Speedometer calibration, or knowing how fast you're going, can be pretty important. We all know a ticket is about the biggest buzz kill on earth! So if you're about to get bigger tires than stock or already have them installed, its time to calculate your true MPH.
Bigger tires will cause your speedometer to register a speed that is slower than your actual speed. For example, if your speedometer says 35 miles per hour on 20-inch diameter tires, changing to 23-inch diameter tires will make your true speed 40.25 mph, even though your speedometer will still show 35 mph.
Speedometer ratio adjustment calculator. Ever wonder how far off your speedometer is with your new bigger tires? I'm using a simple ratio calculation, so this info will only approximate your corrected speed and depends on the accuracy of tire size diameters (assumimg nothing but tire size has changed).
Changing the stock parts, including the vehicle's tires, can throw off the speedometer calibration; larger tires cause the speedometer to read a speed slower than the vehicle's actual speed while smaller tires cause the speedometer to read a speed faster than the vehicle's actual speed. You may recalibrate a speedometer in a few simple steps.
How to Fix Your Speedometer for Different Size Tires. Have you changed your tire size or final-drive gear? You just made your speedo and odometer inaccurate.
But, if a car is modified, its speedometer may need to be recalibrated. Changing tire size is one of the most common things car owners do that can affect speedometer accuracy. That’s because larger tires cover more ground in one complete revolution. Consider the example below.
Ever wonder how far off your speedometer is with your new bigger tires? I'm using a simple ratio calculation, so this info will only approximate your corrected speed and depends on the accuracy of tire size diameters (assumimg nothing but tire size has changed).
Tire Size and Speedometer Accuracy . If you are planning on, or have already replaced your original equipment tires with a different tire size, it is important to remember that changing your tire size can affect the accuracy of your speedometer.
When you put different size tires on your vehicle, your speedometer will no longer read the correct speed. Larger tires cause the speedometer to show slower speeds than reality, and smaller tires show faster speeds. You can reprogram a vehicle when changing tire size using an on-board diagnostics system, or OBDII. ...
I replaced my 29.875 +- tires with 32.5 inch tall tires. I put 32.5 into the gear/tire calculator and committed to the changes. The speedo reads 42 when I am doing 40 by GPS. I changed the tire size in the calculator again to try and compensate and I noticed that in the general table, it changes the General VSS pulses per mile, but does not change the VSS output pulses per mile.