Is this tire switch crazy?
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Senior User
Joined: Aug 2010
Posts: 107
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From: Arizona
Is this tire switch crazy?
I have a F350 CC LWB 6.7 on the way. It has the 3.31 gearing with the stock LT275/70R18 tires... 33" dia 626 revs/mile. I'm thinking about swapping these out for Toyo LT285/75 R18 tires... 35" dia 591 revs/mile. I understand they will fit the factory rims and won't require a lift. That's a 5% change in RPMs. Is that gonna significantly affect my power. I'm not towing anything heavier than a 20 ft bassboat... although we do have some pretty steep grades out here in Az. And... will I have to have my speedometer/odometer re-calibrated? Sorry... don't know much about this stuff... always stayed with stock tire sizes on my duallies.
Elder User
Joined: Mar 2007
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From: at the truck pulls
I don't think there will be a noticeable difference going to that size, not with these engines. Don't know about recalibrating for tire size, I think it was an option in the manual but can't remember.
If not I wouldn't worry about it, just expect it to be about 2 or 3 mph off and go with it.
If not I wouldn't worry about it, just expect it to be about 2 or 3 mph off and go with it.
Logistics Pro
Joined: Oct 2008
Posts: 4,818
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From: Avilla, IN
Close enough, lol. You should be able to pull that easily. I've pulled 44,000 lbs. of corn with my old truck. 215 HP, 425 lb.ft torque, 3.55 gears, E4OD, so yeah you should be fine. lol. I could only pull it 15 mph, that's about as fast as you pull 2 gravity unload grain wagons no matter how much HP you have.
Lead Driver
Joined: Sep 2003
Posts: 5,844
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From: Albuquerque
YES...you should be able to tow your boat very easily...with little change in power. You might lose mpg, but that should be expected with larger tires and "high" gears.
YES...the dealer will recalibrate your speedo. The revs per mile are all you need...and you should be good to go. The importance is really two fold on the recalibration. 1st is for the correct speed reading and mpg, but also to keep the tranny happy shifting. Many...and I mean many a tranny has burnt up with larger tires as a primary reason, because it changes the shift pattern.
Good luck...your SD should look sharp with the setup in mind.
biz
YES...the dealer will recalibrate your speedo. The revs per mile are all you need...and you should be good to go. The importance is really two fold on the recalibration. 1st is for the correct speed reading and mpg, but also to keep the tranny happy shifting. Many...and I mean many a tranny has burnt up with larger tires as a primary reason, because it changes the shift pattern.
Good luck...your SD should look sharp with the setup in mind.
biz
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Joined: Sep 2010
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From: The Thumb, MI
I have a 2011 F-350 SC Dually with 3:73's that I plan replacing the 245's with 6 new 285/75/17's. Using a specialy made spacer for the rear wheels. With 3:73 gears like I had on the previous truck I don't think I will loose any mpg's or power. Actually the truck got better fuel mileage with em.
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Joined: Apr 2005
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From: Anchorage, Alaska
3 MPH is 5% of 60 MPH. So the speedo would read 57 MPH @ 60 MPH.
So if you plan on a top speed of 80 MPH, 5% of 80 MPH (4 MPH) is going to read 76 MPH @ 80 MPH. So if your are cruising with a speedo reading of 80, your vehicle speed will be about 84.25 MPH.
I would program for the shifting purpose at least. the 5% sin't going to affect speed much at lower speeds, so driving over the speed limit isn't going make a difference. Even at 84.25 MPH, if the speed limit were that, you aren't even going 5 MPH over.
So if you plan on a top speed of 80 MPH, 5% of 80 MPH (4 MPH) is going to read 76 MPH @ 80 MPH. So if your are cruising with a speedo reading of 80, your vehicle speed will be about 84.25 MPH.
I would program for the shifting purpose at least. the 5% sin't going to affect speed much at lower speeds, so driving over the speed limit isn't going make a difference. Even at 84.25 MPH, if the speed limit were that, you aren't even going 5 MPH over.
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Senior User
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From: Arizona
Logistics Pro
Joined: Oct 2008
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From: Avilla, IN
Why not just recalibrate it and be done with it? Why do you want to have to think about how fast you're really going everytime you look at the speedometer? I mean what does it cost and how long does it take to recalibrate the speedometer? I doubt its expensive or takes all day. You might even be able to recalibrate it yourself.
Cargo Master
Joined: Jul 2008
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Why not just recalibrate it and be done with it? Why do you want to have to think about how fast you're really going everytime you look at the speedometer? I mean what does it cost and how long does it take to recalibrate the speedometer? I doubt its expensive or takes all day. You might even be able to recalibrate it yourself.
I don't and Won't worry about it. It's so minor of an issue.
Shane