Tire Size

[Missy]

I’m running 285/65/20 which are 34.6 in size. I changed the tire size to 35 in the tuner. I was still 4mph faster than my speedometer. I changed the tire size to 39. Now I’m 1mph faster than my speedometer. I’m going to change it to Size 40 tomorrow and see what happens.
This is on a 2018 F150 3.5 Eco
Is it normal to be 4/5 bigger than what I’m actually running?
Am I supposed to be changing something else in the tune to reflect 35 size tires?

[mikel]

That sure does sound odd.
Please let us know if setting it to 40" dialed it in and well look into it.

[Missy]

Changed it to 40 and speedometer is faster than actual speed.
Changed it to 39.50 and speedometer is faster than actual speed.
Changed it to 39.25 and speedometer is faster than actual speed still by 1mph. Sometimes 50mph & below I’m spot on

I’m leaving it at 39.25 for now.
Is this normal? I don’t think so.

Let me know. Thanks.

[Missy]

Okay it was still bugging me so I did 38.75 and now it seems to be more spot on. So I’m leaving it now at 38.75.
But I would still like to know if this is normal or if there’s something wrong.

Thanks

[Missy]

So.. if your using Waze to calculate your speed, you cannot connect the phone to Apple car play. For whatever reason it’s using your speedometer speed. As soon as I used Waze without Apple car play my speedometer was 10mph faster than my actual speed.

I have now changed my tire size to 33.75 and my speedometer and Waze is now matching.

What a pain, but I’m glad I figured it out. Just wanted to let you know.