Change tire size to correct speedo?

It's all about the visual. What the computer says I'm doing doesn't matter, I care what the speedo reads I'm doing. I'd rather have the speedo and my GPS be within 1mph from 0-80mph and be a few miles late on an oil change. Then to be constantly reminding myself that when I'm in the Tacoma, I need to set my speedo fast.

 

The speedometer needle can read different from the scangauge and the odometer because the needle itself could be off by a degree or something on the pin, the odo and scangauge readout would be driven off the ECU so they should be exactly what the ECU thinks.

However, unless the ECU determines speed with GPS, which I doubt, it will use either the ABS wheel-speed sensors or a sensor on the transmission output, both of which will change accuracy based on tire size unless you change the tire size constants in the ECU programming that the ECU used to calculate speed and distance from RPM.

With 285/65/18s my speedo reads around 5% slow compared to GPS.

 

I know this thread is from a few years back but I wanted to post my experience because most people kept telling me to get the right tire size and gear combo to get the speedo correct. Also there is a option called a yellow box from yellrbox.com but I was no good with cutting wires unless I had to.


So recently I upgraded my 2006 TRD with gears(4.56) and arb in the front. Before the install I check my speedo with gps and it was dead on accurate. So it is true that Toyota doesn't program the correctly from the factory. Bottomline 265/75/16 will correct the speedo. So after the install I just knew there was no way my speedo could be correct with jumping from 3.73 - 4.56. But it was dead on which I now know that the speed must becoming from the wheels. So then I went and purchased my Goodyear Wrangler Kevlar M/T R(if you can't tell I love them!) 285/75/16.


Now this is when the big problem started because at first I thought my gas mileage went way down and started thinking the gears might not have been such a good idea. But then I tested against the gps and found that I was about 5mph slower than I was actually going. Therefore the reason for a huge drop in gas mileage. So I ordered the adapter from hypertech (http://hypertech.com/ProductManuals/InLine/730107_VSS Instlltn Prcdr.pdf) which was very easy to install. The problem came in programming. Since I knew the gears didn't effect anything I didn't worry about programming for the gear change, but I did change the tire size. Since we all know that 265/75/16 made the system accurate I used that as the old setting and the adjusted to 285/75/16 and the speedo was very close. After trying to get it even closer I finally just used inches and found that 31.3 for the old and 33 for the new was about dead on.


Bottom result I didn't care what anything else read, I wanted my speedo to be at close to GPS as possible.