Speedometer calibration and tire size

[Ghurst]

Long story short…. I purchased a 21 king ranch in February. Window sticker and door label shows truck should have 275/60/20, but it actually has 275/55/20. Side note is the tires are Michelin LTX M/S 2, so may not be original tires? Since the truck only has 46000 miles and the tires are worn out, I’m guessing they are OE possibly( lol… or off a GMC). Anyway… I was comparing my Speedo against gps. With the truck on cruise at 60, gps shows 58. In my thinking….. if I replace the tires with the 275/60/20,… is the speedo going to be further off, or am I thinking backwards? Looks like the 60 is about a 1/2 in taller than the 55’s.
Thanks

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[HammaMan]

Safe to say your mileage is off by the delta in circumference + time they've been on it. Looks like the truck is leveled, consider 275/65/20s to fill out the wheel wells better

 

[Ghurst]

Safe to say your mileage is off by the delta in circumference + time they've been on it. Looks like the truck is leveled, consider 275/65/20s to fill out the wheel wells better

Actually…. Truck was leveled, but removed the kit and going to remove the rear blocks (truck has the max tow package). I liked the way it looked, but I like to keep my truck in the garage…. Especially in the winter lol. Ya … I figured my actual mileage is off too! That pic was the day I picked it up.

 

[HammaMan]

4x4s come with blocks -- that's factory.

 

[fordtruckman2003]

Stock tires and my speedometer doesn't agree with GPS either. I'm 1 mph slower by GPS than what the truck says on dash.

There is a way to adjust tire size in Forscan if that is something you want to experiment on.

 

[z-man]

I think the factory programming is off from what I have heard. Forscan can get you spot on.

 

[HammaMan]

Stock tires and my speedometer doesn't agree with GPS either. I'm 1 mph slower by GPS than what the truck says on dash.

There is a way to adjust tire size in Forscan if that is something you want to experiment on.

Factory indicated speed is intentionally shown faster than actual speed. Truck is still counting mileage correctly.

 

[fordtruckman2003]

Factory indicated speed is intentionally shown faster than actual speed. Truck is still counting mileage correctly.

I figured that.
Government allows +/- 10% at 50 MPH.

 

[zx6roclet]

Off topic but I noticed my truck digital speed readout against the actual analog gauge reads 2mph differently. If the center dash says I’m at 50 the analog gauge says I’m at 48 lol

 

[fordtruckman2003]

Off topic but I noticed my truck digital speed readout against the actual analog gauge reads 2mph differently. If the center dash says I’m at 50 the analog gauge says I’m at 48 lol

Ford math.

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