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4.56 Gears and 32 Inch Tires

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willfish4money

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Discussion starter · #1 ·
I would like to install 4.56 gears to get back into the correct RPM range for my engine. Will the 4.56 gear be to low for 32" tires? I am presently running a 265/75R16 tire (31.65") and 3.73 gears. I compared my actual driving RPM's with the gear calculator for the set up I have now and it is just about dead on at 55 and 70 MPH, not sure what happened at 45 MPH. According the gear calculator using the 4.56 gear at 70 MPH the RPM's should put me back to stock, within reason. I use my Jeep more on the road and towing my 1700 lbs bass boat and need the 4 wheel drive for the sand here is Florida.

Anyone else running this set up that could give me their actual RPM readings so I would know if my calculations are correct would be appreciated.

I have a 2003 Jeep TJ with 4.0 6 cyl., a 4 speed automatic transmission with overdrive, and NP-231 transfer case. I have Dana 30 up front and AMC Model 35 in the back. I have an OME 2.5 inch suspension lift installed. I have also changed my speedometer gear so I am about 2-3 MPH slower than what the speedometer reads.

Have I overlooked anything?

Actual Driving Results / Road Test
265/75R16 (31.65")
Gears 3.73
45 MPH = 1600-1750 RPM
55 MPH = 1500-1600 RPM
70 MPH = 1900 RPM

Gear Calculator Gear Ratio Calculator
Tires on Jeep now 265/75R16 (31.65")
Gears 3.73
45 MPH = 1230 RPM
55 MPH = 1503 RPM
70 MPH = 1913 RPM

Gear Calculator Gear Ratio Calculator
265/75R16 (31.65")
Gear 4.56
45 MPH = 1503 RPM
55 MPH = 1837 RPM
70 MPH = 2338 RPM

Gear Calculator Gear Ratio Calculator
Stock Tires 225/75R15 (28.29")
Gears 3.73
45 MPH = 1376 RPM
55 MPH = 1681 RPM
70 MPH = 2140 RPM
 
#2 ·
With the 4-speed auto, I'd seriously consider 4.88's over 4.56. The 42RLE is a different animal with it's 0.69 overdrive. It loves low gearing. Especially if you plan to tow that boat.

A re-gear is more than just RPM matching back to stock. The larger tires and wheels add rolling resistance and unsprung weight. To fit those larger tires, we add suspension/body lift that places our vehicle higher into the windstream increasing drag. We add armor and accessories that increase the overall vehicle weight. All these factors can be mitigated somewhat by placing the engine into a higher, more efficient RPM range.
 
#5 ·
4.88's on 32's.:rolleyes:You guys are nuts.The only difference is OD and who uses that?I never used it with 30's.
 
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Discussion starter · #7 ·
2502 RPM's is what the calculator comes up with for 4.88 gears at 70 MPH and 265/75R16 tires.
From everything I have read 4.56 is the perfect set up for 33" tires and 4.88 is for 35" tires.
I just don't want my engine RPM's to high when I am running 70 MPH.
 
#9 ·
willfish4money said:
2502 RPM's is what the calculator comes up with for 4.88 gears at 70 MPH and 265/75R16 tires.
From everything I have read 4.56 is the perfect set up for 33" tires and 4.88 is for 35" tires.
I just don't want my engine RPM's to high when I am running 70 MPH.
Click to expand...
Those recommendations are for the manual trans, not the 4-speed auto. From our experience of running this trans for 4 years, the 2500 RPM's at 70 is perfect.
 
#10 ·
willfish4money said:
2502 RPM's is what the calculator comes up with for 4.88 gears at 70 MPH and 265/75R16 tires.
From everything I have read 4.56 is the perfect set up for 33" tires and 4.88 is for 35" tires.
I just don't want my engine RPM's to high when I am running 70 MPH.
Click to expand...
You need to take what transmission is being used into consideration. For any of the manual transmissions with the 4.0 (NV3550, NSG370, AX-15), you're right. For the 42rle auto, the OD is so tall that one must gear a step lower than normally accepted in order to not lug the engine at highway speeds while in OD. For the 3 speed auto, the opposite is needed, ie one step higher than what's normally accepted. So:

If you have:
4.0/NV3550/33" tires--4.56
4.0/42rle/33" tires--4.88
4.0/32rh/33" tires--4.10

For 35's and the 42rle, ideally you'll want 5.13's but you're limited to 4.88 with the stock axles, unless you have a Rubi. 4.88's will suffice nicely, though.

Lastly, people need to stop fearing RPM's. I see the line "I don't want to be running the RPM's too high when on the highway." You guys are used to big trucks with V8's that don't require higher rpm's (engines make their power down low) or Hondas that are aerodynamic and have tiny tires. The 4.0 makes the torque down low but runs best in the mid RPM's (2500-3000), especially when pushing big tires. Those extra couple hundred RPM's allow the engine to run without lugging. Don't fear RPM's, fear ENGINE LOAD. That's what lowers the mileage.
 
#11 ·
imped4now said:
You need to take what transmission is being used into consideration. For any of the manual transmissions with the 4.0 (NV3550, NSG370, AX-15), you're right. For the 42rle auto, the OD is so tall that one must gear a step lower than normally accepted in order to not lug the engine at highway speeds while in OD. For the 3 speed auto, the opposite is needed, ie one step higher than what's normally accepted. So:

If you have:
4.0/NV3550/33" tires--4.56
4.0/42rle/33" tires--4.88
4.0/32rh/33" tires--4.10

For 35's and the 42rle, ideally you'll want 5.13's but you're limited to 4.88 with the stock axles, unless you have a Rubi. 4.88's will suffice nicely, though.

Lastly, people need to stop fearing RPM's. I see the line "I don't want to be running the RPM's too high when on the highway." You guys are used to big trucks with V8's that don't require higher rpm's (engines make their power down low) or Hondas that are aerodynamic and have tiny tires. The 4.0 makes the torque down low but runs best in the mid RPM's (2500-3000), especially when pushing big tires. Those extra couple hundred RPM's allow the engine to run without lugging. Don't fear RPM's, fear ENGINE LOAD. That's what lowers the mileage.
Click to expand...
Couldn't have said it any better! :thumbsup:

For this engine and trans, 2500 is just at the lower end of the usefull RPM band. You'll still have plenty of room for passing and acceleration. Heck, I'd even be happy with 2500 RPM's at 65.

As so aptly said above, we'd be geared lower if not for the 4.88 limit of the D30.
 
#19 ·
I use my OD all the time with my 5 spd and 31" tires. It settles in around 2000 rpm at highway speeds with a lower OD. I always wish it was turning around 2400-2500 at 75mph/115kph. It just seems the right spot for good mileage and power. Hills would never be an issue and no need to bounce in and out of OD.

Besides with 4.88'''s your crawl speed would be noticeably improved.
 
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