If I move from 225/65/17 to 235/65/17, will there be an adjustment on the speedometer?

[davidg4781]

I'm probably going to need new tires in the next 6 months. I know I can get 245s but I'm really leaning more towards 225s just to keep the original specs.

Since Ford also offers 23565/R17 as an option on some trims, will the speedometer be off? Or would it be easily adjusted?

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[B-Dog15]

I had a local shop calibrate my speedo through Forscan. I'm assuming it's easy to do yourself I'm just too lazy/busy to actually put brainpower towards it.

As for if the speedo will be off or not... I'm not sure if Ford has the speedo set at 235/65 for models that come with those tires or if they just leave them all at 225. I know my shop said that Forscan showed 225/65 but that was the stock size for me. Would be interesting to see what Forscan shows on BS's that come stock with 235/65 though.

 

[cprcubed]

I went from 225/65-17 Continentals to 235-65-17 Michelin Cross Climate SUVs and the speedometer is only 1, maybe 2 mph slow at 70 mph. Cheers!

 

[jkernitzki]

Tire circumference is held in the Body Control Module and measured in millimeters. This is what’s used to calculate speed, distance, and fuel economy.

If you change tire dimensions, the new tire circumference is different, and the value must be changed in the BCM to properly calculate those metrics. Errors going up one size are minimal, but they’re still there.

Some dealers may do this for you, or has been noted, any decent shop can do it. You can also self-perform the change with Forscan, but if this is the only thing you’ll change, just have someone else do it.

 

[jkernitzki]

I'm not sure if Ford has the speedo set at 235/65 for models that come with those tires or if they just leave them all at 225.

The BCM circumference will always be as-built. Pretty sure it’d be both an FTC and EPA violation to do otherwise.

 

[Rockboz]

So if you are going from a 245mm width to a 225mm width, the tire circumference does NOT change. So no adjustment necessary.

 

[Mark S.]

So if you are going from a 245mm width to a 235mm width, the tire circumference does NOT change. So no adjustment necessary

Yes it does. It's not a lot, but it does change. You can see how much here:

https://tiresize.com/comparison/

The "65" in the tire spec refers to aspect ratio. It tells you how tall the sidewall is as a percentage of the tread width. That means a wider tread with the same aspect ratio yields a taller sidewall.

 

[Rockboz]

Yes it does. It's not a lot, but it does change. You can see how much here:

https://tiresize.com/comparison/

The "65" in the tire spec refers to aspect ratio. It tells you how tall the sidewall is as a percentage of the tread width. That means a wider tread with the same aspect ratio yields a taller sidewall.

He is not changing the sidewall size: 235/65 going to a 225/65

 

[B-Dog15]

He is not changing the sidewall size: 235/65 going to a 225/65

This still changes sidewall size. He will be going from 65% of 235 to 65% of 225. Like Mark said it is not significant but it will still change.

 

[PaulOinMA]

As mentioned, it's circumference in mm. There's a thread in an F-150 forum on how to go in and change it, if you do a search.

 

[jkernitzki]

So if you are going from a 245mm width to a 225mm width, the tire circumference does NOT change. So no adjustment necessary.

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[Bill G]

Ordered my BS withe the optional 235/65 17’s and have found my speedo is consisgently off by 2mph. 37mph on my speedo is 35mph in reality ( have checked it on half dozen different radar speed checkers). From this I suspect odometer readings are also off about 6%. Haven’t worried about this so far, but will probably have it adjusted when/if I put on 70 series tires (237/70r17).

 

[jkernitzki]

Ordered my BS withe the optional 235/65 17’s and have found my speedo is consisgently off by 2mph. 37mph on my speedo is 35mph in reality ( have checked it on half dozen different radar speed checkers). From this I suspect odometer readings are also off about 6%. Haven’t worried about this so far, but will probably have it adjusted when/if I put on 70 series tires (237/70r17).

So, consumer speedos aren’t the most accurate measurement devices out of the box, but they should at least be consistent. Note that police spec vehicles have “certified calibrated” speedos, while civilian spec is “good enough”. That being said,

- Is the error consistent across several speeds by amount or percentage? In other words, is that 2mph difference also at 70, or is it the 6% you stated? A constant 2mph error wouldn’t be tire related. A percentage error would.

- Have you compared your observations with other speed measurements, like GPS or a simple timed run between mile markers? I’ve seen some wildly inaccurate roadside radars, likely intentionally so.

 

[Bill G]

Yup, measured at different speeds, always 2mph high. Have checked it with several different “speed clocks”, all have been the same, with one exception — it showed a 1mph difference.

 

[jkernitzki]

Yup, measured at different speeds, always 2mph high. Have checked it with several different “speed clocks”, all have been the same, with one exception — it showed a 1mph difference.

Well, that’s interesting.

Do you have an all-digital display or combo analog/digital? If combo, do they concur with one another? (My Soul has both a physical analog dial and digital readout, both are in concurrence and match well to local roadside radar displays)

I’m wondering if it’s a “set point” value, or just normal variance from the BCM to the display.

What I do know is that it would drive me nuts.

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