So,my buddy just recently switched tires on his 2002 F150.He went from P265/70/R16 to P265/75/R16.He told me yesterday that he was driving next to his mom (I have the slightest idea why),and his speedometer was reading 5 mph under what his mom's was.He called me up asking why this was,and I told him usually when you upgrade to bigger tires,your speedometer needs to be re-calibrated.However,i'm confused,because he really didn't go to that much bigger of a tire.Is his speedometer that sensitive that the slightest change in tire throws it off?Or does he have some other underlying problem with the speedo?My other question,is how exactly do you recalibrate a speedo on a truck this new?I know on my F100,you have to swap speedometer gears.Do the newer trucks still use speedometer gears on the cable,or are the speedos on these trucks computer controlled?Thanks in advance.I'm not so good with the Gen 10 and newer trucks.lol.If it makes any difference,the truck has a 5.4 and it's an automatic.

Shaun,

Original tire 265/70-16 is 30.6" dia, 659 revs per mile
New tire 265/75-16 is 31.6" dia, 637 revs per mile

The larger tire results in the speedometer reading 3.4% low & odometer reading 3.4% fewer miles than actually traveled.

The computer gets an input signal based on a tire with 659 revs per mile, since the new tire turns fewer revolutions per mile the computer is get incorrect data. To recalibrate the speedometer the computer must be reprogrammed. This is usually a dealer item, unless your friend owns a tuner that is capable of recalibration.

Tire Calculator www.miata.net/garage/tirecalc.html

Okay man,I appreciate it.I'll pass the info on to him.He has ties to the local Ford Dealer,so i'm sure he can get it done.

Yeah I have to recal mine when I flash the computer. Mine is out 10% from factory.

lil_dq said:

Yeah I have to recal mine when I flash the computer. Mine is out 10% from factory.

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Okay thanks Doc.I told him bout it,he didn't seem too happy.lol.

All I have to do is swap speedometer gears.

It is a fairly cheap fix. Talk to your local speed shop and tell them you need a custom tune. They should be able to point him to a place that could do jsut the recal on the speedo.