2010 XC70 Speedometer error

[Mark144]

We just bought our new to us 2010 xc70. So we decided to take a day trip. After a few miles I noticed we were being passed with too much regularity. I finally realized after a few measured miles and then a check with the g.p.s. unit, we were traveling 4 mph less than the speed on the speedometer. Varying the speed seems to indicate a consistent 4 mph error. My question is, Is this a fixable issue by computer adjustment or will the head have to be pulled and adjusted? I am planning to fuss enough about it as a "certified car" purchase that they will take care of it.
AND
I have been offered the extended warranty package for 2700$, covers everything, on a 2010 with 67000 mi on the clock, should I? The warranty will carry me another 48000 miles.
On the up side, it rides great, looks good, and wife is happy.
thanks for your input in advance.

[jimmy57]

There is no physical adjustment of today's electronic speedos. The speedo can be adjusted via settings that can be changed using VIDA. It might be a software download.

That is likely a good deal on the warranty and they can be negotiable on those warranties.

[Mark144]

jimmy, thanks for the reply but I suppose I should come clean with the rest of this adventure. I feel like the dumb one in Dumb and Dumber. First rule of buying a used anything is to see if it has been changed in any way from stock. After I looked at the tire size and the recommended size, I eventually found out that the tires on it would change the speedo 3.6 mph. I guess I will deal with it until I go back to original size. I thought the tires looked a bit thin for an XC model.

[oragex]

Indeed, it is most probably because the actual tires have a smaller diameter compared to the original ones.

As for the warranty, it's certainly appealing but I find it a bit expensive. Around $2000 if negotiable, it would be a better deal and also, quite important to read what exactly is covered and what is excluded.