i have 16" rims (16x7) with bridgestone potenza s-03 tires (205/50/16)... my speedometer will be off i think, so my question is... by how much will my speedometer be off? how much will it be off by percentage accuracy? i tried doing the math from the info they give at tirerack but i think my solution was incorrect so if anyone knows please help me n tell me thank you

 

Will it really be off by much at all? The JDM cars have 215-45-16, so it seems to me that 205-50-16 would be pretty much correct, wouldn't it?

 

You can use a tire calculator like the one at http://www.powerdog.com/tiresize.cgi

It shows that your size is 2.7 percent larger. That means that your actual miles will be 2.7 percent higher than the miles shown on your speedometer and odometer. Also, that the speedometer and odometer will show 2.6 percent less miles than actual. (2.6 percent = 1 - 1 / 1.027)

 

Ahh. So 60 on his speed is closer to 62, and 80 on his speedo is closer to 82.

I guess in some circumstances that could make the difference between passing through radar or getting a ticket.

Thanks, Ken.

 

205/50/16 Diameter
[205mm x (.50 x 2)] / 25.4 = 8.071" + 16" = 24.071"

195/55/15 Diameter
[195mm x (.55*2)] / 25.4 = 8.445" + 15" = 23.445"

Difference - [24.071 / 23.445] * 100 = 2.67%

This is how you calculate it without the tire size calc that Ken provided.

Take the width, times by twice the aspect ratio (50 = 0.50 or 50% of the width of the tire), change to inches, add the diameter of the rim. Compare/divide with the stock diameter... there you go.

At ANY speed, with 205/50/16 tires, you will be 2.67% SLOWER on the speedo than you would with 195/55/15 due to the increased diameter of the tire. (therefore less rotations per minute, so it appears you are going to slower to the speed sensor)

Diameter comparisons.
205/50/15 is 1.7% smaller than 195/55/15. (speedo reads higher)
195/50/15 is 3.3% smaller than 195/55/15. (see above)
215/45/16 is 0.7% LARGER than 195/55/15. (speedo reads SLOWER)

While the differences are minor (195/50/15 is only off 3.3mph at 100mph, half that at 50mph), these tires change your effective final drive ratio.

A tire like a 195/50/15 is so much smaller that you can feel the car moving through the gears quicker, similar to a higher final drive.

 

Actually, you will be 2.60% slower. When one number is 2.67% larger than another number, the second number is 2.60% smaller than the first.

 

Fixed the 215/45/16 error.

But about the above quoted part - I don't understand. The calculation says 2.67%, not 2.60%.

You won't actually be 2.67% slower, your speedo will read 2.67% slower then your actual speed.

 

No, no, no. It's a math thing. The percentage increase does not equal EXACTLY the percentage decrease, although the two numbers are very close to each other when the percentages are low.

Let's use some numbers where the percentage difference is bigger. Let's say one tire is 10 percent larger than stock. That means that when the speedometer and odometer think the car is going 100 miles, the car is actually going 110 miles. The odometer and odometer are going to read 9.1 percent (NOT 10 percent!) less than actual, because 100 miles is 9.1 percent less than 110 miles.

For the same mathematical reason, when the tire is 2.67 percent bigger, the speedometer and odometer read 2.60 percent less than actual.

 

whoops. My mistake. forgot about that in my haste to post somewhat uselful info.

thanks.

 

Ahhh ... maybe that's what's wrong with the 4.9 vs. 4.4 FD thread (on that other board). Hehe!