Forester 6MT - Throw some ideas at me!
 

So 3 years on this car and I mostly like it. Apart from the AC that hasn't worked since day one and still doesn't work after 6 warranty trips. I do miss the easy 43MPG of my old Focus. I also think gas prices will be headed back up, high and quick so trying to toss some ideas around.

In the summer I get about 25MPG commuting, winter 18 or so. In the summer I can eek out 34 or so if I take it easy on the round trip to my parents. That's a 140 mile round trip, state highways with a max speed of 55. Seems that once I exceed 55, the MPG drops sharply.

The AWD system in the MT subarus doesn't decouple and there are three differentials doing their thing. It also has 4.44 gears, so even with the .6xx 6th gear ratio, it's still screaming on the highway.

The transmission and differentials don't use "light" oil (ATF, etc) but I did switch out the gear oil for 75w80. Which is acceptable as a replacement for the factory 75w90. I am running the spec'd 0w-20 engine oil.

I'm thinking about a grille block. I can monitor coolant temps via torquepro. I can't find operating temperatures of the FB25 engine, so I'm not sure at which point I'd be overheating the engine by getting the coolant too hot. These engines are externally plumbed for coolant, so they shouldn't blow the head gaskets as easily. I did consider a block heater for winter, but it's a pretty major install that I'm not sure I want to get into with this car.

I've done my best to try to chart shift points. This engine does NOT like running below 1500RPM under any load. Even cruising, it'll use less fuel at 45MPH in 5th gear than 6th gear.

Ultimately I know I should probably trade it on a used Mirage which would probably have working AC, but I do fit the Subaru marketing image so it works well. I travel off the beaten path with this and it's a pretty suitable replacement for my Cherokee which is now a dedicated trail rig. It also serves as pickup duty and I've towed 2000 or 3000 pounds with it a few times. Not sure a Mirage would like that.

Can't quite go electric yet. My house just doesn't have the electrical service to charge a car.

What model year is this? If you have a warranty I assume it's FB25 powered?

Whoops. 2018 Forester. FB25 powered, correct.

The 3/36 ran out a few months ago. So I'm on my own for the chronic AC problems.

Is it due for tires? Go tall and narrow, the RPMs will like it.

Odd. You say 5th at 45mph is @1500RPM? Are you sure? Do you know what your AFR and timing are at that? Do you have a ScanGauge? Does this have variable valve timing or fixed?

It's geared way lower than that. 5th gear at 45 in 5th gear is close to 2000RPM. 6th gear at 45 is 1600. But it definitely gets less mileage in 6th than 5th at those speeds. At 65 it's almost at 2500 RPM in 6th gear.

It does have some sort of VVT system on it. I should be able to bring up fuel trims, not sure if the car will output AFR or not. And I can check ignition timing. My guess is when cruising at lower speeds in higher gears it just has to pull timing too much.

Tires are still the original. I've run winter tires each year on it so they never really get worn at all. I could get some ecopias but I think that might offset fuel costs.

FB25 is dual cam phasing, and it's a low-revving economy engine so it should be tune pretty well from the factory. The AWD you're stuck with and the car is never going to be lightweight, so aero is probably the only thing you can try improving.

Your gearing isn't that short considering the size of the car and the drag, you can try taller tires but I wouldn't expect too much.

This just doesn't make sense. It's a low revving economy engine that doesn't get good fuel mileage when revving low. If that's true wouldn't putting taller tires just make it worse?

Timing advance becomes less important at low revs and higher loads (e.g. going 45mph in 6th gear). The flame is faster and the piston is slower in that case. So unless your timing is retarding below 7-10° BTDC or so I don't see how it would affect fuel mileage that much. Timing advance makes up for slower flames and faster pistons.

Have you done a compression test recently? Maybe you have too much blowby. ??

Maybe I oversold the concept? The tall, narrow tire has aerodynamic advantage even if the gearing is unchanged.

Tire pressure also.

I had a 03 Forester X with the 4-speed auto and 2.5L NA engine (4EAT + EJ25) and my best efforts of basic aerowork, tire pressure, and driving speed of 50mph got me 33mpg. Otherwise, mid-high 20's was the best I could do.

I now have a 07 Forester XT with the same auto, but 2.5L turbo (with a few mods) and my best tank so far has been 27MPG doing 60-80mph. I haven't really had a chance to give it further road tripping for better, so tanks are 18-23MPG otherwise.

All this is to say: Good luck! Subarus were built with many things in mind, but economy isn't one of them. (or headgasket life)