Bigger tire to save gas, will it work ?

Perhaps.

But will it be enough to offset the cost of the tires?

Maybe, but perhaps a tuneup is in order instead.

maybe check to see if it has a lock up torque converter and verify it is working (don't know if it ahs one, but if it does, this could cause problem).

I would agree with #2, and add that the car may use more fuel, since because of the higher final drive ratios, the driver will have to give it more gas to make up for the taller gearing.

30mpg isn't bad for the automatic.

I'm with the tune-up camp. How many miles? Have the O2 sensors been replaced? Have you ever connected an OBD-II scanner to it? Not all OBD-II trouble codes will turn on your check engine light. Have you had any engine or emissions system diagnosis done?

It'll also be punching a bigger hole in the air.

In news: snipped-for-privacy@posting.google.com, JW being of bellicose mind posted:

The fuel mileage you father is getting is NORMAL for that car. LOTs of things affect fuel mileage ... tire pressure, low ambient temperatures, thick motor oil, wheel alignment, oxygenated fuel, excessive idling, dragging brakes, running the AC compressor for cooling or for dehumidification, ..... on an on. Your notion of changing tires sizes should be abandoned because while theoretically accurate, when applied makes no measurable difference.

Is this a homework assignment from high school auto shop? ;^)

I don't know the answer to your question, but I offer another thought about this... unless he plans on changing the tires himself, many auto tire shops will not put taller tires than specified on vehicles. This can change the performance characteristic of the car, a liability they may not want to take.

The problem is that for steady-state cruising aerodynamic resistance is the biggest consumer of the motor's power, followed by tire rolling resistance. Engine efficiency varies with RPM, with a peak somewhere in the middle of the RPM band. Changing tire diameter will shift the engine RPM for steady-state cruising, but without some dyno info it's hard to predict whether or not that shift will move the engine RPM toward or away from the efficiency peak. Also, larger diameter tires will reduce the torque at the drive wheels, which means that more throttle will be required for a given rate of acceleration.

To increase mileage you could increase the pressure of the tires, that will decrease rolling resistance but may affect tire wear negatively. Narrower tires would also reduce rolling resistance, but will decrease braking and cornering traction. Part of the reason why the car in question isn't getting as good gas mileage as it might would be the auto tranny, it uses power just to run its internal hydraulic pumping system. Lowering the car and adding a front air dam would reduce aerodynamic resistance slightly. Since air resistance increases dramatically as speed increases, simply reducing cruising speed will increase gas mileage.

To sum up, reducing engine speed won't really affect mileage since it is vehicle speed that is the main determinant of fuel consumption.

JazzMan

Bob H wrote in rec.autos.tech

Any change would probably not be very noticeable. But he is running a pretty low profile tire, and I know from experience that the tire dealers will not change to non performance tire on some cars. I wanted to go to a different tire on one car I owned, and they said no, the car was built for a performance tire and the handling and braking would degrade if it were changed. Apparently the P185/65R15 tires are standard on the Corolla.

Not too bad.

that is too bad...i have a 1990 corolla and I frequently get 32 mpg city/hwy mixed. actually, most of my driving is city. usually when my mileage 'dips' to 28 or 29, I know it is time to re-pressure my tires,,,when the tires get below 32psi, mileage suffers.

good luck. mikey. btw, 218,000 miles and still chugging along.

JW wrote:

The car has only got 65K miles on it, which is nothing for a corolla. Trying to change the O2 sensor is a good idea. will try to find out how much it cost to have it changed myself. I heard you have to rent a special socket or something ?

Okay, other than the O2 sensor, what other parts can affect gas mileage ? The EGR system ? PCV system ? Fuel injectors? The engine on this car is really a gem, quiet and rev happy, too bad the 3 spd tranny really sucks.

Or buy (around $15). Unless something else is wrong, I wouldn't expect the O2 sensor to have a problem that soon (but what do I know? you get what you pay for with advice). I'd be inclined to buy a $120 ODB-II scanner, or take it to a garage for diagnostic work. As you pointed out, there are a lot of things that could cause the problem.

"JW" wrote

_If_ you were allowed to take the bigger tires, gear ratio would increase by 6%. That's not very much.

If your typical urban driving is now at a speed where you drive in 3rd gear and the bigger tires would get you back to 2nd, this would make a noticeable difference.

So changing tires (or gears, spoilers, ...) won't pay off. Make proper maintenance and perhaps try to optimize the driving style.

Thomas

you don't get something for nothing. the taller tire will be harder to turn, so you'll burn more gas. in fact the taller tire will probably squish more than a low profile tire. the squishing is what eats gas up (that's why trains have steel wheels - no squishing, low rolling resistance). in my 4runner, i switch between 30x950x15,

31x1050x15, and 32x1150x15 depending on the terrain. i get nearly identical gas milage with the three different sizes. if anything i get slightly worse milage with the bigger tire because of the rolling resistance.

mike

The amount of tire contacting the pavement is what counts. Air the tires up for sure, but a narrow profile tire with all other variables equal will always result in better gas mileage......small but better. As you said if we could ride on the rims we'd see better gas mileage, it wouldent be worth the hay wagon ride though.

When magnetic levitation comes to cars we'll all be in hog heaven.

Dale

|> My father has a 97 corolla with a 3 spd auto tranny. He's getting |> around 25 mpg in town mileage and roughly 30 on highway which is very |> poor for a economy car. I have a question: if I change the tire from |> the stock 14-65-175 to 14-70-175 or even 14-75-175, will he be able to |> get better mileage ?

not anything measurable. Plus, the additional width will add rolling resistance. If you have to change tires, look for a skinnier tire with the same diameter, and the lowest possible rolling resistance. Usually the latter is found in top-of-the-line tires from Michelin or comparable.

|But will it be enough to offset the cost of the tires?

I'd bet "No".

Truthfully, you aren't going to get much better milaage in that car. The automatic is the culprit. You have to stick your toe deeper into the wick to get that transmission fluid stirred up.

Rex in Fort Worth

In news: snipped-for-privacy@news.txol.net, Rex B being of bellicose mind posted:

Rex: The term "lockup torque converter" means their is no slippage thru the converter after the lockup clutch engages (45 mph in the

3spd model in this discussion). Also, 4AFE and 7AFE (1600/1800 cc) engines mated to those 3 spd units were usually geared to run about 3,000 rpm at 60 mph. THERE is some of your mileage loss compared to the same engine running the 4 spd auto or a manual box.