Tire change for the better???? - Ford Powerstroke Diesel Forum
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post #1 of 5 Old 01-27-2014, 04:42 PM Thread Starter
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Tire change for the better????

Here's my current issue on my 12 F-350 2wd w/3.31. I have hated the stock tires (Continental) 275/65/18. Hard riding and poor wet traction. Two different GPS units always showed a 2 MPH error: driving 70 MPH was always actually 68 MPH. After some reading and wheel/tire calculations I found that the 275/70/18 tire size would perfectly correct the 2 MPH error and the truck would be going 70 MPH when the speedo says 70 MPH. So I purchased a set of Michelin's at Costco in the 275/70/18 size. Quiet, excellent traction on wet, and ideal in my towing needs. And, YES the MPH is correct between the GPS and the speedo! The 275/70/18 is actually a standard size on Ford 350 4x4 and the 3.55 rear. The MPG has dropped some but is now probably more realistic. The 3.31 with the new size is equal to a 3.41 ratio do to size differences. The tire is 1" taller but the tread width and section width are the same as the stock tire. The new tires also increase the weight capacity for the truck by 900 LBS total at the 60 front/80 rear inflation per sticker. Or I can run a lower pressure but still have stock weight capacity AND a softer ride and I think I'll like that.

As I've read the computer cannot be altered on our trucks after 2011 but does it really matter in my case? Anybody see a problem or issue for me in this switch? I haven't talked with Ford or the dealer about their take on this switch. Your thoughts will be greatly appreciated!!!


2012 F350 2wd srw 8' Lariat 6.7
2004 Ranger FX4 Level II
2008 Mustang Bullitt dhg
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post #2 of 5 Old 01-27-2014, 06:40 PM
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Seems to me your good to go. If you like it stick with it.

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post #3 of 5 Old 01-28-2014, 10:09 AM
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How do you figure that a taller tire increased your gear ratio? Pretty sure it's the other way around...

2012 CCLB F250 Lariat 4x4 6.7L
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post #4 of 5 Old 01-28-2014, 11:19 AM Thread Starter
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Originally Posted by 5-0stank View Post
How do you figure that a taller tire increased your gear ratio? Pretty sure it's the other way around...
You could be right! Not my best area of knowledge.
Facts: 3.31 axle
Old tire: 32.1 height
New tire: 33.2 height

Whats the NEW effective axle ratio???


2012 F350 2wd srw 8' Lariat 6.7
2004 Ranger FX4 Level II
2008 Mustang Bullitt dhg
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post #5 of 5 Old 01-28-2014, 12:10 PM
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Yep, increasing the tire size would actually decrease your effective ratio.

If you went to a smaller tire, it would aid you in acceleration, and give you higher rpm at a given speed. The opposite with a taller tire, slower acceleration and lower rpm.
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