How do I know what my rolling radius should be?

I know how to work it out with my current 17" tyre sizes but........

My inside door sticker says it came with 16" wheels at 225/50 front and 245/50 rear.

I have a GZ Aerotop Twin Turbo L Reg.

I didn't think this came with 16's? On the FEATURES link under the RESOURCES heading at the top of this web page it says 17's also?

There are two things that concern me. The minor one is the speedo misreading but more importantly the front of the car will sit 5.95mm higher on the road if the tyre is bigger than it should be. Likewise if the rear tyre is slightly smaller then this will have my headlights pointing at the moon!

My calculations work out that if it came out of the factory with 16's then the Rolling Radius should be 315.7mm. My rolling radius for the current front tyres (235/45/R17) is 321.65mm.

So really I should be using 225/45/R17 front and 245/45/R17 rear (which goodyear don't make) is this right????????

Help.

Try this, very very helpful!

Its from the American Supraforum.

http://www.geocities.com/steve_jarvis_me/Fitment.xls

hold on, link not working. . . . . . . .

Ok try this,

Go Here http://www.geocities.com/steve_jarvis_me/

Then click on SUPRA WHEEL FITMENT GUIDE. Will prompt u to download.

Justin, DON'T worry about the SPEEDO ! !

ALL Soops (unless corrected with a Convertor that CAN correct it!)

read quite HIGH/ OPTIMISTIC ! !

ie. ~5-10% high.

As long as you're RAISING the ratio (slightly!) you're only actually

getting a MORE acurate speedo ! !

with my original 16's on (std' tyres) mine read ~3,000=90mph

then with std 17's (std' tyres) it STILL read ~3,000=90 ! ! !

(so probably only raised it negligibly ???)

THEN, I swapped me REAR 255/40-17s for 285/40-17s ! ! !

NOW it reads ~3,000=85/86 and is much nearer a TRUE reading

(just looks like a tractor from certain angles:stupid: )

FatS.

(ps. if FRONT and REAR raised by same amount.... just brings it

up to UK ride height anyhow ???..... and,obviously only REARS affect

Speedo - gearbox/driveshaft sensor)

Originally posted by Steve W

Justin, DON'T worry about the SPEEDO ! !

 

ALL Soops (unless corrected with a Convertor that CAN correct it!)

read quite HIGH/ OPTIMISTIC ! !

 

ie. ~5-10% high.

 

As long as you're RAISING the ratio (slightly!) you're only actually

getting a MORE acurate speedo ! !

Very good point, I have 275/40/17s on my standard rears and at 80mph on the speedo my Origin b2 says I'm doing 75mph, if I every have enough cash for the 19's I'm hankering after I'd probably try and address this with tyre sizes.

My Pete Betts BOX has an adjustable 'reo' in it to give ~+/- 10%

in ~1% steps..... ! ! ! !

but, I've never been arsed to do it:D

(then, when I found the Tractors had 1/2 done it FOR me.......... )

I'm pretty sure there was VERY little difference between me Jap

16s and the UK 17s.... even though Justin's right, theoretically

there IS a slight increase in 'height'/diam. etc ???????

I didn't notice ANY diff' whilst lookin' at the speed v revs.

(UNLIKE after fitting the 285's:blink: )

Cheers guys but as I say speedo don't worry me.

It's the ride height and propulsion 'angle' that bovers me. With my setup the rear is sitting down and the front is pointing up and could be over 10mm in difference from stock which is quite alot when you think about lowering springs etc.

I don't wanna be catching more air under the front than I should be especially at drag races, top speed runs etc.

It may explain why my Blitz Nurspec gets intimate with every flippin speed hump around too!

Anyone help me find out what R/R my car should have???

Good question that! I can't believe that Toyota would have different suspension settings or anything for cars with 16s and 17s, and the 17s have always been an option I believe. I guess 6mm at the front and 7 at the rear is just too small to matter. You've got the same size as the standard 17s anyway (assuming you've got 255 40 R17 on the back). Supras do look like they "sit up" a bit at the front anyway.

EXACTLY...

I'd say they were ALL made to take 17's and some just have 16's

'instead'.................

so, don't worry about it mate.

thing is there must be loads of people running around with possibly upto 5mm difference in rolling radius just due to tyre tread depth differences, plus on spirited acceleration i'd guess that you get a lot more front lift than 6mm.