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Car Forum / Driving, Maintenance, Tuning / Maintenance and Repair / July 2007Tire Size vs Speedometer
Math never was my strong point. Curious how much my speedometer will be off now that I have went from P215 75R15's to P215 65R15's. Anyone who can help, thank you. I'm thinking, after driving a couple days that speedo showing 75 means I'm doin about 65ish. Try telling that to the police. Use a stopwatch and highway mile markers to see how far off the speedometer is. > Math never was my strong point. Curious how much my speedometer will > be off now that I have went from P215 75R15's to P215 65R15's. Anyone > who can help, thank you. I'm thinking, after driving a couple days > that speedo showing 75 means I'm doin about 65ish.  SignatureMike Walsh West Palm Beach, Florida, U.S.A. | > Anyone who can help mysterios ( Mr know all / con men ) deserve no info > | > Anyone who can help > mysterios ( Mr know all / con men ) deserve no info beep chow Uranus SignaturePostulate a group whose intent is to destroy the United States from within via anarchy and bankruptcy. The actions of the United States Congress are completely consistent with the actions one would predict from such a group. > Math never was my strong point. Curious how much my speedometer will > be off now that I have went from P215 75R15's to P215 65R15's. Anyone > who can help, thank you. I'm thinking, after driving a couple days > that speedo showing 75 means I'm doin about 65ish. I recommend a GPS or measured mile to check. How much it is off compared to the other tires really doesn't relate to the actual road speed unless the previous setup was calibrated first. Other than that when I want to find out, I go to the tire seller's sites and get the real height or diameter of the tires and compare. Mike 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's - Gone to the rust pile... Canadian Off Road Trips Photos: Non members can still view! Jan/06 http://www.imagestation.com/album/pictures.html?id=2115147590 (More Off Road album links at bottom of the view page) > Math never was my strong point. Curious how much my speedometer will > be off now that I have went from P215 75R15's to P215 65R15's. Anyone > who can help, thank you. I'm thinking, after driving a couple days > that speedo showing 75 means I'm doin about 65ish. More like 80 indicated = 75 69 indicated = 65 58 indicated = 55 http://pw1.netcom.com/~sgalaba/tiresize.htm -- Steve W. Near Cooperstown, New York > Math never was my strong point. Curious how much my speedometer will > be off now that I have went from P215 75R15's to P215 65R15's. Anyone > who can help, thank you. I'm thinking, after driving a couple days > that speedo showing 75 means I'm doin about 65ish. It's because a smaller wheel spins faster to cover the same distance as a bigger wheel. The rim size of your tire is the same, but the sidewall is shorter (lower aspect ratio) so the overall diameter of the tire is less. But I'm curious to know if this is an issue with modern-day cars, most of which use an ABS wheel speed sensor for the vehicle speed. (It probably still is an issue.) Years ago it was, with a gear in the transmission driving a semi-flexible shaft that went to the speedometer housing in the instrument panel. >> Math never was my strong point. Curious how much my speedometer will >> be off now that I have went from P215 75R15's to P215 65R15's. Anyone [quoted text clipped - 11 lines] > transmission driving a semi-flexible shaft that went to the speedometer > housing in the instrument panel. Most cars use a vss (vehicle speed sensor) which is still in the transmission. A lot of cars can be reprogrammed to the new tire size. >> Math never was my strong point. Curious how much my speedometer will >> be off now that I have went from P215 75R15's to P215 65R15's. Anyone [quoted text clipped - 10 lines] >driving a semi-flexible shaft that went to the speedometer housing in the >instrument panel. Yes, it is still an issue. Both systems simply measure the car's wheel rotation, the latter directly and the former indirectly via an axle ratio. Both make an assumption of the wheel/tire's circumference in their speed calculations, which has decreased... so the same number of rotations will result in less distance driven. Imagine replacing the wheels w/rollerskate wheels. Your speedo would say 100MPH and you would be doing like 5MPH --Ken SignatureKen R. Dye an optimist is a guy | Chicago, Illinois that has never had | www.geocities.com/MotorCity/Track/8746 much experience | dye1146 at sbcglobal dot net archy | How much will my speedometer be off now after I changed from P215-75R15's to P215-65R15's. ______________________________________________ The following method works for all tire substitutions: 1.) Measure: Diameters of the new and the stock tires. 2.) Calculate: Ratio = NewDiam/StockDiam. 3.) Calculate: Actual Speed = (Speedometer Reading) X Ratio. EXAMPLE: I bought new tires 30 inches diameter to replace stock tires of 31 inches diameter. I calculated my ratio as: Ratio = NewDiam/StockDiam = 30/31 = .968 Now when my speedometer reads 75 MPH I am really traveling at: Actual Speed = (75) X .968 = 72.6 MPH. Good luck. Rodan. > How much will my speedometer be off now after I changed from P215-75R15's > to P215-65R15's. ______________________________________________ [quoted text clipped - 19 lines] > > Rodan. Rodan, you monster! Go here: http://www.miata.net/garage/tirecalc.html You even get a cool little picture! (I did a spread sheet that calculated all that, but darned if I can remember where I backed it up to...) > Math never was my strong point. Curious how much my speedometer will > be off now that I have went from P215 75R15's to P215 65R15's. Anyone > who can help, thank you. I'm thinking, after driving a couple days > that speedo showing 75 means I'm doin about 65ish. The diameter of the tire (not the wheel) governs how far the car travels for each 360 degree rotation. The basic formula 2*Pi*R will give you the circumference of the tire. A much simpler method would be to ask the tire dealer whether the tires now on your car are the original diameter. > Math never was my strong point. Curious how much my speedometer will be > off now that I have went from P215 75R15's to P215 65R15's. Anyone who can > help, thank you. I'm thinking, after driving a couple days that speedo > showing 75 means I'm doin about 65ish. Go here: http://www.miata.net/garage/tirecalc.html On Jul 10, 1:51 pm, Hachiroku <Tru...@AE86.gts> wrote: > > Math never was my strong point. Curious how much my speedometer will be > > off now that I have went from P215 75R15's to P215 65R15's. Anyone who can [quoted text clipped - 4 lines] > > http://www.miata.net/garage/tirecalc.html Thank you so much, gotta love that site! |
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