the VIN starts 20 so I'm pretty sure the title is wrong
First two characters in frame No. for -78 standard model CX 500 are 20, so you have a -78 model.
it has a 110-90-18 on the rear and a 90-90-19 ont the front. is this the standard size and can a wider tire be used?
For a -78 model CX 500 3.25 x 19 front and 3.75 x 18 rear are the specified tire sizes. What you have on now is as close as you´ll get with "low-profile" tires.
Later standard models (that to my knowledge never came to the US) had a somewhat wider rear rim and 4.00 x 18 tire. The rear swing would easily accomodate a 130 tire, as on the Custom and Deluxe models (16 " wheel there). A 140 might be squeezed in too (think I´ve seen it somewhere). If I can´t find 4.00 rears for my -81 standard models I always choose 110/90 x 18. I´ve seen 120/90 x 18 on many bikes too (can´t say if they were -78 or later).
Even if a fat / wide rear tire may look great, it doesn´t improve cornering. Wider tires also get the wrong profile on a narrow rim. Better stay as close to the rec. sizes as possible.
For one of my SRX 600:s I went the opposite way, after recommendations in an old bike test article. I went down one dimension from standard, both front and rear, to get better cornering, Great idea! That bike is real fun in the bends now!
That she may look skinny from behind doesn´t bother me much! I sure don´t!! :-)
Regards
Sture .