Thoroughly disappointing highway MPG: the norm or exception?

Hi all,

I just spent a few days driving from my home on the east coast of Canada to Miami & back again. I ran the Cobb economy map, and used cruise control set @ ~5MPH over the limit - 70 or 75 the very large majority of the time. A/C is deleted, so not a factor.

Even on the ridiculously flat states (VA and further south), it was uncommon for me to see above 22MPG. What gives?

If it helps, I've got a new O2 sensor, 4x new coil packs + spark plugs, and 2x new injectors, all OEM.

Been asking myself the same question. Typically in the spring and summer I had been easily getting 27+ on my highway trips. Even last winter 24-25 wasn't to tough to get. This winter for the same exact trips, I'm consciously driving easier and still struggling to get over 22mpg. You said your trip started in Canada, and I imagine pretty big chunk of it was through states with winter gas blends. Wondering if they put some extra winter stuff in this year :o

I was just about to do plugs, pcv valve, Chevron techtron fuel treatment, and clean the MAF in hopes something helps. I'm at 95k miles.

What has been your typical highway trip milage before? Reason I ask is its tough to compare to others here. I've seen people that 18-20mpg is "normal" for them and others 26+. Have to compare to yourself to eliminate the variables of others driving habits, locations, gas quality, tunes, and mods.

Which brings up another question, what kind of gas stations were you using. I see a pretty big mpg difference from say a shell to a Hess station.

My 09 Spec B still get's 25-26mpg when we take it to VT on weekends skiing. It's Cobb stage 2 catted DP.

My wagon has been only getting 22mpg since I put the 18" wheels and big Mishimoto radiator in early Nov. Before that it was getting 24-26 mpg on the DD to work, mostly highway. Even on the drive to/from VT it's hard to get 23mpg.

It's been so cold I haven't had the urge to clean the MAP sensor and reset the ECU to see if that helps.

I also notice a improvement in MPG when I fill the tank with different gas every now and then. Not sure why that is ?

Check your tire pressures and run more then stock. I noticed that my gas mileage would drop majorly when it got cold, increasing the tire pressures to be 42psi front and 40psi rear has helped me regain that mileage (make sure to never go over max sidewall pressure). Do keep in mind that tires will also be higher psi after driving.

Other then that there are a lot of things that could be as variables. Sure your going 70mph, but do you mash on the throttle to pass cars? I mainly drive 70mph, but will every now and then speed up to 75mph to pass a car, but then go back down to 70mph.

Finally, I started a thread to log my research for everything that has helped me improve my economy right here.

I just bought Drew's Spec B and drove it from LA to Bend Oregon. 32 mpg. And that's not flat, quite a bit of mountain driving in the Sierra Nevada, Siskiyou, and Cascades.

Not sure why ya'll are getting such bad mileage...I was going 70-75 almost the whole way.

When I drive mostly highway at 70 I get around 26-27mpg. Any other combination of driving and I get 22mpg

This is more along the lines with what I was expecting.

You said your trip started in Canada, and I imagine pretty big chunk of it was through states with winter gas blends.

 

What has been your typical highway trip milage before?

 

Which brings up another question, what kind of gas stations were you using. I see a pretty big mpg difference from say a shell to a Hess station.

I thought about this, but it really didn't change much the whole trip. This is the first really long trip I've taken, but 24MPG stands out in my mind as "normal". Over my whole ownership period, I'm at 22.2MPG - including plenty of autocross & a few road courses. So somewhere in there I WAS getting more. Been using Shell the whole way.

Check your tire pressures and run more then stock.

 

Other then that there are a lot of things that could be as variables. Sure your going 70mph, but do you mash on the throttle to pass cars? I mainly drive 70mph, but will every now and then speed up to 75mph to pass a car, but then go back down to 70mph.

Running 40PSI F and 38PSI R - checked at every fuel stop! Stock wheels running my winter tires: 225/50/17.

With the economy tune, I'm ridiculously gentle on the throttle. Never go in to boost, never drop into 4th for a pass. I also want to mention that this was strictly highway, my overnight stop (15 hours day one + 17 hours day two) was right off the exit. The ONLY other times I stopped was for gas.

I just bought Drew's Spec B and drove it from LA to Bend Oregon. 32 mpg. And that's not flat, quite a bit of mountain driving in the Sierra Nevada, Siskiyou, and Cascades.

 

Not sure why ya'll are getting such bad mileage...I was going 70-75 almost the whole way.

The six speed with the long highway gear makes a big difference.

Also OP did you have a bunch of weight in the car? My wagon gets about 24mpg highway with 18's, bigger tires, etc. Were all those new parts you put in trying to mask an even bigger problem? Compression and leak down done?

Also OP did you have a bunch of weight in the car? My wagon gets about 24mpg highway with 18's, bigger tires, etc. Were all those new parts you put in trying to mask an even bigger problem? Compression and leak down done?

Right! Should have mentioned that too. I had the car weighed at an autocross event, came in at just over 3200lbs. Add to that: 150lbs of me, 4x wheels+tires (my summers, used them at a road course event), 3x small tool boxes, and my luggage for the trip. Definitely under 3600lbs - I pack light.

Parts were replaced because they were all original @ 120000 miles. Side note: replace your coil packs, my northern brethren! Our stupid ******* salt eats away at them. The difference is night and day.

Compression test done last summer (due to dreaded P0304, which turned out to be injector related) came back just fine. Can't recall the exact figures, but the first thing I did after getting it checked was compare it to average values from other lgt.com members.

I find that speed and my roof rack have huge impacts on mileage.

at 65 I get about 26mpg, at 75 that drops to 21, with roof rack installed that drops further to about 18-19 at the higher speeds.

I'm averaging 22.2 MPG with a fairly even split between highway and city miles. About 10-15 miles per day total.

On long highway trips I could see 25ish MPG.

See, I swear I'm the only one to not experience a hit on my mileage with a rack. I leave my crossbars with no wind fairing on my Honda 24/7, and haven't had any mileage impacts.

Probably has to do with the way the air moves over the car(Subarus are generally taller and IIRC, have slightly higher Cd). On my Honda, the difference crossbars made were negligible(no fairing either)....and this was a boxier 88 hatchback(had basically the equivalent of an SOHC Si engine with short-ratio gearbox; I was able to get over 45mpg highway with that car even though it sat at about 4000rpm cruising around 70mph).

On my LGT wagon, I see about a 2-4mpg loss depending on speed but with a snowboard carrier mounted up top(never measured with actual snowboards in there but those figures are with fishing rods in there). Both of them had Yakima bars but with Railgrabs on the Subaru while the Civic had Q-towers. Though, there were OEM Aero bars for the Subaru but it's hard to get; but there's plenty of aftermarket available(Whispbar, Aeroblade, etc).

Good to know I'm not the only one. I run SST towers on my Civic (only towers that will fit a curved roof) and they're fairly tall. Yakima round crossbars.

I'm gonna get some railgrabs soon for my wagon, I hope I can just drive around with the crossbars without getting too much of a bad effect on my MPG. I do mostly city mileage, since it's either stop-n-go or short 5 or 6 mile freeway trips.

If you're not going highway-speeds often, I doubt crossbars would make a difference. My daily commute involves going on the highway a lot(average of 70mph; and I live 20mins from school and 20mins from work, and the Subaru is way more comfortable than the Civic)...so they only go on when I need them.

One thing with my car is there's only one plastic underbody panel left(never reinstalled the front one and one side got ripped off in a snowstorm a while back). Not sure what effect it has but I need to get that repaired soon.

See, I swear I'm the only one to not experience a hit on my mileage with a rack. I leave my crossbars with no wind fairing on my Honda 24/7, and haven't had any mileage impacts.

I only see 1/2 mpg loss with a Thule cargo box and rack with fairing on the highway. No difference in the streets.

When I drive mostly highway at 70 I get around 26-27mpg. Any other combination of driving and I get 22mpg

What he said...