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235 tires vs 255

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Old 11-26-2000, 03:51 PM
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Post 235 tires vs 255

I want to upgrade my tire size from 235x70x16 to 255x70x16. Would like to hear from someone who has done this on a V6 automatlic as to how it affected performance.
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Old 11-26-2000, 08:28 PM
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Let's assume for the sake of calculations that you now have Goodyear Wrangler RT/S P235/70R15, which are 28.2" tall -- and you wish to go to their P265/70R17 32.0" tall series (Goodyear doesn't list a 255/70R17 online at http://www.goodyear.com/cgi-bin/tire...mpleselect2.pl ).

The simple ratio of new vs old would have it turning 88% slower at any given speed -- which effectively changes your axle ratio by that amount.

For example: If you now have a 3.55:1 final drive ratio, your engine (and speedometer) would think you then had a 3.13:1 ratio.

In order to keep your seat-of-the-pants-O-meter happy, you'd need to change your final drive ratio by 13.5% (eg: swap a 4.08:1 gearset for your 3.55).



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Toreador Red, Keyless XLT SC SB 5.4L E4x4 4wDisc/ABS, 3.73LS, Skid, HD 7700# Towing, LT-245's on Chrome, Tube-Steps, Captain's, 6CD, Tonneau, named: "Nick"

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Old 11-26-2000, 08:46 PM
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DOH! A marvelous answer -- but for a different question (had your tire sizes wrong in my head).

When I again looked up those tire sizes on Goodyear's web (http://www.goodyear.com/us/tires/tirecatalog/LighttTruckLighttTruckWRLRTSTS.html ) I noticed that they list the P235/70R16 at 29.1" whereas the P255/70R16 is listed at 28.5" -- for .6" LOSS in diameter -- and .3" loss in truck height !?!

OK, so let's go back to your assumption that you wanted a 3" diameter increase -- and let's use the 29.1" of the Goodyear Wrangler RT/S as a starting point.

29.1" / 32.1" = .907 -- so your 3.55:1 axle ratio becomes 3.2:1 -- a full drop in axle ratio -- and you would need 3.9:1 to remain "even" after the tire change.



------------------
Y2K™ 7700 4x4
Jim - N8JG@Hotmail.com

Toreador Red, Keyless XLT SC SB 5.4L E4x4 4wDisc/ABS, 3.73LS, Skid, HD 7700# Towing, LT-245's on Chrome, Tube-Steps, Captain's, 6CD, Tonneau, named: "Nick"

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