It will be a % that both are off. In my LX with 286/75/16 Toyo MTs IIRC the error was around 4% under. i.e. both the speedometer and the odometer would read about 4% lower than actual.
You can get new speedo gears to correct that (or get close) from Slee etc. Or you can get fancy and get a Mark's speedo correction box or similar. Or you can do what I did if you have a Scangauge II and have it display your correct speed. Or you can just know in your head about how far off you are
Since not all tires are the same the best way to know for sure how far you are off is to use a GPS and then check at various speeds and do the calculations so you know. If you track you MPG then you need that to "up" the number of miles driven to get an accurate number.
IMO knowing what I know now having done most of the above I'd just go straight to buying a speedometer correction box. Buy one that allows you to change the settings so if you go up to 35s or regear down the road you can change the settings to match. Not such a big deal with 33s but sending the correct speed to the ECU does help with transmission shifting etc