If you don't have a MAF or it's not readable for some reason you need to be able to read RPM, MAP, IAT, and VSS to get a MPG calculation. Note that if you are at a stop your VSS will be zero and as a result so will your MPG.
If you do have a MAF all you need is MAF and VSS to calculate MPG.
The problem comes in when you have a MAF, and consequently no MAP or IAT, but it can't be read via the standard PID. Unfortunately, in this case you're out of luck.