Calibrated my speedometer.. Or did I?

[MrBusses]

So few months ago I upgraded my tires size from stock 265/65R17s to 265/70r17. I noticed a slight hit in mps but my speedometer was dead on per my GPS. Today I decided to program the correct tire circumference with forscan really to see if I gain back the mpgs but I did notice that my speedometer is about 3 mph slower now.

50mph is really 47 which I guess isn't a bad thing but it got me wondering are stock trucks off this much? I never checked mine before so just wondering.

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[MountainGoat]

I have 2439 mm as the tire size in forscan and it's dead on with GPS. Same tire size as you. ?‍

 

[MrBusses]

I have 2439 mm as the tire size in forscan and it's dead on with GPS. Same tire size as you. ?‍

The default value was 2360 took a screen shot so I would have it. when I punched in the tire size to get the circumference I got 2520. If I put in a 265/65r17 the stock size I get 2439

 

[MountainGoat]

It's not exactly right on mine. I have 265/70/17 and 2439 equals the gps. I have in my notes subtract 3.2% from tire size in mm to get it accurate.

 

[Joeiconic]

I reprogrammed for my 265/70’s using the specs provided by the tire manufacturer (Cooper) and GPS fluctuates from 1-2 mph slower than my Speedo. Of the things I have to worry about, that ain’t one of them.

 

[12Bravo20]

By federal law, vehicle speedometers can read up to 10% faster than actual speed. BUT they can not read any slower than actual speed.

Motorcycle speedometers are the worse about reading faster than actual speed. Most motorcycles read between 8% and 10% faster that actual speed.

50mph is really 47 which I guess isn't a bad thing but it got me wondering are stock trucks off this much?

@MrBusses if your speedometer is reading 50 MPH and your actual speed is 47 MPH, then you are okay. Your speedometer is reading 6% faster than actual speed.

 

[Frenchy]

A lot of it will depend on the tire make and model on top of the size. You can take 10 different tires in the same size that are different brands and models of said brands and have 10 different sizes. Also there is this thing called tire wear.

I will also state that many GPS systems are not entirely accurate. I have seen this with Google maps for example and TOMTOM

 

[MrBusses]

I changed it to 2439 and I'm 1-2mph off so I am happy with that

 

[got3fords]

Aside from the speedo accuracy, what are the pros, cons of going one size bigger than stock? I think I might go from the stock 255/65/17 to 265/65/17 next time which is coming up.

 

[12Bravo20]

@got3fords

Going to 265/65R17 tires won't hurt a thing considering the FX4 Rangers come from the factory with HanKook DynaPro 265/65/17 tires.

Though most of us will NOT recommend going with the Hankook tires since they are junk.

 

[Michel Jeanneau]

I think when I changed mine (FX4), it was set from the factory to 2380mm, and I measured the actual roll circunference of my actual tires (Cooper Disco AT4 265/70/17) and got 2405mm; so I entered that and my speedo is 1 mph off by GPS, meaning 60 mph indicated is 59mph on the GPS. I am about to correct it again, see what happens
Correction, I used 2437...am about to change it to 2439mm

 

[JasonTremor]

Mine reads three mph fast with stock tires and wheels. I just assumed that was Ford's way of running the warranty clock out quicker.

 

[Wytchdctr]

Mine was dead on stock. A bit off after the 265/70R17s. Used the LMS box that uses rotation per mile vs size. I'm now indicating 1 mph fast on the dash vs GPS from 20 to 80. Dead on under 20. Weird it stays the same as speed goes up.

 

[Michel Jeanneau]

I just spent the last hour messing with speed cal...I went to 2439 and 50 mph indicated was 48 mph on GPS, went back to 2435, (50/49), 2430 (same) and are now at 2425, I'll test that tomorrow. I'm probably going the wrong way, who knows? too late in the day to do the math!

 

[Michel Jeanneau]

Update, 2425 gives me almost 50 mph (fluctuates between 49-50 on GPS and setting cruise to 50). So, 2420 might do the trick, or I might just leave it where it is now

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