If you are good with mathematics and physics use the Bernoulli's law. You can find all the information you need in wikipedia. This is not an easy task and, by the way you ask, it seems to me that you will be surprised with the results. Inside the exaust tube there is, allways, negative preesure. If you suceed the task keep us informed.

There is another way: Just drill a hole in the exaust, in the point you want to know the pressure, and plug a device as used in turbocharged cars, start the engine and read the information. You will read pressures bellow zero.

Good luck