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General Forums => Air Speed Lounge => Topic started by: Matt on June 13, 2005, 05:24:32 PM

Title: Speedo Calibration
Post by: Matt on June 13, 2005, 05:24:32 PM

So the wife and I took out the 67 on the freeway this afternoon, and before I knew it I was doing 85 mph. But it didn't feel like I was going that fast. I am running 135's on the front. Is their a ratio to figure out what different tire sizes do to speedo calibration?  

Title: Speedo Calibration
Post by: Shane on June 13, 2005, 07:02:17 PM

get the wife to follow you in the pick up.. that should tell you

Title: Speedo Calibration
Post by: silas on June 13, 2005, 08:02:11 PM

you could calculate the circumference of your original tire by adding the size of the rim (15\") to 2 x the width of the sidewall, and then multiply by pie (3.14). this will give you the circumference of the wheel & tire in inches.

then divide 63,360 (= to 1 mile) by the circumference of your original tire to find out how many revolutions per mile it makes.

then do this with the 135. then find the percentage difference between the two. and then multiply that percentage by your mph reading. the 135 will be spinning faster (more revolutions) so the reading on the speedo will be faster than you are actually going.

165/50-15

sidewall=3.24\"s

2 x sidewall + wheel size = 21.50 diameter

21.50 x 3.14 = 67.53 circumference

63,360 divided by 67.53 = 938.25 revolutions/mile


135/50/-15

sidewall = 2.66\"s

2 x sidewall + wheelsize = 20.31 diameter

20.31 x 3.14 = 63.82 circumference

63,360 divided by 63.82 = 992.79 revolutions/mile

...so the 135 is spinning about 5.8% faster than the 165.

...so 85mph x 5.8% = 4.93

4.93 - 85 = 80.07mph (your actual speed)

now, 2 things i'm a little unsure of. 1 - my mathematical skills. 2 - the actual sidewall size of a standard 165 and a standard 135.

hopefully someone can chime in.

also, i dont know if this is the easiest way or the right way, but it seems to work. someone please correct me if i'm wrong.  B)
 

Title: Speedo Calibration
Post by: Matt on June 13, 2005, 08:18:04 PM

Holy shit professor. :o  

I thinks me unedumacted ass will just get the wife to follow me in the truck as you just confused the hell outta me. :huh:  

Title: Speedo Calibration
Post by: Mike Scott on June 13, 2005, 10:49:26 PM

165/50 and 135/50 those are some low pros  ;) . The sidewall percentage is much higher on standard 165 and 135's. The difference is 135 read about 12-15% faster, easiest way use a fabric tape and measure the circumference  of both tire then divide the two for the percent difference.  

Title: Speedo Calibration
Post by: silas on June 14, 2005, 05:21:32 PM

Quote

Holy shit professor. :o 

I thinks me unedumacted ass will just get the wife to follow me in the truck as you just confused the hell outta me. :huh:

i re-readwhat i posted and confused myself.  :lol:

Quote

165/50 and 135/50 those are some low pros  . The sidewall percentage is much higher on standard 165 and 135's. The difference is 135 read about 12-15% faster, easiest way use a fabric tape and measure the circumference of both tire then divide the two for the percent difference.

well that was a hell of alot easier.  

Title: Speedo Calibration
Post by: Shane on June 14, 2005, 05:38:27 PM

Yup Matt that is what I would do.....Get the wife to follow you down the  road. and you dont have to multiply shit