Ok, I am board here at work, so I decided to re-arrange some formulas to figure out how fast I am really going (MPH) with 26" tall tires. I come up with a couple different answers not far from each other, but well worth a ticket if I choose the wrong one.

 

So now I come to you folks. What is the MPH offset I will be seeing running 26" tires? I am close to 5 - 6MPH (9.1%) difference (faster) at 55MPH on the speedo.

http://www.intercotire.com/site62.php that should help

I calculated something like 11% off with my 26.1" tires.

 

Never got me out of the ticket tho

 

-Brian

Cool, I was close.

with my wore 215/75-15 tires (close to 27") when my speedo reads 50 the car is going 60 in a 88 GL wagon Scott

with my wore 215/75-15 tires (close to 27") when my speedo reads 50 the car is going 60 in a 88 GL wagon Scott

 

 

 

yup thats just like are's

Shouldn't it just be a simple ratio of the original tire size to the new? (original being the size the spedo' is set to)

 

if v = w*r (v= velocity, w = angular velocity, r = radius), and we give em subscripts (1 = original, 2 = big tires)

thennn...

 

v1/r1 = w = v2/r2

soo...

v2 = (r2/r1)*v1

yeah, that's right, i'm a physics major, and proud of it!

so i guess if you wanted the 'percentage' you could just take the ratio of the tire diameters... seems right to me

I've been pulled over, speedo read 40, cop said inbetween 50 and 55

 

215/75R15 tires

 

also, there are these little cop boxes that radar you and display your speed as you approach them.. 35 speedo = 42 actual

 

Just my experiences..

I bulit an Excel spreadsheet that has the formulas in it. Interested?

 

In the short term, your stock size is 23.2" (185/70R13). Your new size is 26", so your speedo is reading slow. When you are indicating 50, you are actually doing ~56. Its a percentage expressed by (old tire diameter)/(new tire diameter). In this case 23.2/26=1.1206. Multiply by your current diff ratio and you get a ratio that brings your speedo back into line. 1.1206*3.7=4.15. If you want to use a really good set of calculators, go to www.4lo.com . They've got the best.

Greg, try this, http://www.miata.net/garage/tirecalcold.html

 

Ken

^ an excellent resource. However, since every tire is a little different (not all 205/65/14s are the exact same height, for example), I've just resorted to figuring out the difference myself.

 

(New tire diameter / stock tire diameter) * indicated speed = actual speed

 

So, my stock tires are 22.65" and the new ones are 24.5". 24.5 / 22.65 = 1.0816, an 8.16% difference. So, at 60 indicated MPH, I'm really going 64.9.

 

When I go to 27" tires (27 / 22.65 = 1.1921 = 19.21% difference), 60 indicated will be 71.5 MPH.

 

You can do the math with either diameter, radius, or circumference, the results are the same.

See what happens when I am board at work? Now look what I have started.

 

 

 

Thanks folks for the replies. I have may answer, a close one anyway.