I have an ScT X4 tuner for my 2013 F150 3.5 Ecoboost. I have loaded tunes to it multiple times trying to find the best combo of settings. Recently I found that after a tune the MPH was 10-15 MPH slower than my GPS. I think it has something to do with the needle bouncing around during the tuning process. When I start the engine the speedo needle would elevate to "0". Now it doesn't move at all until the truck begins motion. I have programmed two different tire sizes in the Rev per mile setting. The larger tire setting brings the readings closer together but not more than 5 mph. I was hoping that when I returned it to stock that everything would return to original. It was reading 15 mph less than GPS speed.
I was thinking that the needle bouncing around during the process caused it to shift on the post making it read less than what it actually should. Has anyone ever had this happen? If so what did you do to correct it?
THank you in advance for your help,

 

What size tires are on the truck?
any changes to axle ratio via tuning, or physical gear changes?

 

275/70/18 and 3.31 gears. I didn’t fo any upgrades as far as tire size change or gear change.
I think that the needle position changed somehow when the ECU was get programmed. It bounces a lot when the tune is getting uploaded.

 

Can the x4 give you a speed readout? If so, compare the readout to the GPS and the speedometer. If you have the correct tire size and axle ratio programmed, the readout should be just a bit higher than the GPS - maybe 2 mph at 60. The readout will show you what speed the computer thinks you are going, for proper engine and transmission operation.

 

The X4 doesn’t have speed as an option that I can recall. I’ll look through it though to make sure. It may be part of the gauge/data readings.

 

[According to a Google search]
2013 F-150 tire sizes
17-Inch 235-75-17 255-65-17 265-70-17 245-75-17
18-Inch 265-60-18 275-65-18
20-Inch 275-55-20


Your tires are larger than factory, and will therefore show a slower speed reading. Double check your door jam sticker for the original tire size for your particular pickup.
If the X4 needs tire size in "Revs per mile" you can google that info for your specific tire size as well


The needle bouncing during programing is "normal" during that time. The cluster is powering up/down as the PCM is being flashed with software.

 

Tire size isn't going to make the speedo off by 10 to 15 mph. If you can get a readout on the x4 and it corresponds relatively closely with a GPS, it's a cluster issue. If the readout corresponds with the speedometer, you have a programming issue.

 

going from 265/60R18 to 275/70R18 would be about 10% difference. Showing 55 when you're actually doing 60. Showing 80 when you are actually doing 87