Changing Tyre sizes

Suggest you go for a drive with a GPS on the dash and check your current speedo against real - the larger tyres could actually correct it.

Gday
It will put your speedo out by up to 5 kmh.
I am pretty sure Marks Adapters make a correction device you can buy.
Not sure how much.
Try here
MARKS ADAPTERS
I had 285/75x16 on my 80 series and wouldn't have them again. When they were worn out I was going to go back to 265/75x16. I had nothing but balance trouble and speedo was out. Besides the 265's are cheaper, more common and easier to acquire out in the middle of nowhere in an emergency.
BTW I don't need to worry now as I sold the 80 and bought a Prado. That brings me into more tyre dramas with 17's. I have converted it to 265/75x16 as well.
Just my thoughts
120scruiser

OK ..... the difference is that the diameter will increase from 791.21mm to 833.88mm, a difference of 42.67mm or 5.12%.
The speedo error will be 5.398% too slow.

Oh, & BTW you are pretty brave putting Coopers on after the bagging they have had on this forum regarding excessive wear & chipping.
Unless they are ATs & you are staying on the blacktop!

I did exactly what you're planning to to with my 105 series.

The speedo now reads about 4km/h too slow (at highway speeds). This means that if the speedo says 100km/h I'm actually doing 104km/h.

To compensate I just drive at 4 to 5 km/h under the speed limit (according to the speedo).

Regards
Stephen J.

I went from 29" OD 235/75/15 on my pajero to 235/85/16 (32") which is supposed to be 10%. Speedo reading at 100k is now actually 102k, so it must have been reading 96k new. The bigger tyres corrected the original error. Of course, being a pajero, it may be more prone to error :-).

The real pain is the trip meter, since it is out by 6%. Reads 94k for 100k travelled.
I know it is a different error to the speedo. Can't work it out yet.
Just did plenty of testing and apply a mental correction when I need.
But then, I am servicing at 5300k instead of the indicated 5000k too. But engine revs are less anyway so not so worried.
Gee, to think that the speedo is now reading as 246,000k, and it has really done 260,000k. :0)

Of course, with 12mm deep tread on the tyres when new, that changes tyre OD by an inch when worn, which is 3%. So pick your error and live with it.

I put some 33's on my 80 series.. went from the 275/70/16 (the same as yours) to 305/70/16 which is the same rolling diameter as the 285/75/16 (both 33's)

My speedo was then out by 7% by the GPS so now when the speedo reads 100 I am actually doing 107... I am not going to bother changing it at all though I just keep in mind that I have to put a couple of K's on what the speedo is reading.

Hope that helps