Before I start, I realize this is not a Prius. But I've owned an 08 USB ISF prior to this that regularly got 17 city/25 hwy MPG . Right now my 2011 ISF with ISSForged QED is getting 12 city/22 hwy on average.

I thought that it might be due to my right foot from wanting to hear the exhaust roar, but even when I am careful I can only manage 14 mpg combined at best. I checked my tire pressures to make sure they're at the factory spec so I don't think underinflation is causing my poor mileage.

I also had a CEL for bad O2 downstream sensors. I replaced them with new Denso units and cleared the CEL and it hasn't come back. My MPG has stayed the same though. No other CEL's.

What else could I change/check to make sure my car's engine is running as efficiently as possible? Air filter? Spark plugs? Should I run fuel injector cleaner at my next fill up?

Anyone else get really sub-par MPG even for an ISF? I feel like I'm getting as bad mileage as my friend's old C63 which has an even bigger engine than ours.

 

4 words.

Crappy California Summer Fuel

I've dropped from 19.8 to 17.2 in my RCF since the change 6 weeks ago.

 

Are you in traffic a lot? Many times the idling will cause the MPG to decrease significantly. Coupled that with the heat, one will most likely want to use the A/C, which also attributes to low MPG readings. Also, when was the last time you changed the oil? I do have a friend with an F with the QES exhaust, and he's getting 17 MPG mixed. But he's ripping like crazy and wants to be loud as often as possible. 14 MPG combined is pretty low, unless that lead food is constant on the gas pedal.

 

Could you explain that, please?

 

+1 what he said

 

I've always heard this but always thought people were overreacting. That would **** me off haha

 

Carbon Fiber right foot...

 

If the codes are clear. Check air filter. Spark plug (wich i doubt). Clean maf. And check if all 4 wheels spin freely. And i don't know how much your wheels weight but it can drastically change fuel economy if they are heavy. Im heavy on the foot and not near that mpg.

 

I get ~17 on average per tank full stock. Should I be expecting more?

I did get 24pmg on a 2hr highway trip last weekend with some spirited driving.

 

Ok I stand corrected. The actual opposite is true. I just asked my buddy who works for Valero and this was his engineering response:

Nope. Actually the opposite is true. Summer gasoline (low RVP) gets better MPG than winter. Winter will drop by about 2-8% because we can use high RVP butane in the winter which is a smaller chain hydrocarbon and breaks down and evaporates quickly. Thus, butane also produces less energy when burned.

 

California has regulations that are different for cold weather and warm whether fuel additives, right now (since about the end of April beginning of May) we are using summer blend. now typically summer blend will give you better MPG, the air is warmer so the vehicle is combusting less fuel (leaner air/fuel mix) ,think the difference between a cold startup high idle and a warm start up lower idle.

As a result CA has regulations (so do many other states) that require a different blend for summer, less oxygenation additives, etc.

The problem is that the weather is not cooperating, here is SoCal we are approx. 8°-12° below our average normal temps for May/June.. instead of it being in the mid to high 80°'s it is in the high 60° lower 70° for the majority of the day, only heating to the mid 70° after mid day. This results in colder startups and a richer air/fuel mix with a fuel not designed for it.

as it starts to warm up you should see your MPG rise. then when winter comes you will see it fall again.

 

I regularly get 27.5+ mpg hwy and in warm weather I can get over 30. You guys must be heavy on throttle!

 

Does it have factory size tires? One thing people often overlook is when they upgrade tiresize, you're going to get reduced mpg. I used to get 26mpg highway and upgraded to 245/275 tires and it dropped to 24 mpg highway

 

I am in traffic often but don't have the AC on ever (although I usually leave the car on the half defroster/footwell vent mode no AC on), and my commute is 80% city 20% hwy but 12-14MPG combined is still pretty bad compared to my old car. I changed my oil about 4000 miles ago, no improvement before or after.

Even when I drive like a granny and the econoboxes are accelerating faster than me, I cant manage the MPG i used to get on my old ISF

Thanks that's what I thought I should check. My wheels are the stock 2011 wheels.

My tires are the stock sizes I believe. 225 front 255 rear.

 

Im from sweden and my tank average is on 17 mpg, i've ( read my girlfriend ) has managed to get it to 26mpg, and i've managed something around 24 but no patience driving slow/planning my driving, guess i've got a Heavy foot then? haha also when i drive in Town its around 16-17 mpg,