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*The Electrician*
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900 Posts
Discussion Starter #1
Car in question/theroy, 2017 Mazda 3 GT 18inch wheels(215/45/R18). Theoretically, say I wanted to run 205/50R16's next summer. Tire gap? bah who cares, not me. The speedo will be reading like 10km/h lower than actual speed. With modern technology, could I not take it to Mazda and have them adjust the speed sensors? Are they programmable like that yet? In the ideal world you plug into the ECU and just change the tire size on the programming and presto chango, magic, the speed reading is automatically adjusted to the tire size. Should take all of 2 minutes tops to do said change. Does that exist? Yes Id be paying twice yearly for the speedo to be adjusted and having tires mounted twice a year(until I have money for 2nd set of wheels) but honestly for 2 minutes of theoretical work for the speedo adjustment, should be like $60 TOPS for the adjustment.
 

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Hasan Tayanc
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14 Posts
You don't need to change any variables on the car. According to my dealer, speedo will adjust the visible speed automatically. The car will be calculating/comparing the speed via data from NAV and tire. If you have NAV hardware, SD card is not necessary for calculating.
I have 2017 Mazda 3 with 235/40/19 - Mazda6 wheels. The car adjusted the speed automatically.
 

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67 Posts
Anyone else have experience with this? Cuz I'd really like to go up in tire size if it won't effect the speedo
 

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Registered
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346 Posts
You don't need to change any variables on the car. According to my dealer, speedo will adjust the visible speed automatically. The car will be calculating/comparing the speed via data from NAV and tire. If you have NAV hardware, SD card is not necessary for calculating.
I have 2017 Mazda 3 with 235/40/19 - Mazda6 wheels. The car adjusted the speed automatically.
I had a 2014 and now a 2018 and didn't experience this auto correction / adjustment when I change my tire size...

205-60/16 - Speedo 120km/h - GPS - 122km/h
205-55/16 - Speedo 120km/h - GPS - 118km/h
225-45/17 - Speedo 120km/h - GPS - 119km/h


I found it quite surprising that the speedo reads low with stock size tires... rare as normally they show you 1/2km lower than actual. Really looks like they change from 205/55 to 205/60 without changing the speedo calibration. The 2012 we've got that comes with 205/55 stock reads the same as above...
 

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I just swapped my 18s for 16s with blizzaks for the winter. The computer adjusted for it because I'm running the same speed on my phone using waze. Nothing to worry about.
 
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