Changing tire size...
 

Soooo, my Michelin LTX A/S are at 6/32 on all 4 with 32,000 miles on them with regular rotations. I wont be buying a new set in the immediate future as in my climate I can run them down to 2/32 with no problems. However I am starting to explore my options, and kick around ideas because I cant leave well enough alone....that and the options in my current 265/60/18 size are very VERY limited.

I was thinking about moving up to a 275/65/18. This will give me more variety, and the Nitto Terra Grapplers that I have used before and loved are also available in this size. I understand this will change my speedo readings and whatnot, Im not too worried about that as my speedo already reads 2 mph fast (reads 60, GPS, and road side speed radar thingies read 58), this may or may not work in my favor and I may just get it recalibrated just so its correct depending on how much my OCD bugs me. Anyway, with the tire change supposedly at 60 (speedo) I will actually be going 63. I know they will fit without issue as a co worker has a 2wd crew cab short bed just like my truck and that's the stock tire size on his. It fills in his wheel wells better and just looks better as well.

My concern is my gear ratio which is 3.31. Above mentioned co worker has 3.55s on his, with the V-6 non ecoboost engine. Has anyone done a size change like this, or would know how that would affect power with the small gear ratio going from a 30.52" tire to a 32.07" tire?

Also, should I decide to get speedo recalibrated if I do go up in tire size, where can I get this done, and what to ask for? Thanks in advance for any tips, thoughts, advice, or ideas.

Obviously take-off and acceleration would be affected slightly, however, with that small of a change (5%) shouldn't be a big deal.

MPG may be (?) slightly less due to the increased wheel weight and wider contact patch.

FoMoCo dealership service will be able to flash the PCM with their IDS (Integrated Diagnotic System) - change Tire Size > 650 (from 683).

Another thing to note ... Without the Tire Size change your shift points will be off.

------------------------------
http://www.gdmjoe.com/images/goth150-0.jpg
-click- the pic'
Goth 150
< This space intentionally -blank- >

Thank you. I didnt realize it would change shift points like that. Im still on the fence about it, one minute I want to leave well enough alone, the next I go crazy in my own mind. The real reason I would change is due to the lack of options regarding tires in my current size. The only 2 that I have found and actually like, that isnt chinese garbage is the Cooper Discoverer ATP (which is what will go on if I decide to stay stock size) and the Yokohama Geolandar ATS. Moving up in size opens up a whole new world of options.

I doubt you will even notice a 1.5" overall diameter difference. Plus it will look better and fill the wheel wells more. Just my .02

is your truck 2wd?

275/65r18 is a stock size used on 4wd crew cabs with the chrome package and 5.0 w/3.55's

guess i'm saying that fitment shouldn't be a problem.

Yes it's 2wd.

Okay, so I finally had time to pay attention a bit and research a bit more. By changing my tire size from 265/60/18 to 275/65/18 I would be going from a 30.52" tire to a 32.07" tire (DTD online calculator) with my 3.31 gears. By doing this I am effectively changing my gear ratio from 3.31 to 3.15 (Wallace Racing online calculator).

I do have a question though......I see a lot of 2009+ F150 4x4s, (again my truck is 2wd) and I have had the chance to speak with a couple owners and check their trucks out. So far I have seen 4 that have the 3.31 code on the door sticker as well as the 275/65/18 tires size on same door sticker. Shouldnt their gear ratio actually be 3.15 like my trucks would if I do decide to change the tire size?

Through using the calculators, there is also one available that tells you what gearing you need to effectively change the tire size and replace gears to keep what you have now. For fun I punched in all the numbers just to see. With the jump in tire size I am aiming for, I would need 3.47 gears to keep the 3.31 final ratio. Does Ford even make 3.47 gears to install in above mentioned 4x4s to be able to honestly say that the final ratio is 3.31? This is just thoughts jumbling around in my head, but would make one think.

I believe the closest ford makes to the 3.47 range would be 3.55's. They also put your 3.31's, 3.73's, and 4.10's in the f150's.

Might want to check with aftermarket, they may have something close. This isn't my strong point, so i can't be for sure.

3.31 is the ratio of the rear-end, not the "final ratio"... and that's what the door sticker indicates as well If you want a true "final ratio" you would have to take into account tire wear, what gear the transmission is in, etc.

your truck with 3.31 gears and 275/65r18 tires would have the same final drive ratio as any other truck with 3.31's and 275/65r18.

that is to say, that at any given speed, your truck would turn the same RPM's as the 4x4's originally equipped with that gear, and that tire. the only difference is that your speedometer will be off unless you have it corrected.

you're kind of overthinking it a bit.

Your truck had more over-all gear than the 4x4's because of the shorter tire.

The 4x4's could put on your 265/60r18 and have the same overall gear as your truck, but their speedometer would be off the same amount (but in the other direction).

if you put on the 275's, your truck will feel the same as theirs. if they put on the 265's, theirs would feel the same as yours.

if you upsize the tires, just get the PCM flashed as indicated above by gDMJoe

make sense?

Just went from LT235/80R17 to P265/70R17 on my 2011. Definitely not the same sizes as you but I can tell you the entire truck feels different- in a good way. Those skinnies were just too tall and narrow for the truck. Looking forward to going a little bigger when these wear out!

On a side note, does anybody know the door sticker inflation pressure for supercab trucks that come with P265/70r17 tires?

I had really bad luck with the Geolanders on my Expy. They rode nice and they wore evenly but they wore much faster than they should have. A 60K mile tire was done at 37K. :-X15

I should have gone back to Yoko to see if they would help me out with this issue but that would have meant a new set of Yokos and that was not an option for me.

If you check out www.tirerack.com you'll find five pages of stock 265/60/18's and pages of the 275's that are stock on the 4x4's.

My co-worker just mounted a set of Bridgestone Duellers on his 4x4 Tundra. Nice tire.

Not over thinking it, just keeping myself informed. Thank you for the explanation.

We have put a set of the Geolandars on my wifes 05 F150. They were at 8/32 I beleive with around 30-35,000 miles on them. Although I have no complaints on them, I doubt I would use them again.

I wont buy another set of tires from Tire Rack due to the shipping charges, when other places are shipping free. On Discount Tire Direct I was looking at Cooper ATPs and really like those, but Ill probably end up going with Toyo Open Country A/TII. They are the closest I can get to a Terra Grappler and for the same money as almost any other tire.

I wouldn't use them either, just saying that it's a great resource to spec out tires.