When I recently purchased my 04 XLT 2x4 I had them upgrade the wheels to a set of Lariat wheels they had just taken off a new truck. This took me from 17" to 18". The service manager said the difference was so small I did not need to reset the computer for mileage. Others have since said I should have. Does anyone have experience with this? What would you do?
https://www.ford-trucks.com/user_gal...oid=63127&.jpg

 

Can the F150 computer be updated for revs per mile? If so, what's the harm in getting it done? I know I would.

 

I did the wheel up size this summer. Took it to Ford and they adjusted the computer for the wheel size difference. Cost around $42-$43.

 

Not knowing what size tire came off, I will assume that the standard XLT tire 235-70-17 came off. The Lariat tire size is 265-60-18. If these assumptions are correct, the original tire 235-70-17 provides 673 revs per mile, the new 265-60-18 now provides 661 revs per mile. I think this is substantial enough to throw off your speedo.

BTW here is a tire size calculator to help:
http://www.miata.net/garage/tirecalc.html

 

The stock tires that came on the truck were..255-65r-17
New Lariats are 275-65r-18
According to the Tire size calculator link above when my speedometer says I am doing 60mph, I am really doing 64mph! I will have it corrected, and thanks.

 

Hi JiffyJam. The lariet tires I just put on are 265-60-18. 1.25 inches larger in diameter than the 255-65-17 tires I just took off. 1 percent difference. Rather than adjust the speedo with my Superchips, I'm going to leave it. My speedo ain't all that accurate. This will make it closer.
(excuse to get a programmer?)
Chris

 

Very interesting, I just changed my stock FX4 tires,today in fact,(255 70 r17) to FX4 optional take-offs(275 65 r18).According to Miata calculator(great tool KevinM) mine would only be off 3.3%, reads 60mph=actual 62. Not significant enough for me. Great question and responses though!Thanks!

 

jiffyjam -- I checked out your truck pic. GREAT looking truck. Where did you get that awesome front grill?

 

I bought em at the Ford Dealership where I got the truck, and thanks.
( The shine is from using Zaino Show Car polish, its hard to beat. Try their website.http://www.zainostore.com/Merchant2/merchant.mvc

 

Jiffy - the Lariat comes with 265-60-18 all season for the 4x2 or 275-65-18 all terrain
for the 4x4.

 

275-65-18...is what I have, even though I purchased a 4x2. When they wear down I will replace them with Goodyear RTs. BTW, its at the Ford dealer now being re-calibrated.

 

CORRECTION. Boy did I goof, and just noticed it. The 265-60-18 tires are 1.25 inch CIRCUMFERENCE not Diameter. Hoo-boy. The diameter is .4 inches larger.

Tracking with my GPS the speedo is now only over reading by 1.2 miles an hour, not the original 3.6 at 70 mph. So actually it's closer to right.
Chris

 

Update...Ford sevice people said the difference was not enough to worry about, so I'm not gonna worry about it...

 

I did the opposite swap (18s to 17s on FX4 as there are more options for 17s and mud treads as well), and found (apparently) that the speedo was off before the swap. The 275 65 r18 is 31.8", the 255 70 r17 is 31.2" but when I pass a radar at 70mph on the speedo, the radar says 64mph! I have passed several, so I assume they are not all wrong. Thinking about taking mine in for recal since the dealer did the swap prior to the sale. Incidentally I have tons of pulling power, and get great mileage compared to some of the posts I see. 16.5 to 17.8 on highway driving. 16.1 while towing about 1700lbs...

 

I lost 500lbs of rated tow power when I went from the 17" to the 18".