Thinking about larger 20 inch tires for my Excursion and 04 F250.
Do I need to do something about the speedometer, or just accept an inaccurate reading?

Thanks for any replies

ruggit said:

Thinking about larger 20 inch tires for my Excursion and 04 F250.
Do I need to do something about the speedometer, or just accept an inaccurate reading?

Thanks for any replies

Click to expand...

I believe your dealer can program your ex to compensate for the larger tire/wheel configuration.

as long as the overall diameter of your tire and wheel is not bigger than stock i think you'll be fine.

The diameter will probably be more judging from a quick look at the newer rigs with factory 20 inch wheels compared to the 16's on my rigs.

yeah, I'd rely on your dealer to reprogram that stuff. I'm not sure how the newer model works, but I know it's digitally configured. Even my 94 is with a magnet in the differential.

Ryan

Thanks for the replies.

absolutely. Welcome to the site. Stick around and hang with us in the general areas when your ford isn't out of commission.

... which we know is MOST of the time.

Ryan

This is a piece of instruction for replacing a pcm on an 04 f250, (you're not, but you get the idea). Failure to properly program Tire Size in revolutions per mile, (rev/mile - 63,360 divided by the tire circumference in inches) Axle Ratio, 4x4/4x2, and/or Manual/Electronic shift on the fly (MSOF/ESOF) may result in codes: P1635, P1639. You may be instructed to contact the As-Built Data Center for the information needed to manually update the VID block with the scan tool. Contact the center only if the old PCM cannot be used or the data is corrupt. For Ford and Lincoln Mercury technicians, contact your National Hotline or the Professional Technician Society (PTS) website for As-Built data listed under the Service Publications Index. Non-Ford technicians use the Motorcraft® website at www.motorcraft.com. From the Motorcraft® homepage, use the search function to find the Module Programming or As-Built Data.

IIRC the factory PCM can be calibrated down to 601 rev/mil (35" dia wheel). If you go larger than this, then you need and aftermarket device (ie Truspeed)
www.superlift.com/accessories/truspeed.asp

ruggit said:

Thinking about larger 20 inch tires for my Excursion and 04 F250.
Do I need to do something about the speedometer, or just accept an inaccurate reading?

Thanks for any replies

Click to expand...

My 07 has 35's on 17 inch wheels, according to my GPS, my speedo is
off by about 5mph at 55-60mph.

Possum said:

My 07 has 35's on 17 inch wheels, according to my GPS, my speedo is
off by about 5mph at 55-60mph.

Click to expand...

Mine has 36's on 15" wheels and is off by close to 10 at around 60mph.

I use a gps, which is in all our vehicles and that's always worked good for me. Our gps units have the speed limits posted on the screen as well as our own, which has been super accurate.