New tyres up the fuel bill?

[Wixy]

Hi Guyts!!!

Super brains required = )

I’ve got a stock standard 02’ SXD20R Camry, and recently I conducted the following:

• Oil and oil filter change – I’ve always been using Mobil1 5w-50 & K&N oil filters since I purchased the car way way back.

• ATF change – from standard mineral to Castrol Transmax Z

• Tyre change – from Toyo Teo Plus 215/55/16 to Continental Premium Contact2 215/60/16

Here’s the problem, I’ve been getting the good fuel efficiency figures – up until I made these changes, and all these changes were all in 1 week, so I know it’s probably due to one of these factors.

I used to drive in a sprinted style, rev it hard and I would never, ever, ever go beyond 9.8L/100km (9.3L/100km was really the norm). But now I’m driving like a grandpa with 1 or 2 (yes I count now…) pedal plantings to the floor and I suddenly get 10.8L/100kms consistently. This is the 3rd week I’ve had to put in 3 to 4 more liters worth of fuel in the tank than I normally would.

I can only think it’s due to the tyre change – different construction compounds and a move to a greater % change in rotation (55series to 60series), but one would have thought these Continentals weren’t the cause. I did note that the Toyo Teo Plus’s were always boosting their “low rolling resistance” and high silica compound, but I wouldn’t have ever imagined a change in tyres would cause such a dramatic increase in fuel consumption.

Any thoughts?

[DJKOR]

I'm no physicist so what I mention is just my train of thought. I'm thinking more along the tires too.

You mention you moved from 215/55R16 to 215/60R16. This therefore changes your overall diameter of your wheels/tires from 642.87mm to 664.21mm. Now from my understanding, the larger the overall diameter the larger the force you need to exert to get it to change velocity. When cruising at a constant speed you should be fine but it's only during any moment where you increase speed (even only by a few km/h) that you need to use more force than what you typically would.

In the long run, maybe this could be having some influence on your increased fuel consumption.

[Steven]

Changing rolling diameter is the same as altering the final drive ratio (to a degree), so this will have the affect DJKOR just mentioned in that as you've gone larger in diameter, you've essentially removed some of the "torque" from the equation meaning the car needs more effort to accelerate.

The other issue is that as you've essentially changed your final drive ratio, the speedo will no longer be correct. The speedo will now read slower than your actual on road speed, meaning the car thinks you've actually travelled less distance for the fuel used.

[Wixy]

Thanks super brains = )

My stock size was 205/65/15 with the factory rims, so on my prior change I decided to change to Toyo’s with 215/55/16 because the size gave me the lease diameter difference (-0.71% to be exact). During that change, every single tyre man who rotated and balanced my wheels told me I should’ve gone for 215/60/16 instead of the 55series – and that the 60series tyre would be more common and thus save me money. But I told them I wanted to minimize the speedo impact – and everyone single one of them told me I was wrong.

Now I’ve gone for the 60series only because Bridgestone Tyre Centre Box Hill (VIC) were having the Continental Premium Contact2’s on super special at $120 per corner (that btw is a good special), so I decided why not – good tyre for cheap (which btw is made in Malaysia and not the Frankfurt factories).

I was prepared to allow the +2.53 diameter difference from these Continentals, but I just could not have imagined such an increase in fuel consumption for an ever efficient whitegoods car. Today I pumped 37.11 Litres from travelling 344kms of 1 week’s travel.

I have also been told it may be the change from Toyota DIII ATF to the thicker Castrol Transmax Z ATF? When I changed the ATF, I did noticed that the factory Toyota ATF was a lot runny-er, and thinner – compared to the Transmax Z. When waiting for the ATF to chug down the pipe, it took ages compared to when I was using Toyota ATF. Do you guys reckon this may also be the contributing factor? I have doubts because I thought Transmax Z would have given me improved performance.

Cheers

[Cee-Lo]

As someone else has mentioned, increasing your rolling diameter makes the car travel further with a single full rotation of a wheel. So if you are using your trip meter k's to calculate your fuel usage, this could lead to inaccuries in your fuel consumption calculations.

Thicker gearbox oil would increase resistance within the gearbox, making it lose a small amount of power, which could translate into additional fuel usage.

Another thing to consider is tyre pressure. Higher tyre pressures reduce rolling resistance, which is better for economy, but you don't want to go too high in pressure, as uneven tyre wear could result, and the tyre internals itself could be comprimised.

[Wixy]

Thanks for the expertise & brains.

I like what you said KRS-093, that using my trip meter will create inaccuracies with my calcuations, as this larger tyre goes more per km, thought I'm staring at the figures, and the change in diameter from my stock tyre to current size is only 16.77mm more, while circumference wise its 52.90mm more. So I guess we're talking peanuts. But I found links that expand on what you were talk about.

What brought me to closer attention was some of the further well written brains I found on google:

The final variable to the issue of tire size vs. fuel consumption is application. For example, a 4 cylinder engine might suffer more from wider tires than a V-8, due to the difference in torque. The stronger the engine, the less likely that a small increase in load or rolling resistance is going to require more fuel to operate.

From: http://answers.yahoo.com/question/index?qid=20090114001804AA0hOlS

A larger tire would make a vehicle a bit harder to get moving. But, the trade off means the vehicle would take less energy to keep it moving with larger tires. Therefore, if a car is used primarily for highway driving, overall MPG may improve with larger tires. For city driving, with lots of starts from dead stops, MPG may get worse with a larger tire. This has a lot to do with transmission design, where vehicle manufacturers gear the cars for a good blend of city/highway fuel economy, also based upon the power sweet spot of the engine (and tire size!). However, for cars used for a lot of highway driving, significantly better MPG may be achieved by gearing the transmission to use lower engine rpm's at highway speeds. (Especially with diesels). But regearing, if even possible, costs a lot of money. Increasing tire size is a cheap way of doing the same thing. A bigger tire will go farther distance during one revolution, meaning you need less engine rpm's to turn it.

From: http://wiki.answers.com/Q/Does_rim_or_tire_size_change_mileage

So now im content that I've just chosen the wrong bloody size for city styled driving, eventhough within legal limits, because to and from work I am required to go through 25 sets of lights plus traffic, so the increased weight just doesn't help. I've calculated if these tyres lasted 3 years , it will cost me around $430 more in total than running on a 55series tyre - based on an average of 2L more per fill (including tripo inaccuracies) and $1.50/L cost of petrol.

So much for a great tyre deal...