:lol: . ok ok, lets say that the 15in stellies are (i dunno) 15lb's each. and with the tire on them they have a diameter of um...... 20in. now aftermarket 17in rims weighing in at 15lb's with low pros (with the same tread design as the 15 steelies) with a diameter of 20in(same as above). u will see no gauge de-calabration, and keep your speed (although their is a centrifical mass issue, but thats intense so lets not get into that). now lets say those aftermarkets were 17in rims and they had normal tires so the total diameter was 22in, now that will de-calibrate gauges and slow u down. on the other end of the spectrum, lets say (again) 17in rims with super low profile tires, so your total diameter is 18in, you will speed up but your gauges will fall off calibration. ok now those combinations above all weighed 15lb(15in,and 17in), now i'll change the weight... ok 17in rims weighing in at 20lb's, riding on normal wheels , (so diameter is OEM 20in). but now the rim weighs more (cintrifical mass) you will slow down due to the axle having to move more weight. now if they were 17in rims weighing in at 5lb's, riding on normal wheels , (so diameter is OEM 20in). now the wheels can turn the wheel easyer due to the decreaced cintrifical mass, so you gain speed. their whew!