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Thread: Predator and Tires size... do not leave it stock

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    Predator and Tires size... do not leave it stock

    When you buy a new predator you may think you only need to install your tune and can leave everything else stock. I did. But not so. After riding around for a while I was wondering whey my speedometer seemed to be off. Well here is what happens. The Predator is used on both the Harley and Lightning and the Lightning comes with 18" rims (295/45/18). The revs per mile are thus set from the factory at 703. Now on our trucks with the stock setup you need to be at 678 revs per mile. What this means is when your speedometer is reading 60mph you are really traveling at 62.7mph.

    Just go into axle ratio then revs per mile and make the adjustment.

    the two most common sizes for our trucks are

    275/45/20 - stock size - use 678 revs per mile

    295/45/20 - common upgrade ( I have these ) use 662 revs per mile.

    Thought this might help some like me that was not aware that stock setting means stock Lightning not Harley Davidson.
    Last edited by fastmike; 07-30-2009 at 01:57 PM.

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    Did the same for mine to compensate for the 24's.

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    how do you get to axle ratio? and how do you figure revs? I have 275/55/20

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    Any tire calculator, like 1010TIRES.COMŽ — Tires and Wheels Absolute Authority.

    Type in your tire size, and it will show rev's per mile.

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    Quote Originally Posted by harleyhank View Post
    how do you get to axle ratio? and how do you figure revs? I have 275/55/20
    After you select the tune you want to install it will ask you if you want to modify parameters. you select that you want to modify parameters and the first one is axle ratio. when you select axle ratio, you will then get the option for tire size. This is where you input the revs per mile. Your Predator calibrates your speedometer based on the axle ratio and the revs per mile. Axle ratio should be 3.73 which I think is the stock ratio.

    With 275/55/20 you should be at 632 revs per mile.

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    Cool

    Thanks for the info..Now can I change this myself or do I need to let my tuner do it..In other words can I change a setting without involving my custom tune??

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    Read the post above you again, then go plug your hand held in.. it will be 5 minutes worth of work for you.

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    Cool

    Thanks Krix. that was easy enough lol

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    lol I just did mine a couple weeks ago. I finally swapped to a predator, and my tune was setup at JL's for my factory sized 20's. I filled up for the first time after doing the fuel cell and calculated 9 mpg I almost chit LoL! I always get 12-15 mpg in the truck. Then DOH it dawned on me I never updated the predator for the much larger 305/35/22's when I added the 22" reps so my mpg was all screwed up from the speedo being off LoL!

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    Cool

    Now I will be able to really check my mileage.Not that it matters,just more curious than anything.So what do you guys get...in town,on the road..overall average ???

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    Quote Originally Posted by harleyhank View Post
    Now I will be able to really check my mileage.Not that it matters,just more curious than anything.So what do you guys get...in town,on the road..overall average ???
    I get about 11 mpg combined. Pretty crappy...

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    I was doing some reading about the General Grabbers UHP and their website says that my size (295/45R20) has 685 revs per mile. that is quite a bit different than 662 revs per mile.

    So I need to also figure axle size to get the correct revs per mile?
    Yes officer... I am aware of how fast I was going!

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    Quote Originally Posted by steinjr View Post
    I was doing some reading about the General Grabbers UHP and their website says that my size (295/45R20) has 685 revs per mile. that is quite a bit different than 662 revs per mile.

    So I need to also figure axle size to get the correct revs per mile?
    What is the tire height in inches for that tire on their website? If you take this formula you can also get tire revs per mile. All are a little bit different. I would guess the tire manufacturer data would be the most accurate.

    20168 / tire height in inches = revs per mile
    Last edited by fastmike; 07-31-2009 at 09:39 AM.

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    very interesting... I went to the Hankook site and they say my tires the 295/45/20 are 681 revs per mile. I guess Ill change it im my predator to what the manufacturer says. Maybe the tire calculators are off a little. In fact I looked at 3 calcs online and two say 662 revs per mile for this size and one says 683.



    So I would say the best advice is to go to the tire manufacturers site and get the number from them. Otherwise its an approximate number!
    Last edited by fastmike; 07-31-2009 at 11:02 AM.

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    Quote Originally Posted by fastmike View Post
    very interesting... I went to the Hankook site and they say my tires the 295/45/20 are 681 revs per mile. I guess Ill change it im my predator to what the manufacturer says. Maybe the tire calculators are off a little. In fact I looked at 3 calcs online and two say 662 revs per mile for this size and one says 683.



    So I would say the best advice is to go to the tire manufacturers site and get the number from them. Otherwise its an approximate number!
    With your math, you are assuming 100% traction which is impossible. Things to keep in mind:

    1. When you first start moving, your drive wheels can slip ever so slightly before gaining traction. This is a miniscule amount, but over the life of a tire it makes a difference.
    2. When you turn a corner, tires "scrub" and do not rotate with 100% traction on the street. This effects the overall revolutions per mile.
    3. Tire pressure effects tire RPM. As outside temperatures change (and even just driving from cold tires to warm tires) the RPM of the tire changes. Again, this is a very small amount but makes a difference over the life of a tire.
    4. Tires in different positions on the vehicle will rotate different amounts based on turning, etc.

    The tire industry has an equation to calculate tire RPM which takes into account these and other factors. This is how our tire size calculator is set up and why it is not coming up with the numbers that your math is.


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