Need a mathematical genius to answer my question
11-21-2006, 05:54 PM
#1
2nd Gear Member
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Join Date: Jun 2006
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Need a mathematical genius to answer my question
Ladies and Gents,
I just purchased some snow tires and stock 17's and have a question. I went down a couple sizes on tire to save $$$$, but I do have the tuner to compensate for the speedo changes. I believe it says revs/min. on the tuner and need to know what I should set it to. I got 225-45-17's and would like to know what I should set it to. Any help would be greatly appreciated. Thanks!
I just purchased some snow tires and stock 17's and have a question. I went down a couple sizes on tire to save $$$$, but I do have the tuner to compensate for the speedo changes. I believe it says revs/min. on the tuner and need to know what I should set it to. I got 225-45-17's and would like to know what I should set it to. Any help would be greatly appreciated. Thanks!
11-21-2006, 06:14 PM
#2
6th Gear Member
Join Date: Sep 2006
Location: Gainesville, FL
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RE: Need a mathematical genius to answer my question
808 revs/mi, the stock 235/55/17 is 742 revs/mi, your speedo will be 8.1% to fast if you don't change a thing with the smaller tires. So when your speedo shows 60mph your're actually traveling 55.1mph.
11-21-2006, 06:18 PM
#3
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RE: Need a mathematical genius to answer my question
Wow. I'm impressed. I'm not sure, but I think the tuner only goes by 5's. I assume 810 will be close enough. Is that correct? My original tires are 235-45-18's, what are the revs/mile on those? Thanks!
11-21-2006, 06:30 PM
#4
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RE: Need a mathematical genius to answer my question
Do the following
Divide diameter of NEW tire by diameter of old tire (whole tire, not wheel)
Square this number (multiple it by itself)
Multiple this by the speed you will now see on your speedometer to get true speed
For example... suppose your new tire has a diameter of 25", and the old one was 26.5
25 / 26.5 = 0.9433
0.9433 squared = 0.8898 (this is 0.9433 times 0.9433)
New ACTUAL SPEED = Speedometer Reading * 0.8898
53.4 MPH = 60 MPH * 0.8898
So, when your speedometer says 60 MPH, you will only be going 53.4 MPH
Equation can be used in reverse of coure....
Speed you see on your speedometer = Actual speed / 0.8898
So, when you are ACTUALLY GOING 60 MPH, you will see....
60 / 0.8898 = 67.4 MPH on speedometer
DON'T USE MY NUMBERS ABOVE, YOU MUST MEASURE DIAMETER OF YOUR TIRES YOURSELF
Divide diameter of NEW tire by diameter of old tire (whole tire, not wheel)
Square this number (multiple it by itself)
Multiple this by the speed you will now see on your speedometer to get true speed
For example... suppose your new tire has a diameter of 25", and the old one was 26.5
25 / 26.5 = 0.9433
0.9433 squared = 0.8898 (this is 0.9433 times 0.9433)
New ACTUAL SPEED = Speedometer Reading * 0.8898
53.4 MPH = 60 MPH * 0.8898
So, when your speedometer says 60 MPH, you will only be going 53.4 MPH
Equation can be used in reverse of coure....
Speed you see on your speedometer = Actual speed / 0.8898
So, when you are ACTUALLY GOING 60 MPH, you will see....
60 / 0.8898 = 67.4 MPH on speedometer
DON'T USE MY NUMBERS ABOVE, YOU MUST MEASURE DIAMETER OF YOUR TIRES YOURSELF
11-21-2006, 06:44 PM
#9
6th Gear Member
Join Date: Sep 2006
Location: Gainesville, FL
Posts: 11,278
RE: Need a mathematical genius to answer my question
I'm no genius, I use an online tire calculator.
http://www.miata.net/garage/tirecalcold.html
I believe your stock tires are 235/50/18's right? In that case it's 740 revs\mi and 8.4% to fast.
http://www.miata.net/garage/tirecalcold.html
I believe your stock tires are 235/50/18's right? In that case it's 740 revs\mi and 8.4% to fast.
ORIGINAL: swensicle
Wow. I'm impressed. I'm not sure, but I think the tuner only goes by 5's. I assume 810 will be close enough. Is that correct? My original tires are 235-45-18's, what are the revs/mile on those? Thanks!
Wow. I'm impressed. I'm not sure, but I think the tuner only goes by 5's. I assume 810 will be close enough. Is that correct? My original tires are 235-45-18's, what are the revs/mile on those? Thanks!
11-21-2006, 07:19 PM
#10
5th Gear Member
Join Date: Apr 2005
Location: Babylon, NY
Posts: 3,031
RE: Need a mathematical genius to answer my question
Although, some tires of the same size have different revs/mile. Why, I don't know. But go to tirerack.com, if you can find your tires there, look at "sizes and specs" and they list most of the revs/mile for their tires.