# Thread: Crap... now i need a 9500i!

1. ## Crap... now i need a 9500i!

Because I got wheels 2 inches bigger than factory and it would be nice to know my true GPS speed

wil the car read lower or higher than actual?

2. Looks like someone needs to find a 'your speed is' sign.

3. It isn't the diameter of your new wheels that make the difference. It is the circumference of the tire contact surface. The progression is linear so if your new tire circumference is 5% less than the OEM setup, your speedometer read 5% higher (small diameter makes 1 revolution faster than the larger one).

Here's a calculator that will help: http://www.csgnetwork.com/tireinfo4calc.html

4. ## Re: Crap... now i need a 9500i!

Originally Posted by f_399
Because I got wheels 2 inches bigger than factory and it would be nice to know my true GPS speed

wil the car read lower or higher than actual?
Navalert is half the price and has headings (ability to add direction to user positions limiting false alarms from other directions) and a real live database with updates - SIMPLE!

Chris

5. stock
235 45 17

new

215 35 19

215... its much thinner?

6. Although your new wheels are larger, your new tires are actually slightly smaller in diameter, and thus circumference, than your old tires. 1.59% different, so at 50 mph, your speedo will read 50.81 mph.

215 35 19 vs 235 45 17: 1st number is the width of the tire, in mm. 2nd number is the aspect ratio (sidewall height = width * AR / 100, so 215 * .35 = 75.25 mm or 2.96 inches). 3rd number is rim diameter in inches. Diameter of the tire = rim diameter + sidewall height * 2, so your new tire is 19 + 2.96 * 2 = 24.92" in diameter.

Your old tire is 235 45 17, so that translates to 235 * .45 = 105.75 mm sidewall or 4.16". 17 inch rims gives you 8.32" of sidewall diameter + 17" = 25.32", a difference of 0.4 inches.

7. There's probably an inch difference since the rim is 2 in. longer in diameter old-17 and new 19 and you just dropped the side wall by 10. The configuration of the tire size should have been 215-25-19 to be equal to 235-45-17. However, i have not seen a 25 series tire and it would look like your wheels won't have tires - very thin rubber if they are available and the price will be extremely high.
When you increase the wheel size, you are suppose to take 10 off the side wall. So plus 2 in. would translate to minus 20 for the side wall height. Your speedometer reading will be higher but your actual speed will be lower by less than 4-5miles per hour.

8. yeah, even if the rim is biger its the overall diameter of the circle that matters (Wheel + Tire = Total Diameter) if thats the same its all good, and it looks like since the aspect ratio is smaller on the new tires that the speedo will be about equal

on a look up hers how it stands on paper, note that the faster youre going the further off it becomes, but good thing for you the actual is slower than the indicated

20----->19.5
30----->29.5
40----->39.3
50----->49.2
60----->59
100--->98.4

9. Your 19" combination is approximately 24.9" in overall diameter, the 17" combo is approximately 25.5" per Tirerack.com.

10. You have only hurt your car by your changes. it came with sporty 235 17's and you put some pointless bling on it with a 215 tire. Your 19's probably way a ton and you now have less rubber on the road. Its like shooting yourself in the foot. I hope you really like the looks of your new ride because the performance of your car is severely hurt.

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