I'm wondering whether my newly fitted 175/65/14 tyres with minilites will mean that I will have to get my speedo re-calibrated? I know the original ratio is 17/5 and would my wheels make a big difference to this?

I`ve got 175/60/14 on my ute & the speedo is damn near perfect !
A 65 series tyre isn`t all that much larger in diameter. So if you can live with the slight speedo in-accuracy(when you know how far out it is)shouldn`t be too much of a problem!

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

Unless you want the +/- 3km/hr accuracy the fine police in Victoria require.

The speed cameras here now give you a +/- 3km/hr tolerance on your speed...ridiculous in the extreme but who are we the drivers to complain.

Ooops turn off the sook tap, but Dodgeman has it, wheel circumference is a good way to tell.

Or more simply, but more generally. Get a mate with a car that is correct, and follow him on the highway / freeway and call him on his mobile and get him to do set speeds 60 70 80 etc and see the difference between what his is reading and yours.
Note: the car you follow needs to be standard so it might be better following your mum
-Cal

Mum had better have a hands free, otherwise she'll pull over to answer the phone.

To G.T.D.D.
Thats not a very good way to do it, & how do you know that your mates car is accurate?
Using your method will get a result, but it will always be suspect.
Use "Plan A". Do the measured distance test for results that can be verified & repeated.
Chris

just the quick and easy way, not the best way, that was my point, as an indication and nothing more, i agree with you for a solution tho.
-Cal