Evening all,
First post here on the forum, glad to be here!

Here's my issue. I bought a new to me 2014 F150 last week. The speedometer reads higher than actual speed by 12 mph at 70 actual. I tried configuring tire size in forscan by circumference in mm and then by actual tire dimensions and it did nothing. I tried changing the gear ratio in forscan and it did nothing. So I went and bought a hypertech programmer and I programmed it tonight and of course, nothing. I cannot get the speedo to come down with any change. I'm not new to simple programming but this is very strange. Any suggestions or similar issues? It's strange because the slightly bigger tires I have on should be reading slow on the speedo. Thanks guys

 

Previous owner could have change rear end gear ratio, just a guess. I have a x4 tuner and I’m always off 1-3 mphs. 12 is a big difference.

 

Are you saying the speedo is showing 82 when you're actually doing 70? If so that is really odd. Often when guys upgrade to larger than stock tires the actual speed will be a little faster than the speedo. But 10% is a lot. Sometimes you'll actually be doing 77 when the speedometer says 70. But even at that it is usually only 2-4 mph. Sorry, I don't have an answer for you, but I've never seen anything this far off and never one reading faster than actual speed.

 

Thanks for the replies, I tried changing axle rations with both forscan and hypertech in hopes that was the case but it did nothing.

Yes, the speedo is high reading 82 when I'm going 70. I also thought this was odd.. I can't figure it out. It's driving the odo up way to quick which is unfortunate at best.

Anyone else have some suggestions before it goes to the dealer?

 

Does the speedo rest at the lowest point or is it reading high when the truck is off or at rest?

Forscan has been known to not work very well to make this change. But the hypertech speedo fixer should have worked.

 

Check the rear end ratio.
It will be difficult but mark the pinion or drive shaft. Mark the tire or use the valve stem as a guide. Rotate the tire one revolution while counting the number of rotations the pinion makes.
If anything, this will help you determine if the gear ratio has been changed dramatically over stock.
Read the axle code on your driver door jamb to learn what the factory installed ratio was supposed to be.

It should be obvious (i hope) if the difference is about 3.5 rotations or over 4 full rotations. A difference of 3.55 vs 3.73 gears won't make your speedometer read 12 mph too high at 70.

Is the difference in speed reporting based off percentage, such as at 35 mph the needle is only 6 mph too fast? That would be a direct relationship between the computer and gearing. If the differences are not equal, chances are it is not a gear thing. I mean if it reads 12 too fast at all speeds there is an issue. At that point I'd be looking at a speed sensor.
May need to have a shop connect a scan tool and witness wheel sensor activity. They should be able to spin the tire and see a signal on their screen if it is smooth, erratic or otherwise.

 

An open diff might make this tricky.

OP: Post your tire size and your RPM at a (GPS-indicated) speed, and we can probably guess what your rear end ratio is.

The ECU and trans are picky about tire size changes and will reject changes that are out of some mystical unknown boundaries and default to stock.

In Forscan do you see any kind of P160A error in the TCM? Or any kind of "Vehicle Options Reconfiguration Error"?

 

haha oh right!

 

I have verified the rear end ratio in the door jamb at 3.55 and don’t believe ratios have been changed. The speedometer does change exponentially so yes at 35 actual it’s around 5-7 mph off.

 

I didn’t get the P160A code in the PCM as forscan doesn’t show a tcm module form my truck. I thought that was weird because I normally would expect to have to do a pcm relearn procedure. No code in the pcm posted at all