The speedo and odo are the same gauge. So if one is off, the other is off by the same amount. The gas gauge is not effected because it works off a float in the gas tank. Has nothing to do with the speedo/odo. However when you calculate your MPG, be sure to adjust for the error. In your case, 13.2%. So if your odo indicates you have driven 300 miles, 300 X 13.2% = 339.6 actual miles driven. If you used 15 gals and divided that into 300 your ans would be 300/15 = 20 mpg. That number would be wrong. You should use 339.6(act miles driven/15 = 22.64 corrected mpg.
Hope that makes sense.