The speed on the speedometer is not the actual speed; it's less. When the needle indicates 70mph, I am acutally going about 68ish. This also means the odometer is racking up more miles than it's actually gone.

Kia won't do anything; the automotive industry is allowed up to a 5% error.

When it comes time for a new set of wheels, I am going to go one (1) size larger, which should effectivly bring the speedometer back to where it should be. The larger size wheel goes a further distance on each revolution, so that should fix it.

I wonder if the larger wheel will impact other things like speed sensors, computer input etc.

Anyone have any thoughts on this?

Before upgrading to 215/40 on 18 inchers (from 215/45-17 on OEM Koup SX wheels), my speedometer was a shade over 5% off (GPS cross-checked); actual was 94.7 Kph at an indicated 100 Kph. I'm now just over 1% off, actual 98.8 Kph at 100 Kph indicated.

If you stay within 5%, no problems. Even if you go larger than 5% difference, the car and its sensors won't know and won't care as long as all 4 wheels are the same circumference (no front-to-back stagger). Just means your speedometer error gets larger and potentially a problem if the constabularies in your area are particularly fascist...

Use 2PiR (2 X 3.1416 X total radius of wheel and tire) to calculate theoretical circumference for comparison.

Ex:

Koup baseline, 195/65 on 15 inch:

Radius: 15 inches x 25.4 mm/inch = 381 mm + (195 X .65 = 126.75) / 2 =

253.875 mm
Pi is a constant 3.1415929... but 3.1416 is good enough so:

2 X 3.1416 X

253.875 =

1995.15
OEM SX wheels+tires 215/45-17 is 1936.16, which is 2.86% less than

baseline;

My 215/40-18s are 1976.69 which is 0.8% less than

baseline.

The closer to

baseline, the smaller the speedometer error.
Those numbers are theoretical; actual numbers will differ slightly because the tires are not exactly and precisely XXX mm wide with a perfect XX% sidewall height. Go to Tire Rack on line and click to compare the tires you're considering; one of the criteria in the comparison chart is the number of turns per mile for each tire. You can then arrive at some pretty close numbers to base your decision on.

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