The speed of sound in (carbon dioxide + propane) mixtures, with mole fractions of carbon dioxide between 0.938 and 0.998, has been measured at temperatures from (248 to 373) K and at pressures between (8 and 200) MPa. We find that the addition of propane to carbon dioxide is highly effective in catalyzing vibration-translation energy transfer and reduces the sound absorption coefficient sufficiently to permit sensitive measurements of the speed of sound at frequencies in the low-MHz range. In this work, a 2 MHz ultrasonic cell based on the double-path pulse-echo method was used. The cell was calibrated with degassed ultrapure water at T = 298.15 K and p = 1 MPa, making use of the speed of sound computed from the International Association for Properties of Water and Steam equation of state (IAPWS-95). The estimated overall standard relative uncertainty of the speeds of sound measured in this study are 0.035 %. The speed of sound in pure carbon dioxide was obtained by extrapolation of the sound-speed data with respect to mole fraction at each experimental temperature and pressure. The average estimated overall standard relative uncertainty of the speed of sound in pure carbon dioxide obtained in this way is 0.1 %. Comparison of our results with speeds of sound computed from the equation of state of Span and Wagner suggest that the tolerance band of the latter may be reduced to 1 % at pressures up to 200 MPa.