I have a question on tire sizes. I recently swapped out my 17" Elantra wheels for a set of 16" elantra wheels. I did it because I could no longer stand rolling on rocks ( or that's what it felt like). The 16" solved my ride issues but I did notice some oddities: First, both wheels are OEM for the Elantra. 2nd, the 16" are definitely taller than the 17's. Where I used to run 2250 RPM @70mph, I am Now running about 2000 at he same speed. Now knowing the same car can take two different rims, one a limited with the 17's, and the SE with the 16"s, I am curious as to why the big difference in RPM's ( I can feel it in the butt-dyno as well, it is sluggish more so. So off the calculator I go, putting in the two different sizes, and I come up with this: Original Tire Size: 215/45R17 New Tire Size: 205/55R16 Diameter inches (mm) 24.07 (611.4) 24.62 (625.3) 0.55 (13.9) 2.3% Width inches (mm) 8.07 (205) 8.46 (215) 0.39 (10) 4.9% Circum. inches (mm) 75.62 (1920.77) 77.34 (1964.44) 1.72 (43.67) 2.3% Sidewall Height inches (mm) 4.04 (102.5) 3.81 (96.75) -0.23 (-5.75) -5.6% Revolutions per mile (km) 837.86 (520.62) 819.24 (509.05) -18.63 (-11.57) -2.2% Now for my question.... Are there different sizes for the same tire size, and could this be causing the discrepancy in sizes? IOW, If I bought at 205-55R16 from tire company "A" and bought another 205/55R16 from tire "B" would there be a difference in actual size, even by a few mm? It just seems awfully strange that the same alternately installed wheels could have so much height difference on basically the same car. Meaning, I drop the sunroof, leather seating, and 300W stereo (yeah), and My car turns into a SE model with 16" rims. Nothing else is done to the cars engine. tranny, suspension, etc. Now if I drop my next new tire purchase from the 205/55R16 to a 205/50R16, It will be much closer in size to the original 215/45R17. I have time to get the tires, as the wheels I have are direct takeoffs from a new Elantra. So should I go down a size, to the 50 series, or maybe if the sizes are different between manufacturers, to find another set of tires, just keeping the 55's and deal with the speedo error and power loss?, or go down to the 50's?