changing tires/ gearing ?
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Mountain Pass
Joined: Nov 2005
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changing tires/ gearing ?
I am thinking of going from the stock 265/75/17s to 315/70/17s on my 05 F250 with the 5.4 engine. How will that effect the current 3.73 gearing? Will I need to do a gear change also cause the truck will feel under powered? I think the truck is underpowered now, but will this make it worse? Is the fix some 4.30s or what? I realize there has been some info posted on this in the past but what is the simple answer to this question
Posting Guru
Joined: Jun 2005
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From: central Louisiana
there are alot of web sites out there for gearing! check this one out:
ACCUAUTOPARTS
you can get a pretty close to accurate rpm guage too! when i figured mine it was off by about 200. whitch this can't calculate for Torque converter lock up, so that was pretty close i'd say!
ALWAYS! ALWAYS! think bigger..................one day the lift bug will bite again! might want to think about 4.56's or some 4.88's for those 36, 37, or 38's you just might HAVE TO HAVE one day. i plan on re-gearing ONE more time to 5.38's to run some 41 iroc radials...........that way if i get the lift bug AGAIN, i'll be prepared for some 44's!
just my $.02- i'm sure i'll probably get bashed for it though.
ACCUAUTOPARTS
you can get a pretty close to accurate rpm guage too! when i figured mine it was off by about 200. whitch this can't calculate for Torque converter lock up, so that was pretty close i'd say!
ALWAYS! ALWAYS! think bigger..................one day the lift bug will bite again! might want to think about 4.56's or some 4.88's for those 36, 37, or 38's you just might HAVE TO HAVE one day. i plan on re-gearing ONE more time to 5.38's to run some 41 iroc radials...........that way if i get the lift bug AGAIN, i'll be prepared for some 44's!
just my $.02- i'm sure i'll probably get bashed for it though.
Junior User
Joined: Jul 2006
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From: California
Your gearing is still the same but the tire size will effect your speed and acceleration. Yes, it will seem slower, and yes you can get a new rear-end to correct the change. If you have a manual tranny I think you will be able to live with the difference the tires will make, however with an automatic it really sucks the big one in overdrive. I went to 315/75/r16's after a 4-inch lift kit install and ended up selling the truck becuase the lack of power. It would constantly jump in and out of overdrive on the freeway unless I was speeding over 80-mph, extremely annoying.
Posting Guru
Joined: Jan 2003
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From: Back in the TX Coast
Your effective gear ratio is now 3: 42 you would need 3: 94 to get you back to the original ratio. Most guys w/ the 5.4 have found 4:56 to be a good gear set. I plan on going w/ 4:56 when i do my lift. 
you have about a 9% torque decrease according to the site teammudd mentioned.
http://www.accuautoparts.com/calculatorfin.jsp
you have about a 9% torque decrease according to the site teammudd mentioned.
http://www.accuautoparts.com/calculatorfin.jsp
Last edited by Autobahn F-250 Lariat; Oct 9, 2006 at 02:51 AM.
Thread Starter
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Mountain Pass
Joined: Nov 2005
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ok, now I get it. With the bigger tires the truck will look better but will definately be slower unless I change the gears as well. It is an automatic with 4wd. I was not planning on lifting the truck. Now I will figure out what gears I want. Thanks for the help.
Senior User
Joined: Jun 2005
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From: SoCal
How are you gonna get those meats under the wheel well if you don't lift the truck? I have the Fabtech 6" leveling kit on my F350. I have two wheel drive - I think my front end probably sits lower than your truck. Anyone switch to larger tires on the 6.0 diesel? I've got 35's sitting in my garage that I want to put on, but I've been worrying myself over the gears. I tow about 7,000 pounds max.
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Posting Guru
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From: New York
Originally Posted by bullit340
ok, now I get it. With the bigger tires the truck will look better but will definately be slower unless I change the gears as well. It is an automatic with 4wd. I was not planning on lifting the truck. Now I will figure out what gears I want. Thanks for the help.
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Post Fiend
Joined: May 2006
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From: Central Florida
I'm in the same boat. 2002 F250 4" lift, 315's and 3.73. I had planed on 4.30 until I played with the numbers and it sould still be close the 3.73 ratio. I want more umph, so I am going with 4.56 and the required new diffs. 4.30 you can get by with just the gears. Going to 4.56 will require new differentials. The diffs should be broken at 3.33-4.11 and then 4.56+, with the 4.30 fitting the 3.73/4.11.
Posting Guru
Joined: Jun 2005
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From: central Louisiana
Originally Posted by ReAX
I'm in the same boat. 2002 F250 4" lift, 315's and 3.73. I had planed on 4.30 until I played with the numbers and it sould still be close the 3.73 ratio. I want more umph, so I am going with 4.56 and the required new diffs. 4.30 you can get by with just the gears. Going to 4.56 will require new differentials. The diffs should be broken at 3.33-4.11 and then 4.56+, with the 4.30 fitting the 3.73/4.11.
Senior User
Joined: Jun 2005
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From: SoCal
So from what I'm reading here and other places, to go from 265's to 315's I might be looking at a 4.30 gear and a speedo recalculation. This is turning into a big headache for larger tires, but I like the look. Is there a shop in Southern California where I can get it all done? My truck is 2003, but virtually new and I don't want to screw it up.
Posting Guru
Joined: Jun 2005
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From: central Louisiana
on a side note............. "ALWAYS THINK BIGGER"!!
right NOW you're only thinkin of 315's (34-35"). but it might hit you sooner than you think.............. "the LIFT BUG"!! drop in some 4.56's or some 4.88's and that will get you in the range for some 36/37's to 39's!
i thought i'd be happy w/ 6.5" and a set of 39.5's.............whitch i love the looks...........but i still want larger tires. i'm planning on ONE more gear change to 5.38's, and run a set of 41 IROK's! that way IF i ever get the BUG for some 44's i woun't have to re-gear again! i put the 4.56's in and am VERY happy with the results, BUT for bigger tires i need more gear. if i would have thought about larger tires in the past i would have gone to a 5.13 or a 5.38 to begine with, and wouldn't have to do it again! just something to think about............
right NOW you're only thinkin of 315's (34-35"). but it might hit you sooner than you think.............. "the LIFT BUG"!! drop in some 4.56's or some 4.88's and that will get you in the range for some 36/37's to 39's!
i thought i'd be happy w/ 6.5" and a set of 39.5's.............whitch i love the looks...........but i still want larger tires. i'm planning on ONE more gear change to 5.38's, and run a set of 41 IROK's! that way IF i ever get the BUG for some 44's i woun't have to re-gear again! i put the 4.56's in and am VERY happy with the results, BUT for bigger tires i need more gear. if i would have thought about larger tires in the past i would have gone to a 5.13 or a 5.38 to begine with, and wouldn't have to do it again! just something to think about............
Senior User
Joined: Jun 2005
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From: SoCal
Sounds like you don't mind changing the gears. I don't mean that in a bad way. It's probably fun if you know how to do it yourself, but the downside is expense. It totally blows my mind when I price the big tires not to mention the aluminum diff cover, twin coil over shocks, alum driveshaft, electric steps, etc. I can't even imagine how much money some of these guys have in their trucks.
Posting Guru
Joined: Jun 2005
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From: central Louisiana
Originally Posted by drewworm
Sounds like you don't mind changing the gears. I don't mean that in a bad way. It's probably fun if you know how to do it yourself, but the downside is expense. It totally blows my mind when I price the big tires not to mention the aluminum diff cover, twin coil over shocks, alum driveshaft, electric steps, etc. I can't even imagine how much money some of these guys have in their trucks.
i will be doing it the costly way, i "should have" gone higher in gears, BUT, i thought naaawww i woun't want bigger tires............. then when they came out with the 41 IROK radials..........well there went that thought...........
i don't have all the steps, aluminum dif covers, deep trans pan, dual shocks ect..... not saying i wouldn't LOVE to get some of that, but i'm kinda going slow and when i run across something on e-bay i might snag it up, hahahaha.