spartan Posted January 20, 2009 Share Posted January 20, 2009 Well, I have noticed with my GPS that my car is consistantly reading 2-3 mph faster than what I am actually traveling. I am currently running stock sized blizzaks on 7.5" wide rims. What size tire would I need, to get the speedo closer to reading correctly? Link to comment Share on other sites More sharing options...
sactojesse Posted January 20, 2009 Share Posted January 20, 2009 2-3 mph at what speeds? It should be off by a percentage, not by a fixed amount at any speed. The relationship between tire diameter and speedometer reading is an inverse linear relationship, i.e., larger rolling diameter = reduced reading. Assuming there's a 2 mph difference at 60 mph, that is a 3.3% error, so aim for a tire diameter that is 3.3 % larger than stock. Stock 215/45R17 has a rolling diameter of 24.6 inches. 225/45R17 has a rolling diameter of 25 inches, 1.5% more than stock. 215/50R17 has a rolling diameter of 25.5 inches, 3.5% larger than stock. Link to comment Share on other sites More sharing options...
spartan Posted January 20, 2009 Author Share Posted January 20, 2009 Thanks for clearing all of that up. Interestingly, it is not an increasing or decreasing percentage, it always appears to be 2-3 mph off. I have checked at 25 mph and 80 mph and it is always reading that little bit high. Link to comment Share on other sites More sharing options...
el5y Posted January 20, 2009 Share Posted January 20, 2009 You should check into getting your speedometer calibrated then.. Link to comment Share on other sites More sharing options...
el5y Posted January 20, 2009 Share Posted January 20, 2009 or you could be like me and just leave it as a safety buffer between speeding and getting caught Link to comment Share on other sites More sharing options...
sactojesse Posted January 20, 2009 Share Posted January 20, 2009 Thanks for clearing all of that up. Interestingly, it is not an increasing or decreasing percentage, it always appears to be 2-3 mph off. I have checked at 25 mph and 80 mph and it is always reading that little bit high. Then it's not your tire size. Stay w/ 215/45R17 and calibrate your speedo. Link to comment Share on other sites More sharing options...
Jerboa113 Posted January 21, 2009 Share Posted January 21, 2009 I have to say when I saw "speedo off" i had to check if it was in general funny or not. how do you adjust for tire size on these cars? tuner have it? I've never looked very hard. "The penalty good men pay for indifference to public affairs is to be ruled by evil men." - Plato Link to comment Share on other sites More sharing options...
spartan Posted January 21, 2009 Author Share Posted January 21, 2009 I have asked the dealer about recalibrating the speedometer and they said that it could not be done. That's why I figured I would ask here to see if anyone knew anything about it. Link to comment Share on other sites More sharing options...
Jerboa113 Posted January 21, 2009 Share Posted January 21, 2009 orly.. then how is it the spec B has a larger tire diameter? Do they just sell spec Bs with incorrect speedos? There has to be away to calibrate it. "The penalty good men pay for indifference to public affairs is to be ruled by evil men." - Plato Link to comment Share on other sites More sharing options...
spartan Posted January 21, 2009 Author Share Posted January 21, 2009 I was thinking the same thing, especially since the dealer that told me this is quite incompetent. Link to comment Share on other sites More sharing options...
sactojesse Posted January 21, 2009 Share Posted January 21, 2009 Your speedo is likely performing according to factory specs. There's an EU requirement that prohibits vehicle speedos from displaying speeds lower than actual speed, so most manufacturers err on the side of caution by making the speedos read higher than actual speed. I surmise that the calibration for U.S. and EU metric spec speedos is similar. I'd recommend relaxing your sphincter and living with the discrepancy. Link to comment Share on other sites More sharing options...
sooberoo Posted January 21, 2009 Share Posted January 21, 2009 I'd check to make sure your GPS findings are correct before swapping out your tires or recalibrating your speedo. Here in Alberta, the motor club has speedo test sections set up here and there. Measuring the time it takes to go a given distance will provide you with a far more definitive answer of your speedo's accuracy than what your GPS will. Link to comment Share on other sites More sharing options...
Scooby Fan Posted January 21, 2009 Share Posted January 21, 2009 http://www.miata.net/garage/tirecalc.html That will tell you what you need to know. Link to comment Share on other sites More sharing options...
spartan Posted January 21, 2009 Author Share Posted January 21, 2009 The reason I even noticed it was that my WRX was spot on with my GPS, while driving this car it just threw me off a little. Link to comment Share on other sites More sharing options...
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