New Tire Size - Computer Change?
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Tuned
Joined: Feb 2003
Posts: 451
Likes: 5
From: Raleigh, NC.
New Tire Size - Computer Change?
Hey folks, I know this has been discussed before, but was not clear on this topic.
I just put on new tires. Went from stock 265 to 285's. Now, my speedometer is perfect (accurate) according to GPS. I have always known it was about 2-2.5mph fast on the speedometer and now it is perfect.
However, my overhead computer says I am getting 10 mpg either towing or on interstate. It used to very quickly get up to 15 or 16 on the interstate and 11 towing.
Can I just disconnect the battery and it re-learn the new settings or do I have to get a programmer? Wasn't planning on spending the $$ on a programmer, but I do like the overhead to keep an eye on how she is running.
I just put on new tires. Went from stock 265 to 285's. Now, my speedometer is perfect (accurate) according to GPS. I have always known it was about 2-2.5mph fast on the speedometer and now it is perfect.
However, my overhead computer says I am getting 10 mpg either towing or on interstate. It used to very quickly get up to 15 or 16 on the interstate and 11 towing.
Can I just disconnect the battery and it re-learn the new settings or do I have to get a programmer? Wasn't planning on spending the $$ on a programmer, but I do like the overhead to keep an eye on how she is running.
Sit. Stay.
Joined: Oct 2001
Posts: 18,308
Likes: 20
From: Washington State
Even though your speedo is now accurate you still have to reprogram the PCM for your different tires. Weird, I know...a 3.7% change in tire size creates an indicated 33% drop in mileage...
Some tuners won't support tire size changes for a V10. You'll have to explore your options. If you have a good relationship with your dealer he can adjust the REVS PER MILE in the PCM to reflect the new value.
You can try the resetting of the PCM but the only thing it "learns" is shift duration.
Some tuners won't support tire size changes for a V10. You'll have to explore your options. If you have a good relationship with your dealer he can adjust the REVS PER MILE in the PCM to reflect the new value.
You can try the resetting of the PCM but the only thing it "learns" is shift duration.
Freshman User
Joined: Jan 2006
Posts: 42
Likes: 0
From: Tuscaloosa, AL
I guess I'm missing something here...(and I've spent all day watching the kids, so that is entirely possible), but how would changing the width of your tires impact your computer at all? You said you went from stock "265s to 285s"...
285 refers to the section width of the tire in millimeters, so simply getting a wider tire shouldn't impact the number of revolutions.
If you changed tire size--either the rim size, aspect ratio (for example, 70 to 75), then that would have an impact on the overall outside diameter, thus the circumference of your tire and that would impact your computer.
285 refers to the section width of the tire in millimeters, so simply getting a wider tire shouldn't impact the number of revolutions.
If you changed tire size--either the rim size, aspect ratio (for example, 70 to 75), then that would have an impact on the overall outside diameter, thus the circumference of your tire and that would impact your computer.
Sit. Stay.
Joined: Oct 2001
Posts: 18,308
Likes: 20
From: Washington State
Since you know how tires are sized then you probably know that the aspect ratio of 75 equals 75% of 285.
75% of 285 is higher than 75% (same aspect ratio as stock) of 265.
Thus, a difference in circumference.
75% of 285 is higher than 75% (same aspect ratio as stock) of 265.
Thus, a difference in circumference.
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Tuned
Joined: Feb 2003
Posts: 451
Likes: 5
From: Raleigh, NC.
Will check with the dealer next time I am over that way. Anyone care to chime in on what or how they have changed their PCM based upon tire size? What is my best option? If I have to pay so much at a dealer, it might be best if I can get more bang for the buck with a programmer or whatever others think?
Unless you want the other features of a tuner you may just want to see your dealer (moneywise anyway)
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Tuned
Joined: Feb 2003
Posts: 451
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From: Raleigh, NC.
Good point Monsta! Would love to shorten the shift time on the tranny, but that is not a must. Just don't like that floating shift when getting on it and then letting off all of a sudden and it takes a moment to shift back down to normal, but like I say, that is not a must.
Fleet Mechanic
Joined: Aug 2006
Posts: 1,684
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From: Phoenix, AZ
Dang it, ancient thread bumped up... damn search results from the stone age
Cargo Master
Joined: Mar 2011
Posts: 2,066
Likes: 4
From: North Texas
I am glad you bumped this thread, because I have 285's on mine also and must not have searched the same and didnt see it in my searches. I may try the dealership and see what they can do. I just wish Fuelly had a setting to compensate for the larger tires/more miles traveled than indicated.
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