Speedo correction for larger tires?

Changing the speedo calibration would be needed (if you want an accurate speedo/odometer) when changing tire size

I can't make this clickable from my iPad. Please copy and paste...
http://tire-size-conversion.com/speedometer-calibration/

I made the same change but with 17" tires and the change to the speedo was large. I used a programmer to correct the speedo (matched it up to a GPS).

I reckon I just got used to it and it was just like see 100K (here in Canada) and know how fast I was going.

Mostly though, when travelling, I have a GPS that shows me how fast I am going and it flashes red if I am over the speed limit. When speed zones change, the GPS responds immediately to the change. It gets quite excited and yells if I am in a school zone.

I think I have a bit of a sixth sense about how fast feel comfortable, and if I see red, I just back off a bit.

So, why not just program your head, and not spend any money!

Not sure how far back it can be done, but your dealer has a certain range they can set the computer to... or as above you can do it yourself with a tuner.

We changed 245 75 16s for 285 75 16s and are seeing about 4 mph difference.
Haven't had the computer adjusted yet. I have noticed myself controlling my speed using the GPS.

Gives the wife the chance to exercise her wifely duties of telling me to slow down.

You can have the computer re-adjusted to get the correct speed. Ford dealers can do this, but call to check the price first, not just have them do it then pay later.

Also check for a speedometer shop, they should be able to do it quickly and for a small fee.

You can also buy a GPS for about $100, and it will give you the correct speed all the time, allowing you to figure out your actual speed VS the display. So your truck will continue to show say 50 mph while your actual speed is 65 MPH or whatever. So when you drive on a 65 mile trip, the odometer will only show 50 miles traveled. So should you be getting 11 MPG on that trip (66 miles = 6 gallons) you would use 6 gallons but only show 50 miles on the odometer, indicating only 8.2 MPG.

If the difference shown on GPS is only 4 MPH at 65 miles per hour, then changing the speedometer to make it correct seems a bit pointless. It might just burn up $50 quickly.

Fred.

So in the good old days when you got pulled over for speeding, you'd get out of the car, come around to the rear bumper,lean against the trunk and have a discussion with the officer.

"I just got these new tires that are a different size, and I haven't had a chance to calibrate them with the speedometer yet. Do you know where there is a good spot on the highway I can do that?"

"Why sure son, there's a spot that's marked on the Interstate about 10 miles west of here. Get that done, and you be careful now."

"Thank you Sir, I will."

The difference will probably be minor however you can do it. With an older PSD it is not a bad idea to invest in Laptop software from AutoEnginuity and the enhanced Ford option.

http://www.riffraffdiesel.com/autoenginuity-total-ford-enhanced-bundle/

It will give you some great diagnostic tools along with the ability to send commands to the ECM to test various systems. Changing your speedo calibration based on tire size is pretty easy. I've attached a link to some instructions.

https://docs.google.com/viewer?a=v&pid=sites&srcid=ZGVmYXVsdGRvbWFpbnx3b29kbnRoaW5nczh8Z3g6ZmQ4YzhmNTYwODNiYTUx
If it is really bugging you, next time you are near North Alabama give a shout and I can hook up my laptop and adjust your speedo. PS. Hope you already have a good gauge setup to watch your EFT's and trans temps. A 6.0 trans cooler is a great upgrade for TC's and towing.