Speedo correction for larger tires?

I just use my GPS for accurate speed since upgrading to the Nitto Durra 285/75r/17s high load index. Tires have changed speedo between 2 and 3 mph. These tires are beasts. This is my second set of Nittos. Had Nitto Terra before and lasted 48,000 hauling TC, could have gotten more miles on them but wanted more load capacity.

Americamper wrote:
I went from a 265/75 to a 285/75 both 16 inch. Would the height even change?

Diameter increases by 1.2 inches.

Pangaea Ron wrote:
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.

Now, when you open the door and start to step out of the vehicle without being asked to do so, the officer screams at you to "sit back down and keep your hands where I can see them" as he/she draws his/her service weapon.

Been there, done that.

JumboJet wrote:
Now, when you open the door and start to step out of the vehicle without being asked to do so, the officer screams at you to "sit back down and keep your hands where I can see them" as he/she draws his/her service weapon.

Been there, done that.

In my case, it happened when I first started driving. He didn't draw, but was noticably upset and had his hand on his weapon. Oops. BTW, I was 5 over in a small town right after a speed zone decrease but got off with a warning and a lesson!

Chances are that the speedo is already off.
ford gauges are not the most accurate.When I got my garmin,I found my speedo was 2mph off,
when I installed the scan guage,I found that the temp guage was the same spot
from180-205.

Like billyray50 I use my Garmin to check my speed it is much more accurate than the speedo. Changing from 245 to 265 tires changed my speed by about 2 mph. AT 53 I'm doing 55. Ron

According to a Garmin GPS, my 2013 F250 was off about 3mph with the factory tires. It's about spot-on accurate now. Regardless, I've grown accustomed to reading the GPS for speed because the speedometer in my other truck is blocked by my normal driving hand position. Most convenient to glance at the GPS.

I went from 245/75R17 to 285/70R17 tires. The new tires were 4% taller so in theory when the speedometer indicated a speed of 70 MPH I would actually be at 73 MPH, not enough of a difference to worry about.

I found that with a camper load in the back the rear tires were reduced in height by roughly 4% so my speedometer and odometer are now more accurate than they were with the factory tires.

The dealer would not change the settings for the truck for the new tires and I did not want to spend hundred of dollars on a device to make the change myself. Worst case as I said I would be going 3 MPH faster than shown on the speedometer and I could either drive at 62 in a 65 zone if I thought for a second that I was going to get a ticket for being at 68 MPH.

I went from 265/70R17 to a to a 285/70R17 and the error margin at low speeds isn't bad at all. At 70 mph (according to GPS) my speedo reads about 66-67 mph. I talked to my dealer and they said they couldn't make it any more accurate that what it was. Most of the engine programmers can make the corrections.

My truck has the old style speedo so I'm thinking that it is not re-programmable. My MPG has recently went down a little, so If I understand some of the posts correctly then the tires may be the reason.