Increased air in tires to sidewall max -> +10% mpg

[JEDI]

2005 mazda3i, 200k miles

so my tires say 44psi max. that's when it's cold.

its been a mini-heat wave here in may.
temp is in the 80s.

my car is rated 34mpg hwy.
i usually get 32mpg doing 70%hwy/30% city.

but my last 2 fill ups I've gotten 35mpg on the usual drive!

woot!
or is my engine/transmission going to die soon?

 

[dbcooper1]

Depending on where you are, it's more likely the switch to summer blend gas.

 

[Yuriman]

Increasing tire pressure is generally the first (and easiest) way to improve mileage, and is well documented at giving ~10% over at Ecomodder.

You might enjoy this diagram:

^ Only the most hardcore/crazy do some of these things, but every bit it adds up. Basjoos, the owner of "aerocivic", has done all of these modifications and has documented his vehicle as capable of 95mpg when cruising at 65mph on level ground. The EPA rating is 34/40.

Personally, I run higher tire pressure and have done a bit of smooth paneling under my car (it also protects from salt), and in the winter I run a partial grille block. My car came from the factory without an alternator, smooth wheel covers, and wheel skirts.

 

[Yuriman]

It's probably a good idea to add that there are some negatives associated with running higher pressure in your tires. You're more likely damage the tire if you hit a pothole. The contact patch will also be slightly smaller, resulting in slightly less dry traction. There will also be more road noise transmitted through the tires, and your ride will be a little rougher since tires are generally the greatest contributor to shock absorption.

Contrary to popular belief though, belted radial tires do not wear more quickly at the center when running higher pressure. Wear patterns may change though. Also, higher tire pressure makes you less likely to hydroplane (due to smaller contact patch).

 

[JEDI]

your car didn't come w/an alternator?! what?

hm.. wheel skirts.
too bad my car is alloy rims. you cant put hub caps on there.

whoah at turning off alternator and using a deep cycle battery. wonder how much +mpg that gains.

 

[pauldun170]

2005 mazda3i, 200k miles

so my tires say 44psi max. that's when it's cold.

its been a mini-heat wave here in may.
temp is in the 80s.

my car is rated 34mpg hwy.
i usually get 32mpg doing 70%hwy/30% city.

but my last 2 fill ups I've gotten 35mpg on the usual drive!

woot!
or is my engine/transmission going to die soon?

Your tire pressure should be listed in the door jamb, around 32 to 35psi.
Thats what you should use.

The number on the tire is the maximum pressure that the tire should hold before it becomes a safety issue. That's without it being mounted on a vehicle.

 

[Yuriman]

Your tire pressure should be listed in the door jamb, around 32 to 35psi.
Thats what you should use.

The number on the tire is the maximum pressure that the tire should hold before it becomes a safety issue. That's without it being mounted on a vehicle.

You're right in that running higher pressure is outside of a manufacturer's specifications, but what goes into those specifications is rather complex, and has more factors than just vehicle weight and max sidewall. I suggest reading up on Load Index.

 

[exar333]

Increasing tire pressure is generally the first (and easiest) way to improve mileage, and is well documented at giving ~10% over at Ecomodder.

You might enjoy this diagram:

^ Only the most hardcore/crazy do some of these things, but every bit it adds up. Basjoos, the owner of "aerocivic", has done all of these modifications and has documented his vehicle as capable of 95mpg when cruising at 65mph on level ground. The EPA rating is 34/40.

Personally, I run higher tire pressure and have done a bit of smooth paneling under my car (it also protects from salt), and in the winter I run a partial grille block. My car came from the factory without an alternator, smooth wheel covers, and wheel skirts.

The guy in the pic would probably do better to just lose 50lb himself and use that weight savings for better MPGs....

 

[Vdubchaos]

The guy in the pic would probably do better to just lose 50lb himself and use that weight savings for better MPGs....

You can also do "Extreme MPG" by not going anywhere to begin with.

 

[tortillasoup]

If you're concerned with uneven tire wear, grab a piece of chalk and write across the tread. Then roll the car forward and see if the chalk disappears across the whole tread. Typically I've found that increasing the front tires makes the largest difference for fuel economy due to the heavy engine. That sidewall pressure in the rear isn't typically necessary, though an increase in pressure back there as well can still be useful. Copy the same pressure ratio that's on the door so if it's 30 rear 32 front, and you inflate to 45 front, then you do 42 rear.

 

[BlitzPuppet]

lol.

Next up "Why is the center of my tire going bald so fast?"

 

[Zargon]

The guy in the pic would probably do better to just lose 50lb himself and use that weight savings for better MPGs....

the only way that guy loses 50lbs is by dying man, hes not fat, at all

 

[JEDI]

Copy the same pressure ratio that's on the door so if it's 30 rear 32 front, and you inflate to 45 front, then you do 42 rear.

thx for that tip.
will check it tomorrow morning when tires are cold again

 

[Yuriman]

lol.

Next up "Why is the center of my tire going bald so fast?"

Except that doesn't happen with belted radial tires. Maybe in your grandfather's day.

 

[leper84]

Except that doesn't happen with belted radial tires. Maybe in your grandfather's day.

Except that it does. See it with my own eyes on a regular basis.

 

[shabby]

Should be fun when it rains.

 

[jagec]

whoah at turning off alternator and using a deep cycle battery. wonder how much +mpg that gains.

I read a blog by someone who tried it with a Civic (I believe), and reported ~10% gains. The documentation was pretty thorough. That was by completely removing the alternator belt. Impressive, though a bit of a PITA.

 

[tortillasoup]

Except that it does. See it with my own eyes on a regular basis.

Yeah it can happen but it's not typical. Obviously if it happens, then yes ease up on the pressure a few PSI. High pressure on radial tires obviously doesn't have the effects like Bias-ply tires had. That's why they still like to use Bias-ply tires when mudding as tire pressure can have a tremendous effect on the tread footprint.

 

[JEDI]

I read a blog by someone who tried it with a Civic (I believe), and reported ~10% gains. The documentation was pretty thorough. That was by completely removing the alternator belt. Impressive, though a bit of a PITA.

what?
how long does a deep cycle battery last?

 

[tortillasoup]

maybe 600 DEEP cycles. Shallow cycles means it lasts longer. If you keep the battery above 12v during these cycles, the battery will last significantly longer. Fully discharged is around 11.8/11.9V.

 

[Yuriman]

Should be fun when it rains.

Higher pressure reduced your chances of hydroplaning because of the reduced contact patch.

http://www.nhtsa.gov/cars/rules/rulings/TPMS_FMVSS_No138/part5.6.html

Hydroplaning speed = 10.35 x (square root sign) inflation pressure

Under this assumption of water depth exceeding the capability of the tread design to remove water:

At 30 psi, hydroplaning could occur at 56.7 mph

At 25 psi, hydroplaning could occur at 51.8 mph

At 20 psi, hydroplaning could occur at 46.3 mph.

This is presented to show the relative effect of inflation pressure on the possibility of hydroplaning.

This trend continues to higher tire pressures, though with diminishing returns above around 45PSI.

 

[Yuriman]

Except that it does. See it with my own eyes on a regular basis.

How frequently do you see overinflated tires? I didn't think all that many people did it with road tires.

http://www.barrystiretech.com/sae800087synopsis.html

They conducted a 13,600 mile on-vehicle wear test and projected the wearout mileage. The increased pressure resulted in increased mileage projection - again the effect slowed down as the pressure increased.

I hope you noticed how flat the wear index is for the radial tire. I interpret this to mean that conducting a "chalk test" is not a good way to determine what the best pressure is for wear. Put another way - Radial tires are pretty insensitive to pressure when it comes to wear pattern.

In this test, all tires showed less wear with higher pressure at 40PSI vs 30PSI, though with diminishing returns. Additionally, belted radials showed very little change in shoulder to crown wear with increasing pressure.

 

[LevelSea]

I hope I'm never so poor that I have to over-inflate tires to save money lol

 

[xBiffx]

Only the most hardcore/crazy do some of these things, but every bit it adds up. Basjoos, the owner of "aerocivic", has done all of these modifications and has documented his vehicle as capable of 95mpg when cruising at 65mph on level ground. The EPA rating is 34/40.

No way in hell that he gets that MPG with only those mods. That's an increase of nearly 150% without even touching the thing that drinks the gasoline.

 

[Yuriman]

No way in hell that he gets that MPG with only those mods. That's an increase of nearly 150% without even touching the thing that drinks the gasoline.

You can read his build log here and judge for yourself:

http://ecomodder.com/forum/showthread.php/aerocivic-how-drop-your-cd-0-31-0-a-290.html

I believe more recently he swapped the engine for a VX engine (with lean burn), but here is his fuel log with the original 1.5L non-VTEC engine the car came with:

Note that even at the start he was pretty significantly beating EPA numbers.


Now, mind you, my car stock is rated for 61 highway but I find the numbers really conservative, at least in the summer. Simply turning off the A/C results in beating that significantly. It's parked out front right now with "88mpg" showing on the display, which was the result of 33 miles of 65mph driving and 17 miles of 50mph driving, with a few stop signs and traffic lights, though admittedly I've done a few (invisible) modifications too.