I just got a set of 20 inch wheels for my truck and will be installing a set of nitto 295/60r20. I wanted to see if anyone had luck with reprogramming the speedometer for a bigger tire with 3.31 gears. I know that Ford can only program the largest tire available for the gear ratio, but I wanted to see if anyone had success with Ford changing the gear ratio to 3.55 and just programing it to the 275/60r20 size. If so, did that work out?

 

I have 295/70/18 Nitto's with 3.31 gears. My speedo was showing 3 mph faster than what my gps showed before changing tires. With the new tires it shows exactly the same as my gps. You are 1" less in circumference than mine so you shouldn't have to change anything. I think it throws off your mpg calculations but your speedometer should be ok.

 

That's amazing (the different results). My speedo is 3 MPH slow with 295/70/R18.
My road GPS, according to a 2012 Explorer, 2008 Honda Pilot and my topographical GPS, is spot on.

My OEM tires were 2 MPH fast on the speedo direct from the factory.
In engineering mode, the computer was with the GPS though with OEM.
I've not checked engineering mode with the new tires.

Since your speedo is matching your GPS, I bet your truck is actually off.

 

My original tire size was 245/75r17 which is only 31.5 inches tall.

My new tires are going to be 34.45 inches tall.

I am trying to see if anyone has had FORD change the gear ratio in IDS from 3.31 to 3.55 so they are able to select 275/65r20 without getting the check engine light. Are should I just keep the gear ratio the same and have them change the tire size to 275/70R18.

 

What size tires are you running now? Have you recalibrated your speedometer?

 

295 70 R18 now.
OEM was 275 65 R18 ContiTracs.
I took it to Ford with the new tires.
Every option they tried with IDS threw a CEL so we ended back where we started.
Still waiting for a fix so I can have them try again.

 

Did they try changing the gear ratio? how about just increasing the size to a 275/70r18?

Ford is frustrating sometimes.

 

I have heard of 1 person on here that had the dealer change the gear ratio to 3.55 and then set the tire size he needed. It did not throw a CEL. I questioned what effects that would have, but I have never heard anything. I can't remember who did it.

 

I doubt they tried that and that never even crossed my mind.
Had I thought of it I would guess that would have caused a number of issues and wouldn't have done it anyways.
But that's also just an assumption.

I was standing there as the tech was trying for the second round but I didn't see all of the exact options in IDS.
I paid the tech for his time and if I can find a fix they'll simply make the change for me in the future but they didn't have an answer.

I meant to enter engineering mode before leaving the office tonight but I forgot.

 

There was a thread awhile ago that mentioned that they finally just changed the gear ratio to allow them to set a 20" wheel. But, like you said I would be concerned about what else that did. I never heard anything else from this guy about it. I will try to find the thread.

 

I think I am just going to have them adjust it to the largest sized tire for the 3.31 gears and be done with it.

Has anyone heard of procomp's accu pro speedometer calibrator? Or another option was hypertech's in line module but it won't adjust the tcm shift points

 

I'd be concerned that changing the gear ratio would mess with the transmission shift points with all the computer controls it has..

 

I couldn't find the thread that talked about changing the gear ratio. I wouldn't do it anyway. But from reading all the old threads it seems the best best bet is like you said just have them set the largest 18" tire they can. Previous threads have said that got them to within about 1mph. Good enough for me.

 

Let us know if that change saves without CEL.
Do you have a 2012 MY?
If you do and you can change it, there's always a possibility something is slightly different between '11 and '12 in the programming.

 

I have a 2011.