Ditto, 2mph fast from about 35 on. Measured with GPS

 

Mine is a couple mph fast as well.

They are not "calibrated the same." The one drives the other.

 

Speed O vs the OBD2 port the speed O is off about 2 to the low side most of the time.

 

This isn't 1957 and the technology is different these days. Maybe you should do a 10 mile run and check and your odometer as well. You may actually be surprised.

 

I've done longer than a 10 mile test, several tests around 100 miles or so and the ODOMETER is spot on. (maybe off .1 to .2 miles, which is within a margin of error over that distance using a GPS)

The Speedometer always says I am going slower than I am and at the higher end of highway speeds close to 5 mph. Around town I am off the 2 mph or so.

I haven't done this in a while, but I was curious when I got the truck and I was using a GPS app and noticed the difference between speedometer reported speed at the truck and the GPS reported speed on my iPad. So then I checked miles driven... Trust me in that they (Ford) are not going to give you extra miles on your warranty because they are under-reporting the speed.

 

You just verified what I have been saying. There is no connection between speedometer accuracy and odometer accuracy.

If you want to get real technical, the difference between new tires and tires that are ready for replacement can be >2.5%. Of course there is always the minute difference in actual tire height between manufacturers as well. A 3 mph margin of error should be perfectly acceptable unless you can guarantee Ford that you will use the same manufacturer and size of tire while maintaining equal tread depth throughout the life of your vehicle. There are too many variables that Ford does not have control of for you to expect pinpoint accuracy.

 

I was not disagreeing with you... I hope you did not take it that way.

 

That's not quite right. The same sensors drive one stream of digital data to the computer regarding how many revolutions your tires are turning. The computer's speed read-out and odometer are driven by this information, and are either equally accurate or equally off. Your reading of your dash speedometer's needle doesn't always 100% line up with what the computer is reading. I don't know if this is manufacturing variance, but it sounds like it (and sounds like Ford isn't all that tight with their tolerances).

The true speed your truck thinks you're doing isn't available in the speedometer needle but rather digitally through the debug digital readout. I found the digital readout to be 100% accurate, while the needle tends to be slightly fast at speed, around 3 or 4 mph at 70. I was concerned this meant I was racking up miles on the odometer about 3% to 5% faster than reality, and it wasn't until I got the more accurate digital readout that I saw what was happening and was fine with it.

 

I've found that when you add a leveling kit and mud tires and floor it everywhere you have to put more green label in the filler neck.

 

Well, although I am happy with the new tires performance, ride, handling, looks and towing, I'm confused and disappointed with the effect they have had on the speedometer and odometer and mpg readout.

In the last 7 days I have run over 5,000 miles in my truck(sleep? What's that?). Speedo is within 1 mph at 70, but once I got to the interstate and checked against the mile markers, the odo is now off by 4.8%, after 100 miles odo only shows 95.2.

According to the tire size calculator my new tires are 3.79% difference in speed and mileage, so apparently the odo was off before the tire change, and even more incorrect than the speedo.

So, this skews any previous mileage calculations that I have made so a correct comparison to the current figures is impossible, or at the least just going to be a guess.

First full tank of fuel I ran through it as I started out, no trailer yet, checking the mpg's is skewed too, as about 100 miles in it went into regen and of course the mpg's crashed. In the end it came out to 20.7 mpg, which, strangely enough, matched the dash readout. In the past the readout was always optimistic by about .5 mpg. I used the miles from the GPS for the calculations as it is spot on.

After that I was chasing the lowest price fuel so I didn't do any more mpg calculations since I didn't always top off the tank. And there was also a strong 30-40 mph wind blowing for the rest of the miles I put on the truck this past week, this just killed the mileage, especially pulling the big 5th wheel trailers I had on.

So, I am at an impasse as to whether the new tires have affected the mpg or not. And am confused by the effect on the speedo and odo. Why ford would have them not be on par with each other is just totally screwy.

 

it is called tolerances and apparently Ford has some pretty loose acceptable tolerances in that area. Also, were your old tires worn? and I know they wouldn't be 4% worn anyways...

I'd be happy with 20.7 MPG regardless though.

 

Well...yeah, and the fact that 20.7 was with a full regen cycle, probably closer to 22 had it not done the regen.

And not like I can go back, pretty well stuck with what I have now.

If I had a tuner on it I would play with the tire adjustment on it to see if I could get the odo corrected.

 

Ford designs (it appears) the speedometer to be 2-3 mph fast at speeds above 20 to 25 mph. Not sure why, to keep folks from getting speeding tickets?
Your discovery is correct, many posts of exactly same thing.
On my '05 Ex with stock tires. Radar speed traps read same as sg2/gps too, speedometer was faster.
I used odb2 gauge (sg2) to view what the computer sees. The odb2 mph and gps were the same. On a long drive, gps and odometer agreed on distance. I use the mph on sg2 on the highway vs dash.

Most who go with larger tires, find speedometer is now accurate, but don't realize everything else the computer - or you - calculate is wrong...

Nearly everyone reports 'mpg ' drops with 285 vs 265 tires.. (excursion stock size is 265) as you have found, at a given speedometer value....you now are going faster, thus more air resistance, you are also going farther than the odometer reports, both of these cause your calculations to be below the actual value.
Engineers deal with accuracy, precision, and tolerance all the time, most folks don't realize a digital reading is not exactly what it says..

Sent from my Sprint PC36100 using IB AutoGroup

 

Wow, it seems that you actually opened your mind, did some long range testing and realized that maybe I'm not all that crazy. Don't worry, your kite buddy will come along soon and get you back on the straight and narrow. I'll make sure to not waste your time with any of my crazy talk in the future.

 

Well, I am not going faster, always used the GPS to set my cruise speed, but apparently my previous MPG calculations weren't accurate because even before the tire swap the odo was wrong, just more wrong now.

And I'm sure this is why all the reports are of guys getting worse fuel economy with the larger tires, because they have an even more inaccurate odo due to the increased tire height. I really don't think I lost any actual mileage, maybe didn't gain any but no loss either.


I'm all about reality, can't argue with facts, although I still don't have all of them.

And as to your sanity, I wouldn't begin to debate that with you. But, a guy on a ford truck sight with a jeep pic for his avatar just seems unbalanced to me...