I have a 2012 Accord EX-L that I just changed out the wheels and tires. I went from the stock 17's to 18's. I made sure to get the tires as close to the stock rotating diameter/mass as possible but of course you can't get it perfect. The thing I'm noticing is the navi seems to act like I'm going faster than I actually am. Now before I purchased these wheels and tires I did my research and used a couple different tire calculators to check the difference. Each one showed a difference of .22 miles per hour at 65mph. Stock at 65mph aftermarket 65.22.

The question I have is, is there a way to re-calibrate the computer to the size of the new tires? One would think that maybe the dealership might have something as you can get 18's as an option......

 

There is a TPMS threshold writing that is usually done when changing tire sizes, not sure if that will fix the Navi; if there really is an error there.
In general terms, the procedure they do is to enter the TPMS Mode Menu, select Reprogramming for accessory tire, re-write the threshold data, enter the 10-digit tire information printed on the new tire pressure information label (Honda wheels), and adjust tire pressure, etc. for non-standard tire.

Generally, a GPS is not dependent on the tires to calculate vehicle speed; for example, stand-alone GPS units such as Garmin, Tom Tom, Magellan, etc., etc. are not tied into the vehicle for calculation of speed. However, who knows. I would expect the speed error reading would be at your speedometer, not the GPS display.

 

So in the first paragraph you address the TPMS. Can this only be done at the dealership?

Second paragraph, I totally agree. I wouldn't think it would go off of the speedometer at all. I just wonder why if was acting crazy. What it did was I was going to a new place and I inputted the address and it was 74 miles away. It said it would take me 72 minutes to get there. I could understand if I was traveling at over 100mph but I wasn't.....

 

Many independent tires places can program in new TPMS sensors; however, their software may or may not have the capability to access certain menu items with the car's computer.

Sometimes GPS (all makes) are known to get "confused" if they don't know their true position, principally due to satellites reception. In your instance, it may not have have calculated the estimated time to destination based on its actual current position; but, made a calculation based on some previous start position.

 

74 miles in 72 minutes is 62mph, not 100. Still, that doesn't account for slow traffic or residential streets. I don't know what kind of roads you're on.

 

JimBlake, you're correct. I missed that one, didn't do the numbers.

My own GPS got confused a number of times, not so much with estimated time; but, how to get somewhere. One time my GPS had me literally go in a big circle several times, I had to drive away from the suggested route for a while before it gave me the correct route. Another time when I was driving to San Diego, CA, it told me to get off the freeway and then had me get back on.