Absolutely, if your speedo is indicating below actual road speed, you need smaller tires if you want to correct it.

OR

Let your tires wear out a bit, problem solved!

Many moons ago I used to have an old S10 4WD Blazer with extra large tires on it. Took a speeding ticket for me to realize the speedometer would read low with those tires...

 

Except, the difference in the tire sizes the calculator shows to correct the 3mph error is 1.299 inches....that, I believe, is more than the total depth of the tread on the tires. Of course, it would need to have half of that, or .6495, but I don't think there is that much usable tread either.

 

If anyone gets success with having the dealership correct their speedo error, please advise... I for one, would like speedo accuracy. (I'm also the kind of guy that sets every clock in his house @ gps or atomic time.)

Regarding taller or shorter tires, I'm in agreement with others on this thread that say if your speedo is OVERSTATING your speed (and I believe for those who have inaccuracy, this is what they're claiming,) you would want a TALLER sidewall meaning a LARGER overall diameter which also means a LARGER circumference.

In my mind's eye, I picture a cheerio or an inner tube...the inner tube has greater circumference, so at the same revs/min you are covering more ground with a greater circumference. Hence, if your speedo reads 60mph, while your gps/mile markers tell you you're actually travelling 57, you would want MORE circumference or a TALLER sidewall on the tire to bring you up to par.

I'm not introducing anything novel here, it's just what others have stated, but since it got my mind fuzzy until I thought of it this way, I figured it might help someone else in their understanding.

Then again...I could be completely full of bullshot.

 

you would want MORE circumference or a TALLER sidewall on the tire to bring you up to par.

After a good nights sleep I decided not to over analyze this and I am back to the conclusion I had before I confused myself. Any yes I agree (Ken), if my speedo is reading too high I need a tire that caused the speedo to read lower, thus the taller tire.

(Of course that means if I run a 215/70R16 tire and my difference is 6.45% then a 100,000 mile warranty can turn into a 106,450 mile warranty. Might be worth something to some of you.)

 

I got my Garmin M5 iQue and took it out today to check my spedo. It appeared that I am "only" reading 1.5 mph over at 55, 60 and 70. I would have thought from the posts that it would have been more difference than that. I also thought that the error would be a % off so that the faster I went the more off it would be but at those three speeds I checked the error was insignificantly different.

I can't complain about this degree of error.

I am curious with all the info on the NAV why they didn't include the "standard" speed readout.

 

Let's not get too crazy here. I have an Astro Van that I upsized the tires on. I clocked (measured) the travelled distance (roadside signs on the highway) versus the odometer reading. It was within 0.5% (0.25mph at 50mph). I thought this was great. However, as time goes on, the tread wears down, and the odometer starts to creap up. The tread on the van started at >0.5" deep, by the time I got new tires, the ODO was higher, although slightly. Also, the preasure in the tires will have a second order effect to the measured distance.