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Re: How accurate is your speedometer?
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Originally Posted by NeedForSpeed
Tony,
I hope the engineers come in with their calculators, but I believe that the tire circumference is the important number. In my test, the diameter difference may be 2% more, but the rolling circumference is 4% more, if I got it right.
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Circumference is pi times diameter so they are proportional which means the same 2.2% increase for your case
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Now, factoring my tire change:
3.70 gear with 235/40/18 at 3000 equals: 88.29 mph
3.545 gear with 225/50/16 at 3000 equals: 90.18 mph,
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Your 2.2% diameter increase effectively turned your 3.7 into a 3.62. That is one concern about going to a bigger diameter tire. It will send the ratios in the wrong direction for tranny life. On the other side we have speedos that read too high so we don't want smaller diameter tires to exaggerate the problem more. I think they all read high because the spring loses its springiness over time. Plus, the odometer will rack up the miles faster for a smaller tire diameter. Bad for resale.....
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Tony
1996 Polo Green Subaru SVX LSi, 168,XXX miles, Redline D4 ATF, Redline 75W90 gear oil, K&N HP-4001 Oil Filter, Mobil 1 5W50 FS (3qt) and 5W30 High Mileage (4qt) Oil Blend, Motul RBF600 Brake Fluid, AC Delco A975C Air Filter, NGK BKR6EIX-11 plugs, Centric Rotors, Power Stop Evolution Carbon Fiber Ceramic Brake Pads
2005 Gray Acura RL, 165,XXX miles, Redline D4 ATF with Lubegard Platinum Protectant, Mobil 1 5W20 High Mileage Extended Performance Oil
2009 Red Toyota Venza, 123,XXX, Mobil 1 5W30 High Mileage Oil
1992 Red Ferrari 348 ts, 82,XXX, Redline everything
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