bang,
what ur opinion if using 195/55r15 then 195/50r15 on FD...
i can't catch up... can explain more??
here's a gear chart of two tire sizes running the same ratios, but with different FD.
as you can see from the speed at 8000 rpm, the speeds are almost similar. Tire expansion is a factor, but let's assume they are similar models with similar expansion rates, just with different sizes. but in truth, different tire sizes offer different expansion rates, so even gear calculators can't be 100% accurate to real life (of course, they don't mention this)
In short, if you were using 195/50R15 before with stock Y21 and 4.4 FD, simply by swapping tire sizes to 195/55R15 you can actually gain the effect of near +/-0.1FD ratio, depending on which tire you started out with.
notice the speeds at 8K RPM being very close, even with tire swap, because it is offset by a different FD.
in fact, if you wanted to get a feel of what 4.2 FD feels like, you can run 215/50R16 and achieve near 4.266FD ratio, as the chart below shows. That is, if you can get enough fender clearance to fit 215/50R16, he he.
Another factor in play is tire rotation resistance. more surface contact and more softer compound will make the car feel heavier or not, depending on if your engine has enough torque to offset that resistance. A 195/55R15 semi slick is way more heavy to rotate that a similar street compound 195/55R15 tire. Same with 205/50R15, which is wider. Don't forget the weight as well, though the bulk of the weight is usually the sidewalls and tread material, so this may vary, and sizes can't be an accurate measurement.
The reasons FDs are available in a multitude of sizes is because not all tire sizes can fit in the car.