237/75/15 vs 9.5x30/15??? - Ford Truck Enthusiasts Forums



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237/75/15 vs 9.5x30/15???

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Old 03-21-2011, 10:11 AM
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Question 237/75/15 vs 9.5x30/15???

ok thinlin about new tires on the 96 f150 (STOCK HEIGHT) and read that the 9.5 wont rub on lock but how much bigger are they anyone with pics?? how much would my speedometer and gear ratio be off (3.55 now) and how much should my milage drop also i pull car trailer regularly will i notice a losse in power??
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Old 03-21-2011, 10:33 AM
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The 30x9.5 tires are about an inch taller than 235s, that will make the speedo read 2-3mph slower and it will hurt power a little but not a whole lot. See my Album for pics of my truck with 30's on it.
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Old 03-21-2011, 03:07 PM
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you notice much on power
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Old 03-21-2011, 05:37 PM
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Plug in your tire sizes into this calculator and it'll give you a bunch of comparisions:

Tire Size Calculator - tire & wheel plus sizing
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Old 03-21-2011, 06:09 PM
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How would a change in tire size reduce power? and also on that tire calculating site the guy before me posted it says that changing the tire diameter by more than 3% could result in brake failure.. why?
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Old 03-21-2011, 10:37 PM
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Tires are part of the driveline and tire size factors into your final drive ratio. Without changing anything else the larger the tire the higher(numerically lower) your final drive ratio is. As an example if your truck has 3.55 gears going from stock to 30" tires changes the effective final drive ratio to 3.28, 31's makes it 3.18, 33's make it 3.0, 35's make it 2.82.

The tire size change won't affect engine HP but the taller gearing will make it seem like it lost power, it actually does reduce the amount of torque reaching the ground so acceleration will be slower.

The brake failure warning is a bit overblown but larger tires do put more leverage on the brake components and it will make them less effective.. though like everything else the amount of braking power lost is proportional to how much larger than stock the tires are. In the big scheme of things you probably won't notice much difference.
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