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So, I have an 08 300 SRT Design, 5.7L, 33k miles / 54k km, it sat un-used for 6 or 7 years. I've had it for about 6 months with no real issues ( squeeky suspension and needed new brakes ) but my fuel economy is atrocious! I average 18.2L/100km or about 12.9MPG, I've owned very large trucks that got better fuel mileage then that.

I replaced the spark plugs 2 months ago, economy didn't improve, plugs were decent, there's no fuel filter on this car, the car runs well, doesn't hesitate, has lot's of power, drives nice, just the fuel mileage seems really bad.. from what other 300 owners tell me they get much better..

Any ideas what I can do help this? I did do a seafoam into the intake treatment but I wonder if I should add some to the fuel? I use the 89 octane as recommended.

Thanks!
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I get like 11mpg with mine but figured it's just my heavy foot.

Looking at the instant mpg on my phone with the torque app, as soon as my foot rests on the pedal just enough to maintain speed, the mpg drops from 25+ down to 10. Then at wide open it drops to around 1.6 mpg.

I should really get my wide band bungs installed so I can see what's going on

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I mean, driving style definitely has a major effect. I know I can average much much higher on the highway, but anything stop and go is terrible. For me I feel like part of it is my tune, I have my gas pedal set to be really sensitive, so I think it makes it much more difficult to for me to not constantly have my foot in it too much. At the same time I just really like how responsive it feels. I try to have fun when I drive, lol.

I need to reflash my stock tune and test my theory.

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Hi I am in the UK and recently bought a 2007 300c 5.7 touring ,I bought it up in Scotland 270miles from me it showed 19.1 mpg and that is US calibration so here x1.2 for UK gallon =22.9 within 30 miles I was up to over 40mpg at 60-65 mph so I took photos for proof I could visibly watch it increase, on the way home it still averaged over 40mpg,here 95 octane is just over $6 UK gallon, now the but, I have had the resonators removed and 4" end pipes put on(wanted to extract the V8 growl) and the mpg has dropped down into the 20's 23.4 so UK 28mpg this is around our small UK town and small country roads,another cause for low mpg is the caliper pistons sticking on the 300c crd's common fault due to heat the piston warps and does not return again I had this on my 300c crd mpg down in the low 20's but once new pistons fitted (already had new pads put in ) fuel display was showing 34mpg within 5 miles of leaving the garage.Some tyres have poor mpg rating E's and F I try and go for B's and C's .Hope you get things sorted
 

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Hi again another thing I do with all my cars (3 2x 300c 1 Dodge Nitro 2.8crd) is reset the the fuel computer on a regular basis to give me an accurate figure.I have also got delivered today from Amazon .com a Spectre performance panel filter over 50% cheaper than a K&N here and same company ?
 

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So, I have an 08 300 SRT Design, 5.7L, 33k miles / 54k km, it sat un-used for 6 or 7 years. I've had it for about 6 months with no real issues ( squeeky suspension and needed new brakes ) but my fuel economy is atrocious! I average 18.2L/100km or about 12.9MPG, I've owned very large trucks that got better fuel mileage then that.

I replaced the spark plugs 2 months ago, economy didn't improve, plugs were decent, there's no fuel filter on this car, the car runs well, doesn't hesitate, has lot's of power, drives nice, just the fuel mileage seems really bad.. from what other 300 owners tell me they get much better..

Any ideas what I can do help this? I did do a seafoam into the intake treatment but I wonder if I should add some to the fuel? I use the 89 octane as recommended.

Thanks! View attachment 173958
if you read the owners manual you'll find that 91 or 93 octane is required for the 5.7. I've got a 06 300C/ 5.7 ....went on a long 88 day trip and tried running lower octane fuel . Bad mistake !!! 93 octane , 70 -75 MPH on cruise control...AVG 23-25 MPG ...89 octane..same speeds ..15-17 mpg.......87 octane , the motor ran so bad that the numbers were 11-13 mpg. It took 5 refills of 93 octane to clean out the system and push the engine back up to 25 mpg on the highway at speed. Near as I could find out , the lower octanes, cause the knock sensor to change the engine timing , and thus the lousey gas consumption.
At any rate 91 octane is the lowest I've used since and about every 3rd tank It get's 93 , and a couple of times a year I throw a can of injector cleaner in the tank.

Another thing you might try would be to remove the throttle body and clean it with carb cleaner !
 

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if you read the owners manual you'll find that 91 or 93 octane is required for the 5.7. I've got a 06 300C/ 5.7 ....went on a long 88 day trip and tried running lower octane fuel . Bad mistake !!! 93 octane , 70 -75 MPH on cruise control...AVG 23-25 MPG ...89 octane..same speeds ..15-17 mpg.......87 octane , the motor ran so bad that the numbers were 11-13 mpg. It took 5 refills of 93 octane to clean out the system and push the engine back up to 25 mpg on the highway at speed. Near as I could find out , the lower octanes, cause the knock sensor to change the engine timing , and thus the lousey gas consumption.
At any rate 91 octane is the lowest I've used since and about every 3rd tank It get's 93 , and a couple of times a year I throw a can of injector cleaner in the tank.

Another thing you might try would be to remove the throttle body and clean it with carb cleaner !
that was an 8 day trip NOT 88 !! LOL
 

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Discussion Starter #9
if you read the owners manual you'll find that 91 or 93 octane is required for the 5.7. I've got a 06 300C/ 5.7 ....went on a long 88 day trip and tried running lower octane fuel . Bad mistake !!! 93 octane , 70 -75 MPH on cruise control...AVG 23-25 MPG ...89 octane..same speeds ..15-17 mpg.......87 octane , the motor ran so bad that the numbers were 11-13 mpg. It took 5 refills of 93 octane to clean out the system and push the engine back up to 25 mpg on the highway at speed. Near as I could find out , the lower octanes, cause the knock sensor to change the engine timing , and thus the lousey gas consumption.
At any rate 91 octane is the lowest I've used since and about every 3rd tank It get's 93 , and a couple of times a year I throw a can of injector cleaner in the tank.

Another thing you might try would be to remove the throttle body and clean it with carb cleaner !
Everywhere I had read says 89 is the recommended octane for the 5.7, and 91+ for the 6.1.. even on this forum it was mentioned 89 octane, unless there was some sort of mistake..
 

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Take from the owners manual:

3.5L and 5.7L Engines The 3.5L & 5.7L engine is designed to meet all emissions regulations and provide satisfactory fuel economy and performance when using high quality unleaded gasoline having an octane range of 87 to 89. The manufacturer recommends the use of 89 octane for optimum performance. The use of premium gasoline is not recommended. Under normal conditions, the use of premium gasoline will not provide a benefit over high quality regular and mid-grade gasolines, and in some circumstances may result in poorer performance. Light spark knock at low engine speeds is not harmful to your engine. However, continued heavy spark knock at high speeds can cause damage and immediate service is required. Poor quality gasoline can cause problems such as hard starting, stalling, and hesitations. If you experience these symptoms, try another brand of gasoline before considering service for the vehicle.
 

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Take from the owners manual:

3.5L and 5.7L Engines The 3.5L & 5.7L engine is designed to meet all emissions regulations and provide satisfactory fuel economy and performance when using high quality unleaded gasoline having an octane range of 87 to 89. The manufacturer recommends the use of 89 octane for optimum performance. The use of premium gasoline is not recommended. Under normal conditions, the use of premium gasoline will not provide a benefit over high quality regular and mid-grade gasolines, and in some circumstances may result in poorer performance. Light spark knock at low engine speeds is not harmful to your engine. However, continued heavy spark knock at high speeds can cause damage and immediate service is required. Poor quality gasoline can cause problems such as hard starting, stalling, and hesitations. If you experience these symptoms, try another brand of gasoline before considering service for the vehicle.
Yup !! your correct , my bad !! BUT for the last 81,000 miles I've run the 91 octane , changed the spark plugs at 100k and could probably put the old plugs back in for another 50 k miles. I've spoken with other drivers operating the 5.7 's here in New England , and they've also said they use the 91 octane and get the higher mileage out of a gallon of gas. Sort of in a quandry now over 91 or 89 !!! price difference most of the time is 4 bucks on a fill up. cause I don't let the gauge go below 1/4 tank. !!!!
 

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Check or replace the oxygen sensors.

My 2006 300srt is at around 14.5 mpg with a heads/cam setup, 3.91 rear end, 315/35 rear tires and 155,000 miles.

It used to be at 17-18 mpg stock so I would imagine a 5.7 would fare a bit better.

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