View Full Version : +2psi = 2MPG better?
LowFast
11-30-2006, 08:51 AM
Crazy I know, I have always run the spec-b at the reccomended pressure 33psi, but decided, after our day at the track, that 33psi always seemed low anyway and decided to bump it up a bit. So I bumped it to 35psi, car feels better. Previous MPG (at 33psi) around 25 mostly highway. Exact same driving style and type with the tires at 35psi 27mpg! Who knew just a few psi would make such a difference.:icon_bigg
wukindada
11-30-2006, 08:52 AM
Still on the low side;)
lucas569
11-30-2006, 08:54 AM
omg when i 1st read it i thought u were pushing 33psi of boost! lol ;)
jerseyglock
11-30-2006, 09:15 AM
More psi, less tire contact surface = more mpg.
LowFast
11-30-2006, 09:55 AM
"Still on the low side"
That is actually my though as well. I think that the Subaru left the psi reccomendations the same as the regualr GT even though the Spec-B has a lower profile tire. I may try 38psi and see how the car rides and the tires do. What pressures are you guys running in your 18's?
wukindada
11-30-2006, 09:56 AM
40f/38r
I think that the Subaru left the psi reccomendations the same as the regualr GT even though the Spec-B has a lower profile tire.Doesn't the SpecB have the same 215/45 profile just like the LGT, except just larger diameter?
LowFast
11-30-2006, 10:08 AM
^I guess so, I did not realize the GT had 215/45's. Well then all of the tires are underinflated from the factory.:rolleyes:
Deer Killer
11-30-2006, 10:15 AM
The higher the better... (except for your kidneys)
lgt_nube
11-30-2006, 10:31 AM
The higher the better... (except for your kidneys)Wouldn't too high a pressure cause the center tread to wear before the outer parts?
mickeyd2005
11-30-2006, 10:39 AM
Wouldn't too high a pressure cause the center tread to wear before the outer parts?
Theoretically yes, but there are a lot of other factors involved such as sidewall stiffness and overall design of the tire plus alignment. Monitor your tire wear and adjust as needed.
I had my stock tires bumped up to 40/38 and didn't notice any noticeable problems with center wear after about 6000 miles.
Some tires seem to need more tire pressure. With coilovers, the Kumho SPT 235/40/18 tires that I have are really bouncy at low tire pressures. I had to bump them up to 42/39 to get a decent ride out of them. One of the other local forum members said he noticed the same thing with Kumho tires.
BTW, I think you gained a big increase in MPG because your tire pressure was really low compared to what other people are running (disregarding Subaru recommendations). If you increase your tire pressure to 40/38, you probably won't see that much more increase in MPG (maybe a little bit but not much).
LowFast
11-30-2006, 10:56 AM
Thanks for the info, I did not realize subaru did such a crappy job in what they recomend (although my tires did not suffer, as they look like brand new). I will try bumping up higher and see what results I get. Hopefully I will be changing rims out and moving to a wider tire. Another set of spec-b rims will be up for bid soon.:icon_bigg
srgdrum
11-30-2006, 11:24 AM
What is a good pressure for the stock rubber for colder temps? 40/38 like wuki?
AWDxBOOST
11-30-2006, 11:26 AM
more pressure, less drag from the tires=better mpg
ebpda9
11-30-2006, 11:43 AM
even the lowly 2.5i's seem to be better with around 36/34psi f/r
rporter
11-30-2006, 11:49 AM
Thanks for the info, I did not realize subaru did such a crappy job in what they recomend (although my tires did not suffer, as they look like brand new). I will try bumping up higher and see what results I get. Hopefully I will be changing rims out and moving to a wider tire. Another set of spec-b rims will be up for bid soon.:icon_bigg
It's not a "crappy job", it's the way that all mfrs do tire pressure recommendations.
They take into account all conditions, as well as ride comfort and tire wear. And, for most folks (even some on this forum), ride comfort is a high priority.
But, for performance and mileage with a slight sacrifice in comfort, pump those puppies up!!
With a previous performance car that I owned for 10 years, I could tell when the tires were getting soft but a drop in gas mileage (it also did it when the oil was getting old, but that's another topic).
If you are concerned about mileage as well as performance, don't move to a wider tire. The stock size (width) does a fine job, and the extra weight, coupled with a marginal trad width increase, won't help anything except the "look". Matching tread width to rim width, as well as tire composition, will do more for handling.
LowFast
11-30-2006, 11:54 AM
^ I plan to go to a 18x8 wheel with 245/35 tires, so will get more width from both rim and tire. MPG's are not my main focus, I was just amazed as the difference a few psi made. I am pretty tollerant of ride quality so the extra pressure is not a big deal.
PhilT
11-30-2006, 11:55 AM
more pressure, less drag from the tires=better mpg
Also means less contact with the road when it comes to braking.
rporter
11-30-2006, 12:03 PM
^^^ Not really, it was more of an issue with old bias-ply tires.
There is even a strange effect with some radials where higher pressure could wear the edges more than the center. Higher pressures in radials don't necessarily affect the tread contact patch as much as the sidewall flex, which can sap more power.