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I have 14" steelies on my LX, and I bought a set of 15" wheels to replace them. I went to tirerack.com and it said I needed 195/60-R15 (which is also the tire size that comes with factory 15" Focus wheels). But I went to Miata.net's tire calculator and when I changed the tire size from 185/65/-R14 (my factory tire size) to 195/60-R15, it said that it would make the speedometer 3.2% too slow. Can someone help me out? Thanks.
 

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Shiroi kabocha
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you could go with a 195/55 instead and that wouldnt affect the speedometer at all, but thats not much tire and your car would look rather odd. if i had 15s id use either a 195/60 or 205/60, whichever one i could get a better tire in.
 

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Are the ECU's programmed for different diameter tires/wheels from the factory ?

i.e. the circumfrence of a stock tire on a 14" wheel is 73.73 inches and the circumfrence of a stock tire on a 15" rim is 76.07 inches (your 3% difference) so unless the cars with two different wheels are reading speed/mileage differently they would have to be programmed in the ECU.

I know this can be done since I can use an Xcal2 to program a different tire diameter (by changing the tire revs/mile) in my ECU but I was just curious if this is done at the factory as well.
 

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RespectMyAuthoritah!
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The rim size doesn't matter too much, as long as the offset is right. I know you can put 17's 205/45 on my car that had 195/60/15's. The outside diameter of the car didn't change. I can put my 17's and my 15's on top of each other and the overall diameter of the tires doesn't change.

Basically look at this site and type in 195/60/15 and then type in 205/45/17 and you'll notice that there the same exact size through the whole diameter of the tire.

Clicky

Now remember 205 is the width of the tire and 45 is the height from the edge of the rim to the outter edge of the tire and 17 is the size of the rim.

And also here's a sticky so,

Clicky Clicky
 

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Lildisco, your examples are just a tire size coincidence.

My tire calculator says 195/60/15 is 24.21 in tall and 205/45/17's are 24.26 in tall. Maybe the website where you are looking may have them truncated where they both show 24.2 ?


185/65/14's are 23.47 in tall, which is a larger difference (still not enough for me to worry about personally but....)

I agree that the rim size doesn't make any difference as long as it has a similar diameter (3% is similar enough IMO. If I want 3 digit accuracy, I'll just use the Xcal2 to adjust the revs/mile)

Does anyone know the answer to my ECU question ?



width height (in) circ.(in) Revs/mile
195 60 15 7.68 24.21 76.06612134 832.96
225 60 15 8.86 25.63 80.51877234 786.90
205 50 16 8.07 24.07 75.62085624 837.86
215 50 16 8.46 24.46 76.85770374 824.38
205 55 16 8.07 24.88 78.15639361 810.68
205 55 16 8.07 24.88 78.15639361 810.68
215 40 17 8.46 23.77 74.68085214 848.41
205 45 17 8.07 24.26 76.22691151 831.20
215 35 18 8.46 23.93 75.16322266 842.97
205 35 19 8.07 24.65 77.43902206 818.19
185 65 14 7.28 23.47 73.72847956 859.37
225 40 18 8.86 25.09 78.81192279 803.94
235 30 19 9.25 24.55 77.12981019 821.47
 

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RespectMyAuthoritah!
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They are a hair different and the revs per mile are not off that much to notice too much of a difference. The ECU is programed for whatever came on the car. But with the stock sizes you can go up or down a size and as long as your tires fall in roughly the same circumferance.

Here's the Tire Calc from the 2nd link i gave:

Tire 1 - 205/45R17 >Search Tires Tire 2 - 195/60R15 >Search Tires
Section Width: - -

Section Width: 8.07 in 205 mm

Section Width: 7.67 in 195 mm

Rim Diameter: 17 in 431.8 mm

Rim Diameter: 15 in 381 mm

Rim Width Range: 6.5 - 7.5 in

Rim Width Range: 4.5 - 7.5 in

Overall Diameter: 24.26 in 616.20 mm

Overall Diameter: 24.21 in 614.93 mm

Sidewall Height: 3.63 in 92.202 mm

Sidewall Height: 4.60 in 116.84 mm

Radius: 12.13 in 308.10 mm

Radius: 12.10 in 307.34 mm

Circumference: 76.21 in 1935.7 mm

Circumference: 76.05 in 1931.6 mm

Revs per Mile: 857.3

Revs per Mile: 859.1
 
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