Your original 12" tyre was about 552mm in diameter. The rolling radius was less than half of this, but for the purpose of calculus we will stay with published data.

552mm x Pi = 1734mm. Thats the circumference of your original tyre, & we will say that it is the distance your vehicle travels per one revolution

Your 175 x 65 x 14 tyre is listed at 584mm
Using the simple formula above we get 1835mm circumference.
Thats an increase of101mm or in real terms, 4 inches.

Your car now travels an extra 6% [approx] further for each wheel revolution.
Theoreticly this means that your odometer & speedo are 6% slow, however speedo's often read faster than actual speed by as much as 10% so it may be OK.
The idea is to get the odometer to read as close to the actual distance as possible, then any speed variation will be caused by the strength of the rotating magnets or the tension of the hairspring on the speedo needle.
The best way to check is to simply drive to a highway where they have one of those 5k long speedo check lengths & set the trip meter to zero as you pass go & keep tabs on it. Do not do this at warp 3, & stay in the left lane so you can pull up next to the 5k post to allow a carefull analasis of the final reading.

Alternately, take a long drive on a country road, the longer the better [over 100k is good] Compare the odometer readings with the signposted distances & the longer journey will give a more accurate indication of the variation. 3% over or under will be OK, just as long as you know which way it is so that you can mentally calculate your likely speed against the indication.
A 10% error probably needs to be looked at. Suck it & see.
Now isn't that what you need? Another excuse to go for a drive.
Chris