17 MPG mostly highway

[pskelly]

I have bought a used 2009 V6 sedan. It used to be the V8 version but it turns out a previous owner replaced it with the V6.

I have had the car checked out by several mechanics and the engine seems to be in good working order. The problem is that I am getting 17mpg averaged per tank when driving about 66% highway, not ideal. Any suggestions on what I can have done?

Thanks

 

[Mark_888]

I have bought a used 2009 V6 sedan. It used to be the V8 version but it turns out a previous owner replaced it with the V6.

I have had the car checked out by several mechanics and the engine seems to be in good working order. The problem is that I am getting 17mpg averaged per tank when driving about 66% highway, not ideal. Any suggestions on what I can have done?

Thanks

What transmission does it have? Originally, the V6 used an Aisin and V8 used a ZF. The gear and final drive ratios may not be set properly for just switching the engine alone.

Also, since the V8 weighed 200 lbs more than the V6, all on the front end, the front suspension tuning was different, although I don't think that would affect MPG.

 

[427435]

Are you calculating your mpg from actual mileage and gallons used or using the readout on the dash?

 

[pskelly]

How can I tell which transmission type it is?

It's roughly 17 when I calculated the miles/gallon for a full tank of gas, and also roughly 17 on the mpg detector

 

[Mark_888]

How can I tell which transmission type it is?

Can you take it to the mechanic who inspected it? Probably can tell if you get the car up on a lift.

Even if the right transmission for the V6 was installed (Aisin), the ECU that controls both the engine and transmission may be out of synch.

One thing I would do is get on the highway at a fixed speed like 65 or 70 MPH and note the RPM's on the tach and post that here. Then we can tell whether you are able to reach top gear on the transmission.

 

[Statguy_Actuary]

Something isn't right with your mpg because I have a 2014 V8 and I'm getting roughly 21 mpg, probably drive 75% highway. That's based on 6 months of driving. It's been steadily rising from 19 mpg as the weather has gotten warmer in Chicago. I track all my mpg in a spreadsheet: reset the trip every fill up, pump gas until it automatically stops, and track the miles driven and gallons pumped each tank. The mpg detector in the car isn't always right, so I track it myself. Something to do while I'm taking a break at work.

 

[nb_mike]

I have had a 2009 3.8 and now a 2013 3.8
Over the weekend I just put on 760 miles round trip to Chicago cruising at 75 MPH I averaged 30+ per car computer. That is pretty consistent with my 2009. I get 18-20 semi-rural and normally 28+ on highway driving. It sounds like you have some kind of problem.

 

[landtuna]

The problem is that I am getting 17mpg averaged per tank when driving about 66% highway, not ideal. Any suggestions on what I can have done?

You should have no problem reaching 30 mpg at normal highway speeds in the V6. Something is definitely amiss. Posting of the tach data will probably reveal the cause.

 

[pskelly]

Ok this afternoon I checked it on a non-slopped hwy. While cruising at 75mph I was sitting at 2150 rpm. And yes I was hoping to average at least 22/23 since I'm on the highway a decent amount.

 

[danygenesis]

Definitely there is something wrong with your car. 16-17MPG is what I got when I had my G1 3.8 in Miami with heavy traffic but when driving highways the numbers always went higher like 27-28MPG

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[Fredder]

Plugged air filter?

Tire pressures correct?

 

[Gunkk]

Lots of ways to get lousy mileage. Get the FUEL BUDDY app to improve logging of your mileage and fuel usage (lots of nifty features).

Questions for the OP:
Do you use cruise control?
What tires to you have and at what pressure?
What speed do you drive and where?

 

[427435]

Plugged air filter?

Tire pressures correct?

A plugged air filter would cut power on a computer controlled fuel injection system but not have much (if any) affect on mpg as the computer adds fuel based on actual air flow.

However, the computer is not getting the right readings from air flow, engine temp, air temp etc could be causing the problem.

 

[GensisBoss]

Dang thats super bad... 17 mpg? go get it checked out cause somethings def wrong

 

[SixFootSix]

I have recorded the following MPG for my 5.0L, based on multiple 500 miles of highway trips between San Francisco and San Diego:
- avg. 55 MPH = 34 MPG
- avg. 65 MPH = 30 MPG
- avg. 75 MPH = 26 MPG
- avg. xx+ MPH = 22 MPG

IMO, With a V6 in the engine bay of what was a V8, the front of the vehicle will ride 1/2-1" higher. That would cause more air to strike the under cover, and reduce mileage by a higher ratio than normal. The final drive gear may also be creating a greater load on the V6, and higher fuel consumption.

 

[bassjam99]

Definitely there is something wrong with your car. 16-17MPG is what I got when I had my G1 3.8 in Miami with heavy traffic but when driving highways the numbers always went higher like 27-28MPG

These numbers jibe with my 3 weeks usage so far. I've not done much highway driving to see it I get the 30+ MPG others are getting.