well depending on tires, it CAN inflate further 5-10psi during hot.

my tires does just that especially after spirited driving which can rise from 32 to 40psi or so.
then again they aren't meant for eco.

just thought i'd clarify this a bit

Actually, one can calculate how much the pressure will be under hot, using thermodynamic equation
PV=nRT.
As volume of tyre volume remain constant so

P (cold) =nR T(cold)
with
P (hot) = nR T(hot)

so the air inside must be very very hot, if it manage to rise extra 8psi on its own.

Can someone doing the math to see how much temperature rise to have extra 8psi.

i released the air inside before and it was almost scalding like (fingers gotten a bit red lol)

i would assume that it'd be damn hot?

If it rises 8psi on its own, it should be burning already, as temperature needs to rise more than a few ten of degree C to have so much pressure rise.
Again, anyone help on the thermodynamic calculation, to show exact how much temperature has risen that can result in 8psi rise.

Ambient temperature assume 30c.
which is 303 K (273+30)

Pressure 30 psi at 303
Pressure 40 psi at ?

32/303 = 40/?

? = 378K

378 - 273 = 105C.

Please correct me if I am wrong, thermodynamics rusty already.

it didn't felt like 105c though else i'd have burns

iinm i read somewhere about 10F increase would increase about 1 psi, so 8 would be 80F and thats about 26C, +30 would be about 56 which seems about right for the temperature i felt when i release air pressure.

May be I get the unit and formula no right, as said rusty already.

Unit problem.
Tyre pressure = 32 psi *relative* to atmosphere. Absolute pressure is tire pressure + atomospheric pressure
Assume 1 atm pressure = 14.7psi

Therefore
P1 = 32+14.7 psi = 46.7 psi
P2 = 40+14.7 psi = 54.7 psi
T1= 30 + 273.25 = 303.25 K

Since nR and V are constant

P1/T1 = P2/T2
T2 = T1(P2/P1)
T2 = 303.25 * (54.7/46.7)
T2 = 355.20 K = 81.95 *C

Thus temperature increase
T2 - T1 = 81.95 - 30.00 = 51.95 *C increase

If pressure only increase to 38 psi, temp would be 68.96*C. This is for *ideal* gas. Air is not ideal gas, it consists of many types of gas and water vapour (moisture). Air temperature does not increase linearly with pressure. Pure nitrogen however is close to ideal gas behaviour.

My thermodynamics is rusty also. Forgotten how to take in account partial pressure of the gasses.

This post has been edited by sanadi: Sep 17 2013, 05:43 PM

Fuel efficiency had turn into complex physics calculations ...lol...

Maybe should get Soft, Medium & Hard compound tyre, don't forget semi & full wet tyres too change according to road temperature & weather ...rofl...

Cabut....

This post has been edited by jolokia: Sep 17 2013, 06:59 PM

You'd need a pit crew travelling along with you.

Over-inflating by just a bit is quite safe...provided that you know that you are not cornering very hard/fast, and will not be operating the vehicle beyond the recommend loading. You don't need to over-think the "extra load" bit. I'd look at my passengers and cargo to have an estimate. I've never had to worry about that even when there a 6 occupants in my car (Myvi BTW). However, most of the time my passengers are small petite women, so...

But none could replace driving habit. The trouble is, lots of people confuse between driving gently and slowly. Driving slowly will just be annoying to everyone else (unless you're doing it for safety reasons like carrying a s***load of durians at the back seat, then carry on).

There's no need to over-think it if you want to save fuel. Drive at the speed limit, and be patient when getting to that speed limit, just those for starters. There are various driving tips all over the net. I've seen a person from a forum getting 11km/l using a '76 El Camino with a 5L V8 auto, no mods just driving technique. If that person can do that with that car, what excuse do we have driving cars much smaller than that?

ok i'll be honest here, i fail my math during school

some cars though are just not meant for driving "efficiently"

take for example my iswara... 3 speeder auto.
1st shift is at 20km/h, 3rd and final shift is at 40km/h. so if i wanna stay at minimal throttle input/sub 2-3k rpm, id be sitting between 40-60km/h.

surely, it improved my FC from 7-8km h to EASILY 8-9 km/h or so despite i STILL pull off red lines when the traffic permits... but damn it is hard to break a hard revving habit and go 40km/h most of the time.

for reference: http://www.fuelly.com/driver/quazacolt/saga

Sorry, got carried away with the calculations.

I agree with you. The key to eco driving is smoothness. Do not drive too slow or accelerate too slowly. Not only is it dangerous, but the fuel saving will be wiped out when you got rear ended and have to repair the bumper. Maybe only RM300 but can the fuel you saved cover that? Your are saving fuel, but not saving *money*. At the end on the day it is *money* that we want to save. (Maybe environmentalists don't think so?)

My advise for saving fuel and money that give to my friends:
* avoid jams if at all possible
* avoid idling. Need to wait for someone? Shut the engine and wait outside the car. (If safe & possible)
* accelerate normally, at a relaxing pace. Not too slow and not too fast.
* drive the speed limit
* lighten your car by taking out the things you don't need. Your don't need to always put your grass cutter in the boot. Take out. But don't take out your spare tyre or your emergency kit. (If you have to call a tow truck when you have a punctured tyre, expect 3 digit charges, unless your insurance covers it)

fuel saving topic is nothing new.

9 basic steps if you really into it:
http://www.wikihow.com/Save-Petrol

When waiting for someone you can (when save) open the windows too. Lots of wind sometimes, very cooling. Even a bit (so that others can't really attack you) helps. Maybe just open the opposite site so they can't take a knife to your neck.

Hm. One of the reasons why I overinflate a bit is cause I'm scared the tyre gets torn off the rim when cornering Wrong thinking?

An optimum tyre pressure shouldn't have this happen.
Normal car won't be cornering 60~90 degree like F1 at >100km/h.
You turn turtle instead.

Haha, I'm driving on 185/65 R14 though... if it were 195/55 or so I'd agree. I've seen my car on the Nordschleife with a camera mounted next to the front tyres, and it was a scary sight. In terms of bodyroll I don't think that's so easy to achieve (not enough grip from the XM2...), though if I do slalom I would eventually play turtle...

just keep ur RPM below 2 if possible or max 2.5, then you're golden

I know how fun it is and I sometimes find myself a very large empty parking lot to ease off my addiction (I like the feel of throttling up after a very hard corner).
If you really want to save fuel but not change your car I guess you either live with it or dump the habit.

It's got a long ratio; the 3rd gear on iswara is about the same as the 4th of a Myvi manual box. Not a highway cruiser for sure, but I think it won't do too bad at the lights. I'd say tune the carb just to be a bit lean but that would mean you'd REALLY have to say bye-bye to the high-rev habit. Change carb maybe (if you haven't done so already)?

You'd have to "crawl" that thing off the lights but you'd be annoying everyone behind you. I know I would be if I were behind you, eventhough I am a hyper-miler.

it's at 13.5 AFR, pretty lean btw.
my engine is typically running lean so the temperature is a bit on the high side.

well, now i can indulge in some other fun shenanigans which is floor/downshift if someone decided that my crawling is too annoying