My tyres were oversized my MPG went down!
 

So I had my winter tyres on when I started monitoring my FE. I was getting pretty good fuel figures without really trying too hard. No aero mods and a little P&G would get me way above book figure (or EPA as you would say?).

Anyway, I have since realised the winter tyres were 195/65 15" and my car should have 195/55 15" fitted!!! This has made all my fuel readings void! (I go by in car computer not actually tank range).

Last week I get a new set of 18" alloys with 215/35 Summer Tyres, which is pretty much the exact same rolling radius as the standard 195/55 15's. All of a sudden I have lost about 5 MPG!!

Panic sets in! Now obviously, I haven't *actually* lost fuel, but the onboard computer has been reporting incorrectly due to larger rolling radius. But it is still alarming when you are used to seeing 40+ on the computer suddenly drop to mid 30's!

So I had the task of getting the MPG 'readout' back up to the figures I was used to seeing.

Today I put a lower grille block on, nothing fancy, but it does the job. It is 29°C this week and my car ran 2°C hotter :) which is good because motonet (auto shop) sent me the wrong thermostat when my old one failed and this one runs 2 degrees cooler. So that has brought my thermostat up to standard temp! And I also adjusted toe-in as one wheel was way out and the other was slightly out.

Then I went to garage on nearest island which has an air pump guage. The new tyres should be 50 PSI. I checked all 4 - below 30PSI! The drive there I stuggled to get 38 MPG (UK) (ok I didn't P&G I had a ferry to catch), the drive back I got 47 with a bit (and I mean a bit) of P&G :)

I know the old cliché about tyres being pumped up properly, but jesus christ, what a difference, maybe because I have low profile tyres they scrub even worse deflated I don't know.

So now I am pretty much back to the readings I was getting with my oversize winter tyres, only now my readings are true!

I really need to get some polycarbonate to cover the lower grille and fogs properly, I think this is pretty bad area that needs adressing. Here is a pic of same model as mine, see the lower grille and fogs is a gaping hole:
http://memimage.cardomain.com/ride_i...0041_large.jpg. My grille block sits in the centre hole quite a way back, so air is still entering it and then hitting the block, I could really do with something flush just below the license plate down to the front splitter. I think this would be significant improvement :)

*Note that aint my car (exact same year and model (and wheels)) but mine car looks like an off roader it sits so high!

First, be sure you've calculated your mpg's correctly.

Second, you did remember to increase the inflation pressure on your 18" because you now have Extra Load tires - Right?

Third, Summer tire are not noted for their RR properties, so depending on what you actually put on, they could be TERRIBLE for RR - and you will never get good RR out of a 35 series tire. That's just not what they are about.

You didn't lose 5 MPG if all your previous data is invalid. You're just starting fresh, and you're doing a pretty thorough job setting the car up.

Yep, I live in the middle of nowhere. Today was first chance I got to go check the PSI. These tyres I just checked were inflated to less than 30, the sidewall recommends 50PSI, so I took them all to about 52 (Well 3.5 Bar here which I think is about 52PSI).

The difference from 30PSI to 50PSI is simply, astonishing. as my MPG figures showed immediatly as I pulled out of garage onto country lane (resume cruise) was hitting 37MPG on the way, now 47MPG!!! (UK MPG that is..my Volvo is UK Spec).

sorry for intruding- however, i thought the correct tire pressure should be what is on the sticker on the door frame, NOT what the sidewall reads.

in other words, correct tire pressue is vehicle dependant.

Most car manufacturers recommend lower tire pressure for comfort (softer ride). I'm running 50 psi in my tires, I think the door placard states either 30 or 32 psi. It does give the car a little bit of harsher ride, but the handling of the car has improved and my tires seem to last longer with the higher pressure.

I can't say I have ever even looked if that sticker even exists on any car I have owned. It is just common practice to look at the sidewall, you already bending down with the air gun, it makes sense.

What is written on the sidewall of a tire is the MAXIMUM usable inflation pressure, but that doesn't mean that is the correct pressure to use. The vehicle tire placard has what the vehicle manufacturer specifies for tire size and inflation pressure - and that will almost never be what is written on the tire's sidewall.

I tried to look your vehicle up - a 1998 Volvo S40 SE, right? - and my book doesn't list that model because my book is for vehicles imported into the US and that one wasn't.

So look for the vehicle tire placard and see what it says.

But we do know one thing - and that is that you have changed tire sizes. But there is a way to connect the placard to what you are using and that is by calculating the load carrying capacity of the original tire size at the placard pressure and then figuring out what pressure is needed to match that load carrying capacity.

The calculation isn't difficult, but it requires the use of tire load table - which I have - and I would be glad to do that calculation, That way you'll know where you are relative to what the vehicle manufacturer says.

I was taught to reference the sticker. So it is common sense for me.

But even if I wasn't does that make the sticker wrong?

I'm not a car designer / engineerer / manufacturer, so when the car manual (yes I do read those) or the sticker says that the tire pressure should be X then it was deemed so for a reason. Perhaps, saftey reasons, perhaps wear and tear reasons (and if so, is the money saved in petrol worth the extra money in excessive wear and tear with OVERinflated tires?)


according to www dot advanceddrivers dot com/info_01002.htm
Tire Safety Tips from Advanced Drivers of America
What IS the correct pressure to put in your tires?

So now let's go back to the information on the sidewalls of your tires. It is surprising how many people believe this is where one can find out the correct pressure to put into the tires but it is important to read it correctly. What it actually tells us is the maximum pressure at which a tire may ever be used, not the correct pressure for that tire on your particular type of vehicle.

But it is actually the automaker, not the tire maker, which decides the correct pressure for your tires. Why? Because only the automaker can measure the dynamics that their vehicle will apply to the tires and only they that can therefore assess what is needed from the tires, specifically for that vehicle.

If a driver over-inflates the tires it is the central band of the tread pattern that wears fastest because the tire has bulged like an over-inflated balloon. But because the tire is running only on the central band of tread, as opposed to the full width of the tread pattern, there is much less rubber in contact with the road and the result is less grip, especially when grip is most needed: under hard braking. So with over-inflated tires braking ability is reduced.

So lets review;

-Correct tire pressure is not what is on the sidewall.
-Over inflated tires cause improper and excessive tire wear.
-Over inflated tires reduce braking effectiveness which is not safe.

It's interesting how adamantly people will believe that the manufacturer's recommendation is THE right number. Especially here, in a place where we all agree that the way the manufacturer made our car is not the best way to have made it.

"Yeah, the manufacturer didn't design the grille right, and the air dam is too high, and the underbody is a mess, and a lip on that hatch would drop the Cd by 0.01,
BUT
I am POSITIVE that he is EXACTLY right on the tire inflation pressure, and that that single number is correct for all drivers, all tire manufacturers, all tire styles, and all driving conditions."
Just listen to yourself.

*warning - long post!*

Once aftermarket wheels/tyres go on, the door jam sticker is as much use as a one-legged man at an arse-kicking party.

But even with stock tyres on I would be inclined to run at max psi or as near to max as you deem comfortable.

Wiki'd:
"Should a low pressure tire be forced to perform an evasive maneuver, the tire wall will be more pliable than had it been of a higher pressure, and thus it will "roll" under the wheel. This increases the entire roll movement of the car, and diminishes tire contact area on the negative side of the vector. Thus only half the tire is in contact with the road, and the tire may deform to such an extent that the side wall on the positive vector side becomes in contact with the road. The probability of failing in the emergency maneuver is thus increased.

Further, with low tire pressure—due to the side wall being more pliable—the tire will absorb more of the irregular forces from normal driving, and with this constant bending of the side wall as it absorbs the contours of the road, it heats up the tire wall to possibly dangerous temperatures, as well as degrades the steel wire reinforcement; this often leads to side wall blow-outs. In an extreme case of this phenomenon, the vehicle may drive into a pot-hole, or a hard elevation in the road. Due to the low tire pressure, the side wall at the contact area will temporarily collapse, thereby wedging the tire between the wheel and road, resulting in a tire laceration and blow-out, as well as a damaged wheel.

High tire pressures are more inclined to keep its shape during any encounter, and will thus transmit the forces of the road to the suspension, rather than being damaged itself. This allows for an increased reaction speed, and "feel" the driver perceives of the road. Modern tire designs allow for minimal tire contact surface deformity during high pressures, and as a result the traditional wear on the center of the tire due to reasonably high pressures is only known to very old or poorly designed tires."

My opinion on this is,
Bridgestone, Dunlop, Pirelli etc spend a damn sight longer testing tyres tolerances and flex at varying pressures than the car manufacturer. So if the sidewall says it is safe up to 50PSI, then you can be damn sure it is safe up to 50PSI. They will have pressure tested these in multiple different temperatures, humidty & conditions. Max pressure means exactly that, the max pressure the tyre can go to whilst retaining all its design characteristics and effectiveness. IT IS NOT the max pressure the tyre will inflate too, you will probably get 100PSI into a standard tyre!

IMHO, max sidewall pressure is optimum pressure. To think your cars handling is going to be dangerous inflating your tyres above the pressure stated on the door jam sticker is quite frankly, ludicrous. As stated, this is designed more for comfort than anything else. And I would suspect in some auto manufacturers, this is to hide a multitude of sins in the cars chassis/suspension so it is purposeful set to 32 or whatever to hide these flaws.

I am going to disagree with that. The load carrying capacity defined by the placard tire size and pressure tells you a lot about what the vehicle manufacturer was thinking - and he has a lot of expertise in this area. You can disagree with the compromises that he made, but you can't completely ignore what basic information is available.

By the same token, the load carrying capacity of the aftermarket tires can be determined as well. So you have some basic infomation to connect those 2 items. It would be better to know what each of those is.

We can discuss the merits of various inflation pressures, but without knowing the basic information about what the vehicle manufacturer was specifying, and how the aftermarket tire fits into that picture - then, indeed we would have a one legged man.

Bill, that's the funniest thing I've read in a while. :thumbup:

But really didnt lend much to the discussion. :rolleyes:

I was under the impression that we are adults...



Are there any vehicle engineers here who can say that overinflating a tire past sticker value doesnt;

-decrease safety
-increase expense because of faster tire wear

?



I really would like to believe overinflating a tire past sticker is not an unwise thing to do. I don't think one can fault somebody for being cautious of claims made over the internet especially when said claims have pros and cons that could be debated ad infinitum. This is not personal, I was hoping we could exchange thoughts about this in an adult way. Thanks to those who have done so thus far.

I'm trying to figure out what pressure to use in my Firestones on the old Explorer I just picked up. Knowing that the vehicle engineers are more concerned with safety than comfort and ride characteristics, I won't use the numbers on the actual tire as a guide but will stick only with the door sill sticker.

The only engineers involved that got to act as engineers were the ones who set the sidewall pressures. They aren't talking about how much room they left above that number, but you can be pretty certain that the actual limit isn't below that. The vehicle manufacturer came up with their numbers based on how they made the ride feel. That's focus group engineering. I'll worry about my own ride feel, okay?

-Tire wear is a function of the tire, not the car.
-Safety, is your concern loss of traction or blowout risk? If it's blowout, pay attention to the sidewall. If you're worried about traction, don't outdrive your tires. If a few PSI is the difference between a safe corner and a fatal accident, I'd point fingers at the driver and ignore everything else. You shouldn't be driving that close to the edge on public roads.

And yes, I work at a dealership and we treat that sticker and other factory specs like they're laws. That's not because they are, it's because we're representing the factory and that's our schtick.

[QUOTE=Fat Charlie;244256]-Tire wear is a function of the tire, not the car.

That doesn't negate the fact that over inflated tires increase tire wear. If our goal is to decrease the expense of operating a vehicle then how does replacing tires more often because of over inflation fit that objective?

Huh? Safety should be everybody's concern. Should we be put at risk of getting rear ended, run over by somebody who over inflated their tires?

Safety is everybody's concern, and tire safety here boils down to traction and blowouts. You kind of indirectly implied that you're concerned about traction loss. Anecdotally, I can say that my extra 5 PSI haven't turned the highway into a skating rink for me, even with some vicious rainstorms lately. I'll repeat that if softer tires make the difference for traction it's the driver's fault for driving at that ragged edge of performance in the first place.

iskyfly, run your tyres at whatever PSI you feel comfortable with! Never ever over-inflate your tyres for better fuel efficiency, better of with a lighter wallet than dead!

Over-inflation and max PSI are two complete separate entities, you need to realise this. If someone runs at max psi, they are not running over-inflated, they are still within the designed tolerances of the tyre.

Also, I would suggest if your car becomes unstable if you deviate from the car manufactures recommended air pressure, then either a) it is a **** car or b) you have damaged components.

Running at whatever pressure the tyres are designed for, be it at 30 psi or 50 psi, then the car should still handle adequately.

An over-inflated tyre and an underinflated tyre will bother wear incorrectly, but especially the less air in the tyre, the quicker they wear out due to heat build up. Max psi usually gives you perfect wear right across the tread pattern. Running them soft usually causes feathering at the edge and at least causes them to taper off at the sides, which come wet weather, is much more dangerous if you ask me as you have less contact with the road.

But again, it's whatever YOUR comfortable with! YOU make the decision.

Thank Yooo! I'll be here all week!

Instead of rolling your eyes, pay attention and think about it. Try again. I know you can do it.

don't even bother... some people just like to argue even if they know they are wrong.

Define overinflation.

Over sidewall max set by the tire manufacturer, or over the placard rating by the car manufacturer ?

We got an entire forum running higher pressure than what the car manufacturer decided to put on their tire placards !

It doesn't appear we're losing members by the numbers because their tyres explode :)


We should really turn the question around :
What / who made you think it is unsafe ?

I’m 62 and my family of 4 kids and wife have owned a lot of cars. Here’s some empirical “truths” I’ve developed over the years.

Measure pressure when COLD, not after driving, but first thing in the morning. There are 2 tire pressure recommendations on any car:
1) Car manufacturer sticker located on the "B" pillar (between front and rear doors on driver side). This is the pressure recommended for the original tire size (e.g. 205x50x16) which is also on this sticker. This is for "best ride characteristics”… suspension is tuned for this setting. My opinion, this doesn’t result in best mileage or wear.
2) Min – Max pressure on the tire sidewall. Tire manufacturer rating based on tire construction.

I set cold pressure exceeding tire max on sidewall by 5%, especially front tires, where all the weight is. Why? When driving, tire sidewall flexes (squashes) between road surface on each revolution, causing heat. More pressure = less squash = less heat. Heat is calories (energy) and all energy comes from gasoline. I believe I get better mileage. I have never had a blow-out. Heat also warms the rubber and therefore accelerates wear.

I’ve never experienced over-inflation, center tread wear to any measureable degree. Tires wear on edges because each turn causes the outside tire to roll under to some degree. More pressure reduces “roll under” and therefore reduces edge wear. I suspect that if I only traveled in a straight line at 70 mpg, center tread wear might become evident.

So I feel the bumps a little more but save cash on tires and gas. In the spring with the new pot-holes, they get a little extra. I've never bent a rim for broken a cast mag.

I only rotate tires front / back on the same side; never changing the rotational direction. In 1979 I had a pull develop in the steering wheel. An old tire guy told me radial tire casings shift under the tread against the direction of the pull. If the tire is placed on the other side of the car the casing has to shift back. Sometimes the tread separates from the casing when this "reverse shift" occurs. I've learned a lot over the years by listening to old car guys. If you do not rotate every 5000 miles, then don't cross-rotate. I'm sure radial design has improved since 1979, but ya can't teach an old dog new tricks.

Just another opinion, if you get my meaning.

I do the same, with similar results. Except I pump mine up just a bit more - 58 psi front, 56 rear. I measure when cold, it's a safe bet they go up about 4 psi when driving more than a short distance.

My only regret is that under these conditions, my Kumho tires seem to be lasting approximately forever. I wish they would hurry up and wear out so I can replace them with nice Michelin LRR tires. The Kumho's are supposedly "eco friendly" but I expect Michelin LRRs will be better.

Oh yes - about 1.5 years ago I was rear-ended while driving on um, well-inflated tires. Apparently their braking action was quite good. I've had occasion since then to (very rarely) use the brakes very firmly, with excellent results.

And, back to the topic:

If you put larger diameter tires on your car the odometer is going to read fewer miles that the vehicle actually travels. I don't know if this was said, all I see is a bunch of rubbish.

iskyfly,
It's just me......but let me know how many of the other "inflation tire pressure" threads you have read here.

I can appreciate a noobie being all excited and wanting to jump into the fray.....
But this is hardly a new topic here.
By a loooonnnnggggggg shot.

There are some VERY smart tire guys here. Read their posts, then get back.

@ car_guy:
WOW you set the tire pressure over 5% max when cold? I usually fill it to max cold because when it heats up it will be well over max temp. The greater the cold temp the more pressure when it heats up.

Whats the pressure after the tires are heated?
Anyone else doing this? I was kinda freaked out going over max sidewall pressure because my brother destroyed a tire doing this but he put like 45 psi in a 34psi tire.

5% over max. sidewall pressure won't kill a tyre ;)

I've run 3.8 bar (55psi) in a 3.5bar(51psi) rated tyre for a while - couldn't attribute any improvement in FE to it , but the car felt a lot bumpier than at 3.5bar (51psi), so that's what I stuck with.

That's like 33% over ...
What do you mean by "destroyed" ?
Did it blow (due to pressure, not due to wear), or did it wear down the centerline ?