Why did my new wheels change my rideheights?

Please forgive my ignorance, but I just don't get it. Measured the ride heights on my Miata this morning while it still had the 195/60R14 Star Specs on 14X6 wheels: 4.75" front, 5" rear to the pinch welds; just where I wanted it. Then I install my new 205/50R15 Rivals on Kosei K4R 15X8 wheels and the car is sitting visibly higher. Measurements confirm: around 5" front and 5.25" in the back to the pinch weld. Based on the handy miata.net tire size comparison, the tires should only have a 0.05" difference in radius so I would expect the ride heights to change by only that amount (and the new wheels are actually the shorter ones!). A very quick drive around the neighborhood did not change the heights.
So what happened?

How worn were the tires you took off? The tire size calculator assumes a brand new tire, FWIW. If you replaced a really worn set with a brand new one, well, there ya go.

CrashDummy wrote: Based on the handy miata.net tire size comparison, the tires should only have a 0.05" difference in radius so I would expect the ride heights to change by only that amount

With all respect to Miata.net's tire calculator...

Every manufacturer is going to have some variance, so you have to measure YOUR old tires vs. YOUR new tires. No other numbers matter.

Air pressure differences figure in as well.

Edit: I was thinking height from the ground...

also.. have you driven the car since you jacked it up and replace the tyres? It may help to drive the car a mile and then check

Heck, it usually helps to roll the car back and forth a couple feet. The same track width change with suspension travel that makes a nice above-ground roll center will also keep the car from dropping down to ride height after you drop it off the jack.

Knurled wrote: Heck, it usually helps to roll the car back and forth a couple feet. The same track width change with suspension travel that makes a nice above-ground roll center will also keep the car from dropping down to ride height after you drop it off the jack.

mad_machine wrote: also.. have you driven the car since you jacked it up and replace the tyres? It may help to drive the car a mile and then check

quote from his first post:

A very quick drive around the neighborhood did not change the heights

What, I'm supposed to read EVERYTHING?

(throws table, leaves room)

Do a 60 second brake stand, then rotate front to rear, repeat until desired measurement is reached or steel belts show.

Knurled wrote: What, I'm supposed to read EVERYTHING? (throws table, leaves room)

Tire size specifications are approximate, I'm not surprised to see a quarter inch difference in radius. That's only 2%.

If you look at various tire manufacturer's spec sheets, compare the "revolutions per mile" figures for various tires that are nominally the same size, and you'll find a substantial variance. I picked two random tires just now -- Hankook Evo V12s and Bridgestone RE11As in 225/45R17, and they are 832 and 808 revs/mile. That's almost a 3% difference.

wbjones wrote:

Knurled wrote: Heck, it usually helps to roll the car back and forth a couple feet. The same track width change with suspension travel that makes a nice above-ground roll center will also keep the car from dropping down to ride height after you drop it off the jack.

mad_machine wrote: also.. have you driven the car since you jacked it up and replace the tyres? It may help to drive the car a mile and then check

quote from his first post: A very quick drive around the neighborhood did not change the heights

then I suggest a longer drive.. a couple thousand miles should do it

codrus wrote: Tire size specifications are approximate, I'm not surprised to see a quarter inch difference in radius. That's only 2%. If you look at various tire manufacturer's spec sheets, compare the "revolutions per mile" figures for various tires that are nominally the same size, and you'll find a substantial variance. I picked two random tires just now -- Hankook Evo V12s and Bridgestone RE11As in 225/45R17, and they are 832 and 808 revs/mile. That's almost a 3% difference.

Pretty much this.

I like when you get into low-production tires where they actually write the tire's circumference in chalk on the tire, because of tolerances.