Leaky exhaust and bad fuel mileage

[JakePSD]

Simple question, can my leaky exhaust system be the cause of my horrible fuel mileage? No check engine lights, runs like a champ, but I'm lucky if I can manage to get 300 miles out of a tank of fuel. If it isn't that, what other kinds of issues should I look into?

 

[J_C]

Someone should correct me if I'm wrong but the fuel tank capacity is about 17.5 or 18 gallons so at 300 mi. that's 17 MPG, not all that bad for an aging SUV. Check the typical things like air filter, spark plugs, tire inflation. Summer blend and/or E10 gas may lower MPG too.

Exhaust leaking before the O2 sensor could throw the system into open loop mode and generate a fault code and check engine light but after the sensor, it shouldn't hurt fuel economy.

 

[JakePSD]

19 gallon I do believe. Assuming that and going exactly 300 miles running the tank dry equates to 15.789 mpg. I've been averaging in the 14s. I used to be able to pull 17 consistently, nearly 19 straight highway. My girlfriend gets 16-18 consistently mixed city and 2 lane highway with hers and besides the paint color they are twins, both early build 97s, AWD V8. I maintain mine pretty religiously, air filter, plugs, wires, etc. are all pretty new. The only thing mechanically wrong with it is the rotted out exhaust. I do know a bit about engines and the science behind them, and I do know that exhaust flow can affect efficiency based on the scavenging effect of the exhaust pulses. I just dont know to what extent it affects it, if it could cause a loss of 3-4 mpg.