mythbusters tests tyre / tire pressure

[kimer6]

I have been driving my Geo Metro convertible with 40psi in the tires at 70 degrees F. I made 2 recent trips of 730 miles each within 3 weeks taking the same roadways. The first trip netted me 44.1 mpg after setting the tires to 30psi front and rear. I switched back to 40psi and averaged 47.8mpg on the second trip. I held local speed limits on each trip which included stretches of 70mph. The load was myself, my dog, toolbox and overnight bag.

The ride is slightly rougher with the additional air in the tires but it is well worth the added mileage. The car is stock except for the XFI cam and timing gear changes, ATF in the transmission. Driving was with the top up and windows closed on both trips. The weather was similar both times.

I did some maintenance before the first trip and found one tire to be at 30psi, so rather than open the garage again for the compressor I just evened out the tire pressures with the thought of checking mileage changes. This was about as accurate a test as I have done in a long time. I also fill right up to the filler neck to be sure the usage is accurate.

[GdB]

This is a great thread about the 1st mod that everyone should do. I have always inflated to 100% of tire rated max. cold pressure. I believe that is the safest because: (1) low tire pressure builds up tire temperature due to hysteresis, when the rubber is hotter it is weaker, thus the Ford explorer situation (2) a leak will give a bit more time to catch it before it get dangerously low. (2) grip is close to optimum at high pressure, even if slightly reduced. stiffness is high which make the car handle stiff not sloppy. Sloppy handling like old US cars is dangerous.

A trick I use to reduce leakage a lot is grease the tire valve. A tiny dab of grease in the valve when adding air get pushed into the valve seat area. I have fixed many leaky valves this way. It works best on bikes since they loose pressure much fast for a given leak volume. Leaky valves are very common. Yokohama sells a tire that looses less air thru the rubber. I have gone many months without having to add 1psi.

[hu_man]

jakobnev thanks for the 3rd grade education. I was just trying to get my message out that it is not safe to over inflate the tires.

As mentioned before the suspension is not designed to work with a tire that bounces a lot. A over inflated tire on a rough road will bounce. It is the shocks job to keep it from continuing in this motion. If the tire is bouncing it is not on the road. If it is not on the road you have no control of the car.

I have been racing cars for over 35 years now and suspension is what keeps the car on the road. Yes the tires do also but if the suspension is not working properly you can not maintain control in the extreme. And it is in the extreme that I want control. Most of the time everything is fine it is when the idiot pulls out that you want complete control. If your willing to give up control for $77 a year go for it.

Me I will keep my tires at a presure that give me the most control over my car and spend my energies else were. A well tuned engine, driving habbits, and just not driving around to see the sites.

[watercat]

I think a real test would be to really squeeze them and see how firm they are. See if she has them just right before you take them for a test drive. Visually it looks like the set is perfect in diameter but until I can get my tool on them to check them for sure will just have to assume there with in tolerance.

[watercat]

i run my Geo tires at 50 psi and they wear just fine. Tomorrow I'm installing a small lift to the suspension and raising it to clear my new 15" rims and tires so i can turn my 5 gear into a 6 th gear. I did this a few years back and it really helped on mileage but i actually never wrote the numbers down. This weekend I'm driving 1200 miles from Washington to Montana and back with two and three people on board so will see how it goes. I'll post my numbers. I'm hoping to really use the heck out of my kill switch on the down hill runs. I'll also measure the tire heights and post them as well. I have log book now on my first 10,000 miles on this restored and modified 94 metro.
FYI just bought a tracker 1994 4wd gets 28 mpg at 60 mph with just me in it. and over 300 miles it averaged 25.49 towing my geo metro behind it on a tow bar. it turns 3500 rpm at 60mph. it has 205 70 15 on it and tomorrow i'm switching it to 235 75 15 to lower the rpm and recheck the gas mileage. I'll post some reports. its the 1.6 8 valve motor if any one wants to know in a manual and the geo car is a 3cyl 5 speed.

[evtinker]

One point to recall on tire pressure, the effects of heating. I run the tires on my electric Civic conversion at about 40 PSI front and 38 PSI rear. The sidewall rating is 44 PSI, factory sticker says 32 Front and 28(?) rear. The thing is the factory rating is for cold pressure and assumes you may be driving at high speed for hours, which would really heat up a tire and raise the pressure. Since my EV is an in town only commuter and seldom exceeds 45 mph, heat buildup and the resulting pressure rise simply never happens. For all intents and purposes my tires stay pretty cold most of the time which is not what the factory expected. Effectively at the factory settings they may be under inflated.

[jtbo]

Isn't sidewall rating for cold pressure too?

[moorecomp]

Search the old posts before dragging all of this up again!

[wyatta4]

So I gave a quick question to throw in here.. If I were to get the Geo metro tires and inflate them to bout 50 psi then id see a decent increase in gas mileage in general? Ive considered this, along with some pretty sweet Moon hub caps HAHA
Thanks,
Wyatt

[jtbo]

Originally Posted by moorecomp

Search the old posts before dragging all of this up again!

And with energy you used to be an ass, you could of provide simple answer which seem to be very clear to you, wear to your keyboard would been less, also usage of electricity etc.

So you did not achieve nothing else than being an ignorant person.