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Axle Swap and Gearing Question


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20 minutes ago, Eagle said:

 

Those numbers aren't too far off (for an automatic), but they're off. But the OP has a 5-speed, so stock axle gearing is 3.08, not 3.54.

 

The problem with every on-line gear ratio/speed calculator I've seen is that they use the nominal diameter of the tire. In reality, 31" tires are typically about 30.5" in diameter, 32s are typically about 31.5", and 33s are typically about 32.5". Then you put weight on them and the sidewalls flex, so the actual rolling radius is less than half the diameter of the tire. I made up my chart to allow for this, as much as possible. Wherever I could find data, I used a manufacturer's actual published rolling diameter or published revolutions-per-mile (the best number) for doing my calculations.

 

For the OP, 60 MPH with stock tires and stock gearing is going to be just 1676 RPM in 5th gear. Jumping to 33" tires with 4.10 gears, 60 MPH will be 1974 RPM. So the overall gearing will be a slight improvement, but only a tiny bit better than stock tires with 3.54 gears. That's not bad at all for general driving with some mild to moderate wheeling. I was able to do a lot of trails at Paragon in my '88 Cherokee on 30" tires with the stock 3.08 gears.

That calculator allows you to put in the manufacturers revolutions per mile number from their specs.  If you use that instead of the advertised outside diameter the results tend to be pretty dead on.

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   There are drop down to select transmission, TC, etc. I think also metric tires. I would have though a manual trans would want the same RPM as the auto 4.0L. Same motor. I do think the manual trans up to the 2010 gave 'better' power, than the autos though.  A stock MJ/XJ is fairly capable. I would also wonder if the tires grow a bit in diameter at highway speeds. I reference drag cars. 

    The other aspects the OP needs to consider is Noise at Highway speeds and MPG. I think 4.10's and 32-33's would be about stock ratio. Other aspects as C(6 ply) vs E (10 ply) and width or overall weight  would come into play. I think maybe 1 mpg possibly more with bigger tires. I am getting between 17.5 and 18.5 on my 2000 XJ lifted 2", 245-75-16's Auto and 3.73's. I'm on E rated tires. Also, if the OP looks at Transfer case gearing to 4:1. Might help wheeling. 

    So, I think with your 33's and 4.10's we are on the same page with a manual trans. 

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I filled in some data on that spread sheet. I used AX-15, 231 TC, 32's.  @55-1871 rpms. @65- 2211 RPMs. @ 70 - 2381 RPMs. A bit higher than say stock, but not much. The OP could select gears, say 4th and cruise at those RMPs. See what noise factor of NVH is. 

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1 hour ago, Pete M said:

stock auto 2.5s came with 4.56s and they had tiny tires.   :D   don't fear the rpms.  embrace the power.  :comanche: 

I see the rear for that Ratio only for the Dana 35. The front seems hard to find. 

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If I was to regear a ford 8.8 to 4.56, would I have to look for gears specific to an explorer 8.8? I know ford 8.8s are in everything and spline count isn't the same across all the vehicles they use.

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3 hours ago, ktmall07 said:

If I was to regear a ford 8.8 to 4.56, would I have to look for gears specific to an explorer 8.8? I know ford 8.8s are in everything and spline count isn't the same across all the vehicles they use.

I just got done regearing a Explorer 8.8 with 4.56 gears if you have any questions. 

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