My tyres were oversized my MPG went down!

[Hauki]

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:
. 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!

[CapriRacer]

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.

[Fat Charlie]

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.

[Hauki]

Originally Posted by CapriRacer

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.

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).

[iskyfly]

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.

[Ford Man]

Originally Posted by iskyfly

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.

[Hauki]

Originally Posted by iskyfly

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.

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.

[CapriRacer]

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.

[iskyfly]

Originally Posted by Hauki

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.

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.

[Bill in Houston]

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.