Before i installed my 4" exhaust system on my 96 6.5 TD i was getting almost 13 mpg. When i did my fuel milage calculations today i am only getting 12.27 mpg. What gives? This system was supposed to pay for itself! I am not an aggressive driver, i only get up to 1800 rpm when accelerating. I was expecting at least 14 mpg. I am not hauling anything right now.

 

Sure you didnt get on it a little harder enjoyin the sound of your new exhaust?

 

less backpressure is a good thing and your system on there is already flawed, i'm assuming it's not a turbo back 4" so it probably has the awesome stock downpipe on it, if that is the case look at the down pipe and you'll see the answer as to why your mileage didn't change and you have to factor in where you got your fuel, how good was the fuel? when is the last time you changed your fuel filter? air filter? did you idle more than normal? there's alot of factors that you have to take into consideration before you assume that a freer flowing exhaust is going to make mileage worse

 

oh my god- a 0.8mpg decrease!

Seriously, your truck is a 96 model. When did you get the injectors tested the last time for example?
Injector wear, uneven spray pattern, low pressure can cause that.

On the other hand you do have to do your calculations over a longer period of time to get an average.

Most people do not understand that an engine is a complex system. Changing a single item into a better one may not give you the expected mpg increase depending on the engine.
Maybe you have increased your hp, but more hp needs also more fuel- so the fuel saving advantage diminishes.
Less backpressure is a good thing to start with. Now check the engine and proceed with engine adjustments. Your torque curve and your powerband will move to a different spot due to the less bp. Find it!

 

She got a new air intake system when i put the 4" exhaust on. I changed the fuel filter when i changed the oil last weekend. I have been thinking about getting new injectors but i have just had more important things to deal with. My exhaust is a turbo back. I only buy Shell diesel from one station by my house. And no i didn't get on it harder this week. i did that last week after getting it.

 

I knew it had to happen sometime

 

This week she got 12.5 mpg. And i drove VERY conservatively. This ain't worth a ****.