MPH vs taller tires

[heep70]

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.

[soobmater]

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

[TheSubaruJunkie]

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

 

Never got me out of the ticket tho

 

-Brian

[heep70]

Cool, I was close.

[Scott in Bellingham]

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

[soobmater]

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

[Andyjo]

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

[TurboSPFI]

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..

[Scoobywagon]

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.

[Qman]

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

 

Ken

[samo]

^ 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.

[heep70]

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.