Tire size calibration

[Brusky]

I got new larger tires. Which of course threw the speedo off, so no big deal, came home looked tire height up on line. (33.3in) so I reprogrammed with my superchips 1715 to 33in. And it threw the speedo off even farther but in the other direction saying I was doing 3mph faster than I actually was. So I then put a tape measure to them while still on the vehicle and it measured 32in. So I set the programmer to that and it's still off saying I'm doing 1 mph faster than actual. Any help would be greatly appreciated.

All checked with 2 different GPS devices that both read the same

 

[Flightstopper]

Just keep in mind how much its off. Mine says im doin 50 when im really doin 60. I just remember its 10mph off.

 

[GAINMOB]

i think 1mph is good enough...if you want it more accurate then you may need to go to a performance shop or to dealership to make it exact

 

[rocketvette95]

Most of these newer Vehicles the speedo is operated off a speed sensor located in the wheel bearing. When I was installing bigger tires within reason the speedo would not be off but maybe a mph. Now you can't install 44's but if you install a 33 or so the Vehicles computer will compensate

 

[FordandPolaris]

I agree, it is not going to be exact when you change from the stock tire size. 1mph is definitely pretty close, there could just be some error in the gauge too. Mine reads something like 5mph slow at highway speed, but near normal at in town speeds. Makes sense because the gap gets proportionally larger the faster you go.

 

[toms89]

Most of these newer Vehicles the speedo is operated off a speed sensor located in the wheel bearing. When I was installing bigger tires within reason the speedo would not be off but maybe a mph. Now you can't install 44's but if you install a 33 or so the Vehicles computer will compensate

Not following you? Computer has no way to compensate. The pcm needs to know the distance the tire travels per revolution to accurately calculate speed regardless of where the tire rpms are read from. I can tell you with certainty that the first gens calculate vehicle speed via the output shaft on the tranny. So both gear ratio and tire size are input to calculate vehicle speed.

The software in the pcm uses tire revolutions per mile which has a very low percentage of error if you can use the manufacturers published info on a tire. I have access to this via my sct advantage software.

I believe hand held tuners more or less calculate this when you input tire diameter but then there will be a much higher percentage of error there. Diameter is typically rounded off or figures are simply just inaccurate.