Changing wheel diameter doesn't affect speedometer calibration. Changing the overall circumference (pi * tire diameter) of the tires does. The speedometer is calibrated to how far the tires travel with each revolution.

Compare the overall circumference of your original tires to the new ones to determine the difference that you'll see reported by your speedometer.

 

I agree. Changing the rims changes the inside diameter of the tires. But the outside diameter is determined by the other tire parameters.

I'm not sure how to cross the tire size to get from one rim to the other. For example (out of thin air) suppose you have a P235-70R16, the 18" cross isn't just swapping 18 where the 16 is. One or both of the other numbers will change as well in order to maintain the same outside diameter.

The numbers involve ratios rather than exact dimensions, and I always have to look them up. Whoever came up with system of tire designations was just a little different, I think

That said, given the actual designation for the stock 16" tires, it shouldn't be too difficult crossing them to an equivalent 18".

Doug

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You also could use a tire calculator to see how far off your speedo might be. There are many of them on the web, you could try this one..https://tiresize.com/speedometer-calibration/

Sent from my SM-G930R4 using Tapatalk

 

I've done the exact same thing and the easiest way to check is to use a gps devise, easiest is your phone on waze or something similar that shows your speed and compare it to the cars speedo, you'll see just how far off you are if any...

 

Us old school guys would set the cruise control on 60mph and time the distance between the mile markers

Doug

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Not promoting this particular web site but it has a calculator that will tell you what the speedometer should read with different size tires/wheels.

https://www.discounttire.com/learn/...BRBUEiwAwaWjjM73J3lEkZO9k7c7s0q1wissGaIlDkJPS-WyH4QvuJw6NNVK6S2gwhoCUZIQAvD_BwE