the odometer works by counting the number of times the driveshaft rotates and multiplying that what it thinks is the circumference of the rear wheels.
ok. the stock gear ratio is 3.27 right? that means that every time the rear wheels make 1 rotation, the driveshaft goes around 3.27 times. also lets assume your wheels are 245/45/17, which is about 25.5" circumference, so to make the math easy, lets round it to 26".
so if you have 3.27 gears and your wheel circumference is 26", then: 26" / 3.27 = 7.95" traveled each time the driveshaft makes one rotation. now one mile is 5280' * 12" = 63360", so 63360 / 7.95 = 7969.81 rotations of the driveshaft is one mile.
so every 7970 rotations of the drive shaft gets you one mile on the odometer.
if you change gears to 4.10 and leave the wheels the same, you get:
26" / 4.10 = 6.34" per rotation, 63360" / 6.34" = 9993.69 rotations per mile.
so every 9993 DS rotations you go a mile in real life, but the odometer thinks you went 1.25 miles. (9993 / 7970 = 1.25)
if you make the wheels bigger, then the odometer is closer to correct because the rotations per mile is closer to 7970.
i hope this helps.
hey hazmat: are those ported stock heads thumpers? are you happy with them?