Changing tire size on F-350.
 

2000 Ford F350 7.3L PSD 4x4 w/ 4-speed Automatic/OD transmission:

What's the best way make my truck aware of my new tire size IRT shift points and speedometer?

I am going from factory:

235/85-16 tires which are 31.72 inches tall
to
245/70-19.5 tires which are 33.00 inches tall.

I'm changing tire size by 3.88%, and I know I am dropping my overall gear ratio by a bit as well.

My biggest question, is how do I make sure my automatic transmission is still going to shift in the right RPM range; and secondly, how do I correct my speedometer?

I am already running a BullyDog 6-position chip on my truck which am not all that interested in changing.

Can a Ford dealer reprogram my ECU for the new tire size? I know it's not recommended that I run a Hypertech or Diablo tuner (which can change tire sizes) at the time as the external 6-position chip.

I used the Superlift TruSpeed™ Speed Sensor Recalibrator on my 2000 7.3 f250 after upgrading to 37 inch tires.

http://www.superlift.com/accessories/truspeed.asp

You should be able to the swap with dealer input/parts. my 2002 shows on the door lable that it came with 265s stock. Work with your dealer, probably just a quick change to run the 33's accurately. But are you really concerned with less than 4% accuracy difference? i'd just run 'em personally.

I had this on my 6.0 but my cruise control never worked after

http://www.superlift.com/accessories/truspeed.asp

Thanks guys. I am concerned with the accuracy because I tow 12,000+ lbs. I want my transmission to know exactly what it needs to do so it shifts as close to stock as possible. Losing cruise control really scares me.

This is what you are looking for, and it can be stacked with your builly dog, I run mine along with a banks 6 gun on a 6.0l

http://www.sctflash.com/products.php?PID=1&VID=404

The best thing about this unit is the custom "tunes" that can be had all over the internet. I have a tow tune, eco tune, and a looney tune that is ballistic.

Fixed all my shift problems with 38 tires without a re-gear.

Do some reasearch on a fordtruckworld.com, the SCT is the unit of choice because of the custom feature.

Going backwards from you as I'll be dropping from tall 315s down to stock height 265s shortly for the winter. I'll be interested to see if the shifts change (improve) at all.

35" and a superchip on my 250, 7.3

I just got a call back from my local Ford dealer. I have found dealers to be hit and miss IRT what they can do for diesel trucks.

He knew exactly what I needed, and said it would probably be $120 bucks and a couple hours to change the readings on 3 or 4 sensors.

I kind of like this direction better than messing with the ECU program(mer) or the Superlift adjuster.

just curious if you have any lift on ur truck w 315's