

For 33" tires, you'd want to regear the axles to 4.56 to bring the engine rpms back up to where they need to be, close to where they were with the smaller tires. Also, you don't regear the transmission, it is the axles that get regeared for bigger tires.
Click to expand...

So i would have to regear both front and rear to 4.56?
Front being a Dana 30/186mm
Rear being a Dana 35/194mm
 
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