heep70 Posted July 29, 2005 Share Posted July 29, 2005 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. Quote Link to comment Share on other sites More sharing options...
soobmater Posted July 29, 2005 Share Posted July 29, 2005 http://www.intercotire.com/site62.php that should help Quote Link to comment Share on other sites More sharing options...
TheSubaruJunkie Posted July 29, 2005 Share Posted July 29, 2005 I calculated something like 11% off with my 26.1" tires. Never got me out of the ticket tho -Brian Quote Link to comment Share on other sites More sharing options...
heep70 Posted July 29, 2005 Author Share Posted July 29, 2005 Cool, I was close. Quote Link to comment Share on other sites More sharing options...
Scott in Bellingham Posted July 29, 2005 Share Posted July 29, 2005 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 Quote Link to comment Share on other sites More sharing options...
soobmater Posted July 29, 2005 Share Posted July 29, 2005 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 Quote Link to comment Share on other sites More sharing options...
Andyjo Posted July 29, 2005 Share Posted July 29, 2005 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 Quote Link to comment Share on other sites More sharing options...
TurboSPFI Posted July 29, 2005 Share Posted July 29, 2005 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.. Quote Link to comment Share on other sites More sharing options...
Scoobywagon Posted July 29, 2005 Share Posted July 29, 2005 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. Quote Link to comment Share on other sites More sharing options...
Qman Posted July 29, 2005 Share Posted July 29, 2005 Greg, try this, http://www.miata.net/garage/tirecalcold.html Ken Quote Link to comment Share on other sites More sharing options...
samo Posted July 29, 2005 Share Posted July 29, 2005 ^ 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. Quote Link to comment Share on other sites More sharing options...
heep70 Posted July 29, 2005 Author Share Posted July 29, 2005 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. Quote Link to comment Share on other sites More sharing options...
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