Tire OD ? - PowerStrokeNation : Ford Powerstroke Diesel Forum
 
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post #1 of 5 (permalink) Old 11-16-2011, 02:14 PM Thread Starter
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Tire OD ?

Posted in the tire forum, but no success. My truck is a '97 F250HD 4x4, E4OD w/4.10 gears.

My stock tires are 235/85R16. The engines turns 2,250 RPM at 70 MPH and 2,000 at 65.

***What size tire would I need to reduce the RPM's by 250-300?***

Is there an easy chart to determine this?
Also, what is the best method to determine tire diameter? I know there is something about rolling circumference.

Thanks,

DR

97 F250HD (Cal. model) Auto, Ext. Cab XLT, 4x4, long bed
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post #2 of 5 (permalink) Old 11-16-2011, 02:46 PM
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Re: Tire OD ?

Tire size calculator

Of course, you won't see any difference on the speedo, since it measures axle rotation and assumes a tire size. You can correct in your head based on the numbers derived from the site above, or use GPS, or you can re-calibrate the PSOM (can only be done a certain number of times).
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post #3 of 5 (permalink) Old 11-16-2011, 03:06 PM
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Re: Tire OD ?

Here is an awesome gearing calculator. It is set up on the page twice, so you can input your stock stuff on the left, set up anything you want on the right, and then compare the two.
Using that calculator i see you turning 2158 @ 70 mph. To get 200 rpm lower, it says you need a tire that measures 35" tall, which means you need 36s or 37s to get what youre looking for. Take a look at that link, do some calculations, and you can decide what you think will be the best option for your truck


madpogue is right; if you change tire size the speedo wont be accurate unless you re-calibrate it for the bigger tires.


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post #4 of 5 (permalink) Old 11-17-2011, 04:19 AM
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Re: Tire OD ?

Here's a site that has several calculater and charts that with a little brain power can tell you what you want...

http://www.4lo.com/calc/geartable.php
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post #5 of 5 (permalink) Old 11-17-2011, 04:59 AM Thread Starter
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Re: Tire OD ?

Thanks guys. That's plenty of info there!

DR

97 F250HD (Cal. model) Auto, Ext. Cab XLT, 4x4, long bed
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