How much does exhaust (mufflers, etc.) affect gas mileage?

[DailyDriver]

Over the past few months I've somehow "lost" about 3 mpg. I was averaging about 21mpg on my 1995 4 banger/5 speed super cab. I was getting about 425-450 miles out of a tank (20 gallons I believe), now it's about 360. Motor seems to be running fine, no codes. Fuel filter & air filter are always replaced regularly, & I run full synthetic 5w/30. I checked all fuel lines, no leaks.

The catalytic converter has about 100k on it & my cheapo muffler is looking pretty rough & I will be replacing it this week.

Any thoughts?

 

[Kenneth S]

The "honeycomb" inside of the catalytic converter is probably braking apart and choking up the exhaust, my truck did that, I also replaced the muffler because some of the "honeycomb" got into the muffler, and it was ratting around inside there, I could hear the ratteling while driving down the road (it was annoying as hell).

 

[DailyDriver]

I gave the muffler a real good look over, it's got rust holes everywhere & is extremely brittle. Exhaust is shooting out everywhere. It was like a hair dryer under there lol.

 

[Bryan22]

a full tune up wouldn't hurt. I'd suspect if your exhaust was clogged you'd notice some driveability issues, not just decreased milage. But you never know.

When my cat came apart and got into my muffler, I knew. truck sounded like it was under a load all the time, and i couldn't keep it above 45 on the freeway. Downshifting and flooring it just slowed me down.

 

[Kenneth S]

I'd suspect if your exhaust was clogged you'd notice some driveability issues, not just decreased milage. But you never know.

When my cat came apart and got into my muffler, I knew. truck sounded like it was under a load all the time, and i couldn't keep it above 45 on the freeway. Downshifting and flooring it just slowed me down.

A friend of had a car in which a round parabolic sound deflector came loose, we took the tail pipe off, and we found the deflector had covered well over 1/2 of the 2" muffler outlet, the car still ran ok but the gas mileage went down substantially.

 

[DailyDriver]

Tune up was done about 16,000 miles ago. NAPA wires & Autolite Plat. plugs. I pulled a couple plugs last week & they looked good.

I'm really scrathing my head on this one.....

 

[DailyDriver]

A friend of had a car in which a round parabolic sound deflector came loose, we took the tail pipe off, and we found the deflector had covered well over 1/2 of the 2" muffler outlet, the car still ran ok but the gas mileage went down substantially.

Hmmmm, we may be on to something here. The muffler change is going to happen in the next few days. Hopefully this will be the fix & I won't have to shell out $75 for a new cat.

Back pressure isssue???

 

[Wicked_Sludge]

a vacuum or backpressure test will reveal a plugged cat/muffler. cat converters do fail...but not as often as people like to think they do.

 

[shadetree]

Back pressure isssue???

If the exhaust system is clogged somewhere, you will have a tremendous backpressure problem. You don't mention a power loss, or failure to gain rpm, so I kinda doubt exhaust clogging. If it is, it will be minimal. In the last three months gasoline formuas have changed to winter mixtures. This will cause a loss of mpg.shady

 

[jdm90]

I had NAPA wires with the single retaining clip on the coil side. You could lightly bump them and they would come unhooked to the coil. My Autolite wires have a clip on each side and stay attached. Just a thought

 

[pud]

filling up at a different station? they might be rippin you off and youre not getting as much gas as they say.
Its happened before.

 

[Bryan22]

A friend of had a car in which a round parabolic sound deflector came loose, we took the tail pipe off, and we found the deflector had covered well over 1/2 of the 2" muffler outlet, the car still ran ok but the gas mileage went down substantially.

I could agree that a small obstruction wouldn't cause as much problems as i had, but i wouldn't hardly call 3 MPG a 'substancial' loss in fuel economy. When mine was plugged i think i burned through 5 gallons in about 60 miles.

 

[Kenneth S]

I could agree that a small obstruction wouldn't cause as much problems as i had, but i wouldn't hardly call 3 MPG a 'substancial' loss in fuel economy. When mine was plugged i think i burned through 5 gallons in about 60 miles.

Loosing 3 mpg when you have to fill a 25 gallon tank, it's substantial (75 miles less than you normally get). You got around 11-12 mpg, that's a bunch!

 

[Gotta_gofast]

Over the past few months I've somehow "lost" about 3 mpg. I was averaging about 21mpg on my 1995 4 banger/5 speed super cab. I was getting about 425-450 miles out of a tank (20 gallons I believe), now it's about 360. Motor seems to be running fine, no codes. Fuel filter & air filter are always replaced regularly, & I run full synthetic 5w/30. I checked all fuel lines, no leaks.

The catalytic converter has about 100k on it & my cheapo muffler is looking pretty rough & I will be replacing it this week.

Any thoughts?

You say over the last three months? Do you live in a colder area? I live in Wisconsin and for winter the gas stations will switch over to a "winterized" gas. Even though gas has gone down considerably, I only pull 18-19mpg with my 3.0L in the winter. (long warm ups burn gas, too). In the summer its 22-23.

 

[DailyDriver]

I live in about 45 miles north of New Orleans & I park in my garage

Got the new muffler installed today. We'll see...................