Ann, like Mike said/inferred, that method is just a 'proven to be wrong more often than right' seat of the pants method to determine tire pressures. If you want to bring a relatively easy to use method with high degree of repeatability &/or consistency to 'setting' your cold start pressures on 'normal' Car Tires (see Mike's comments re the Kendas above - they are just so generally so poorly/inconsistently constructed that nothing much really applies!!) you can use 'the 4psi rule' (or measuring tire tread temperatures) to work out &/or confirm that your pressures are in the right ball park, & then adjust up or down to suit your own particular ryding needs & preferences. :thumbup:
Briefly, the 4psi rule looks for a 4psi increase above your cold start pressures after an hours ryding (or after 30 mins ryding if you only ryde that long!) - MORE than a 4psi increase tells you that your cold start pressures were too LOW & amongst other things you are risking tire sidewall damage, explosive delamination of the tire tread/carcass, excessive squirm/sloppy steering, soft & spongy ride, poor handling, excessive fuel usage, excessive tread wear on the outside edges of tread, or possibly even rolling the tire &/or bead off the rim; while LESS than a 4psi increase tells you that your cold start pressures were too HIGH & amongst other things you are risking punctures/cuts in the tread or sidewalls, poor traction, hydroplaning in the wet, poor braking, harsh ride, excessive tread wear in the middle of the tread, &/or 'darty' steering/loss of control/skidding etc etc...... but you just
might get phenomenal fuel economy (that is, if you don't die due to those others!!) :shocked: And all you need to use this method is a reasonably consistent tire pressure gauge/TPMS/FOBO & a couple of minutes before & during your rydes for a while (at least until you learn what pressures will work for you, anyway!)
Another method for checking/establishing tire pressures involves measuring your tread temps across the face of the tread immediately on stopping after ryding for 30 plus mins or so.... the tread temps should be within a couple of degrees all the way across the tread face/contact area, but you need to use a suitable infrared remote thermometer very quickly after stopping or use a fairly expensive & fragile purpose built multi probe tread block thermometer to do this, & unless the temps are relatively even to start with, there is then some skill & experience required to interpret & act appropriately depending upon the results you get. BUT, remembering that there is a direct link between tire tread temps & tire pressures/tire pressure increases, generally it's a whole lot easier for the average ryder to use a tire pressure gauge or TPMS/FOBO than it is to carry & use correctly the right type of thermometer & then play with & interpret their tire tread temps properly....
Or if you want, you too could do the study & learn all the tire science stuff, but even so there's still a heap of weighing & measuring & crunching the numbers etc that you hafta do to get a final valid result, & yet, once again like Mike said, due to the massive safety margins involved in running a 'proper' car tire under your lightweight Spyder instead of a Kenda, the fractions of a psi involved in the variations between brands/sizes likely to fit rarely warrant anything more than just going with the pressures we've already recommended.... but all that stuff STILL makes my head hurt just thinking about it & if you really
must have something to guide you more, the 4psi thing is sooo easy to use IF you can ever bother to make the effort to check your pressures! :dontknow: I know the method that I think is the most helpful/useful! :2thumbs:
Ps: despite all the above, some people become quite used to/comfortable with whatever handling & ride characteristics their significantly WRONG tire pressures may provide; so it might take a bit to get used to running appropriate pressures that are better in all respects than those you may have been running before; but in time, your bike, your wallet, & bum/back will thank you if you can persist, & you'll generally be a whole heap safer too! :thumbup: