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Speedo/milage correction for new tires

Old 01-17-2010, 10:08 AM
  #11  
01 P-51G t
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Originally Posted by cliffyk View Post
245mm / 25.4 = 9.64in; * 45% = 4.34in; * 2 = 8.68in total sidewall;

8.68in + 17in = 25.68in overall diameter;

The working diameter, because of loading, will be about 97% of this. 24.9in or so.

For the OP, here's my How Fast Can I Go in Each Gear? calculator, you can select you tranny, rear-end gears, and tire size and see what's what...
Thats an awesome calculator cliffy

Now can you make another one, that has standard tire sizes and a drop down menu for PSI? So we can use it for the track for guys running slicks.

And while your in there, another RPM colum for gt and cobra redlines.
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Old 01-17-2010, 11:29 AM
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Originally Posted by 01 P-51G t View Post
Thats an awesome calculator cliffy

Now can you make another one, that has standard tire sizes and a drop down menu for PSI? So we can use it for the track for guys running slicks.
There are too many other factors, other than inflation pressure, that would impact effective tire diameter, making calculation of same from tire size and PSI a very inexact exercise.

You can however do a forced entry of the tire diameter, just click on the displayed value and you can enter whatever you like.

And while your in there, another RPM colum for gt and cobra redlines.
I added two columns, took it out to 8 grand...
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Old 01-17-2010, 12:06 PM
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Originally Posted by cliffyk View Post
245mm / 25.4 = 9.64in; * 45% = 4.34in; * 2 = 8.68in total sidewall;

8.68in + 17in = 25.68in overall diameter;

The working diameter, because of loading, will be about 97% of this. 24.9in or so.

For the OP, here's my How Fast Can I Go in Each Gear? calculator, you can select you tranny, rear-end gears, and tire size and see what's what...
Oops.. I miswrote, I Meant to say 25.7"

Jazzer now looks foolish

PS. Good calcs though
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Old 01-17-2010, 12:10 PM
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So if you dont change it, will your odometer be off? by a bit?
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Old 01-17-2010, 01:06 PM
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[deleted]...

Last edited by cliffyk; 01-17-2010 at 01:11 PM. Reason: double post
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Old 01-17-2010, 01:09 PM
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Originally Posted by budders View Post
So if you dont change it, will your odometer be off? by a bit?

Both the odometer and speedometer will be affected by running tires that have a different diameter than the stock tires (245/45-17, 25.7" dia unloaded, 24.9" dia loaded).

The amount of the error will be roughly 0.4% for each 0.1" difference in diameter, smaller tires will make the speedometer read fast (as compared to actual speed), larger tires will make it read slower than real speed.

The odometer is similarly affected, smaller tires will make it rack up miles faster, larger tires slow it down...
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Old 01-17-2010, 01:10 PM
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Originally Posted by Jazzer The Cat View Post
Oops.. I miswrote, I Meant to say 25.7"

Jazzer now looks foolish

PS. Good calcs though
Nah, if I got hung out to dry over every typo I've made I'd never post again...
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Old 01-17-2010, 01:48 PM
  #18  
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Originally Posted by cliffyk View Post
There are too many other factors, other than inflation pressure, that would impact effective tire diameter, making calculation of same from tire size and PSI a very inexact exercise.

You can however do a forced entry of the tire diameter, just click on the displayed value and you can enter whatever you like.



I added two columns, took it out to 8 grand...
Allright, thanks, I kinda figured there'd be too many variables. So a forced rear tire dia. entry overrides width,aspect and dia. drop down menus?

And thanks for the added column, I just meant one for red line (5750) so I could figure best ratio/tire combo in the 1/4. No biggy though,I just thought it would be easier than splitting the difference between 5500 and 6000.

Last edited by 01 P-51G t; 01-17-2010 at 01:51 PM.
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