Tyre size vs speedo reading

fordluva, Jan 22, 4:49am
Im currently looking at some A/T tyres for my navara ute, at present it is running 205/65 r15s on which the speedo reads just under 110km/h at 100 km/h (measured by GPS) I would like to fill the wheel arch at the same time as the current looks too small, does anyone know of a good accurate converter that can tell me what my speedo reading would then be? I have tried several online which give you readings in miles and all seem to be different in result, tyres I am considering are 235/75 r15 which Im hoping will fill the gap, improve economy and at the same time make the speedo more accurate, any ideas?

quickbuck, Jan 22, 4:51am
do you mean like this? http://tire-size-conversion.com/tyre-size-calculator/

Just use 62 MPH as 100k.

fordluva, Jan 22, 5:06am
Hmm bit confusing but if Im right that would mean with the new rubber it would be doing around 97km/h at 100km/h on the speedo which I could live with, sound about right? just seems to be a hell of a difference in diameter

quickbuck, Jan 22, 5:09am
Yeah, that is what I make it.

theunicorn, Jan 22, 5:34am
I use http://www.miata.net/garage/tirecalc.html
That shows a difference of 21% between the two tyre sizes. If your speedo currently overreads by 10%, then you added 21% larger tyres, you would be doing about 110 when the speedo indicated 100.

theunicorn, Jan 22, 5:36am
Oops, I keyed in 17 inch for the new rims. At 15 inch the new tyres are 13.3% larger. So you would be doing about 103 at an indicated 100.

pico42, Jan 22, 10:37am
Doesn't that mean you are over the certification threshold with more than 5% increase in rolling diameter?

scuba, Jan 22, 10:19pm
before you do the job maybe you should go to a tyre shop and stand the 2 tyre sizes next to each other. that big a difference will affect your performance and handling - not in a good way.

fordluva, Jan 23, 2:39am
more than likely, the point is at the moment the speedo is out which im picking means it has the wrong size on it to start with, cant seem to find what the factory size is as google is only showing me aussie models which run a 14 inch rim, and the sticker on the door jamb no longer exists.

supernova2, Jan 23, 9:07am
Bung your rego number into the Beaurepairs site and with luck it will tell you what size tyre you should have, or at least give you a couple of options.

sprinter51, Jul 26, 4:00pm
A wheel 3.8197 inches in diameter rotating at 3200 rpm would result in a speed of 36.363636 miles per hour