275x55x20 Calibration?

[Lucky13MN]

Hey all, I'm a newbie tuner and trying to figure out what to put in for my summer wheels.

During the winter I run 265x75x16's and in the summer I run 275x55x20's, funny thing is (according to my GPS) it's pretty accurate as-is with no adjustment, but if I do put in the diameter according to a tire size calculator it throws it off a bit more.

Just wondering if I should leave it or what's going on? The stock setting is something like 30.8 while the 20's are 31.9, so there is a noticeable difference there.

Here's the site I'm using:
http://www.miata.net/garage/tirecalc.html

Suggestions/comments?

[dc96819]

The tire calculator is not very good.My 305/20 were way off.
Try measuring the tire.

[Bill@HPTuners]

you know theres a tire conversion calculator right in hpt right? http://www.hptuners.com/forum/showthread.php?t=5691

[Lucky13MN]

Hey, I did not know that... thank you.

That tool gave me the same output though, 31.9. Nice to know it's there... still confused on why my speedo is off with that number in there.

Edit:
Apparently the factory setting is for 245x70x16 or something (30.81)... according to that calculator:

265x75x16 = 31.6
275x55x20 = 31.9

Now... that said, I know there's a lot more than .3" difference when they are side by side.

I think I'll just have to use a level and get an accurate measurement to be sure and take it from there.

Thanks guys.

EDIT:
Update... I measured the tires, 31 1/8" high, so I put in 31.12 and voila...spot on. I guess some things are just easier when you do the measurements

[EC_Tune]

SS trucks have the same tire and it comes out as 31.04". Tire calculators don't take into account the weight placed on the tire "squishing" it.

[jimmyblue]

Do not believe any calculator based off the numbers
stamped on the tire. The only thing you can trust is
the measured circumference of the tire, drive over a
tape measure and wrap it. Then you can divide by pi
for the diameter, or whatever you need to enter in
the speedo section to drive the auto-scaling.

[EC_Tune]

You can also do a "roll out" by marking the tire & ground, rotating the drive wheel tire 1 revolution and measuring the distance. Then divide by Pi to get diameter.

EC