Is this an automatic? If so, it surely is downshifting from high gear when you go up a hill, causing the engine to speed up. (faster engine speed, more horsepower) And yes, tire pressure, vehicle load, etc may make the transmission downshift sooner. Tire size, yes, but have you changed them lately? Normal wear on your tread will be negligble for your measurements.
If you are on a level with an automatic it should put you in high gear and your rpms should not fluctuate greatly. Most new transmissions actually "Lock In" on high gear to avoid any slippage from the fluid clutch... Not sure on the Rodeo, but I would suspect that the 2000 has this....
If it is a stick, then if you are in high gear there should be no fluctuation between your RPM/MPH measurements... A hypothetical example: a measurement for a vehicle at 3000 rpm shows 70 mph whether going up a hill or not. (unless your clutch is slipping)
