About the woman whose husband wants to put bigger tires on the car for better gas mileage…wouldn’t the bigger tires rub on the car when they turn a corner? And also, wouldn’t the extra weight of the tires and rims cancel out the benefit in gas mileage of the bigger tires? P.S. your phone just rings and rings…no machine.
The tires wouldn’t necessarily rub. Most cars are designed to accept snow chains so they have lots of extra room for bigger tires. Many models offer bigger and wider tires on the same model as an option. If lower profile but wider tires were chosen the diameter would be the same as the 13 inch tires. That would decrease mileage because the wider tire has more drag. The extra weight of the 14 inch wheel and tire would cause a very small decrease in mileage. and virtually no decrease in steady highway mileage.
Hmmm
I was curious about all this myself. you can get larger rims, but put the same size tire, so larger tires must have been put into the mix. Now a larger tire will result I believe in less miles reported, so it would take a real mathematician to calculate change in reported mileage, and may actually result in a lower calculated mpg unless factored in…
"P.S. your phone just rings and rings...no machine."
@robin7–There are no more call-ins for the radio program, which has been running repeat segments for about 2 years. Now that one of the brothers–Tom Magliozzi–is deceased, the likelihood of a call-in format returning is…not good.
robin7
About the woman whose husband wants to put bigger tires on the car for better gas mileage…
I was amazed that Ray didn’t declare the idea ridiculous.
On today’s show, it was suggested that using larger diameter tires would improve gas mileage. What is the limit of this relationship? Can we put on infinitely large tires and get infinite gas mileage? If not, perhaps there is some optimum tire size. Perhaps the car manufacturer is using that tire size.
It appears that changing tire size changes the relationship between engine speed and car speed (RPM vs MPH). This relationship is more easily changed using the car’s transmission. There is indeed an engine RPM that will yield the highest efficiency when the engine is delivering a certain amount of power. The power at a constant speed is that required to balance the power consumed by the various losses in the car such as transmission losses, tire losses, and aerodynamic losses. If, also, you are going up a hill, engine power is additionally converted to “potential energy” that is recovered when you eventually go down a hill (unless you go uphill to school and uphill back home).
Increasing the tire size MAY result in an increase in mileage under certain driving conditions if the transmission is not precisely matching the load to the engine. It’s about as likely, though, that mileage will get worse due to the load mismatch being made worse.
Harold
(Editorial note: I combined @Hallikainen’s comment into the other thread formerly known as “wom” and retitled it so people knew what it was about.)
Thanks !
Bigger tires only affect the final drive ratio if the overall diameter is greater. If it is, then theoretically, it’s like adding a little overdrive. In reality, it seldom works as the motor has to wok harder to overcome the resistance of the bigger tires, the aerodynamics are decreased slightly by the raised height and the acceleration suffers. I suppose if the tires were taller but more narrow and a front fairing was added…maybe.
But still, IMho, it would just be better to get low rolling resistant tires. Now, if hubby just wants the car to look better; that I could understand. I am always in favor of big wheels and tires as tall as will fit…just for looks of course. I hate cookie cutter tires and if hubby does too and wants a little softer ride and more protection from potholes, go for it. Economy would be the least affected in a positive way. But I think cars always look better with bigger tires.
hmm … In an ideal situation the physics says the only reason you need gas at all is when you need to overcome inertial forces ( accelerate , turn), or to overcome friction. So there’s an array of factors to consider switching to bigger wheels and tires. It’s not clear to me if the switch would produce better mpg, or worse mpg. I’d guess probably better for freeway driving, but possibly worse for city driving.
- Reasons for better mpg with bigger tires
- Lower rolling resistance
- Greater 0-60 times (reduced max acceleration)
- Reasons for reduced mpg with bigger tires
- Wheels and tires weigh more (larger inertial forces to accelerate)
- Wheel and tires weigh more (larger rotational inertial forces to accelerate)
- Wind resistance may be higher as the car is higher from the ground
That’s all I can think of, but there may be others too.
Cars and their gearing is pretty well optimized as it comes out of the factory. If bigger diameter tires were an easy way to get better mpgs, we’d all be driving these:
@texases Only one comment for that picture… Donk deDonk-Donk-Donk!
The change in the gear ratio from taller tires that reduces engine rpm at a given speed also causes the throttle plate to be open farther. That reduces the pumping losses on a gas engine caused by trying to suck in air at part throttle. That is partly why diesel engines get good MPG’s, no throttle plate to restrict. It is also partly why GM, Honda and Chrysler have engines that shut off half the cylinders at highway speeds. The engine only has to suck in half the air so the work to do it is half.
I wouldn’t…that’s just ugly. Some vehicles just don’t look good with low profile tires.
"It is also partly why GM, Honda and Chrysler have engines that shut off half the cylinders at highway speeds. The engine only has to suck in half the air so the work to do it is half."
I’d say the engine would be sucking in almost the same total amount of air to the active cylinders, but through a wider open throttle, thus reducing pumping losses. The MPG increase from cylinder deactivation isn’t huge.
I’m not so impressed with the looks of that tire/car package, but in the background, the house and landscaping look great! I wish my lawn were that green and well kept.
Like a great many people, I am skeptical that increasing the overall diameter of a tire (which effectively increases the overall drive ratio) would have any affect, because of all the other things that could change at the same time. What with the change in where on the BSFC (Brake Specific Fuel Consumption) curve you would be - as well as the change in mass and rolling resistance of the tires.
HOWEVER, several websites devoted to improving fuel economy have had people make such a change and the general consensus is that it helps. Needless to say, the amount of diameter change is limited by the size of the fenderwells.
My two cents: The physics is very simple. Increase the tire diameter by 10%, say, and the circumference increases by 10% and the engine RPM for a specific speed decreases by 10%. (This also causes the speedometer reading to read erroneously 10% low)
you can (in theory) reduce the width of the tire at the same time so the weight remains about the same. Or use lighter weight aluminum alloy wheels.
Re MPG, this change will function like an overdrive, which could increase the MPG although not always, the RPM may be too low and the engine lugs.
An easier thing to do is the change the gear ratio if that is possible.
You may ask, why didn’t the manufacturer just change the gear ratio for you, as they are always trying for fuel economy? Or just change the highest gear or add a gear? The answer is (as it is for most things automotive) influenced by marketing. You will get less acceleration… But the reason may be that they have already done that (6 speed transmissions are common now) and further reduction does not help.
Bottom line: leave it as it is. You can get much more improvement in MPH by careful driving habits.
b
I needed tires on a 1991 S-15 pickup and by coincidence had a set of 235-75x15s collecting dust. The original tires were 215-75x15s and other than occasionally rubbing the fender liner the tires worked well enough except that higher rpms were required and overdrive was for the most part unusable. With the 5 speed manual with the speedometer indicating 70mph the truck would not cruise on level highway in overdrive. When shifted to 5th gear(overdrive), the truck would lose speed on level highway on even a very minor incline speed would drop quickly and force downshifting. After several weekdends driving a 350 mile round trip on a parkway with a 50 MPH speed limit and often needing to shift to 3d gear I installed the correct tires and the results were spectacular. while I don’t recall the fuel mileage numbers there was a significant improvement with the smaller tires when I made that weekend trip a few more times and I used the map mileage for calculating the mileage since the odometer was considerably off.
<b>the engine would be sucking in almost the same total amount of air to the active cylinders</b>
You are correct, I mis-wrote. Good catch. The pressure drop is lower but the amount of air is essentially the same. Cylinder de-act doesn’t improve things a bunch but every little bit helps when getting only 15 mpg highway. Comparing my non-de-act truck (15 MPG) to the current model (20 mpg) with de-act and other stuff.
Only recently with our last three cars have I not tried to put the biggest diameter tire on every truck and a couple of cars we have owned for over twenty years. I did it for a variety of reasons, mostly to increase ground clearance and take pot holes and rough terrain better. Never, in all those attempts, had the economy ever improved. Some times it stayed the same, some times it was slightly worse. It was never better. The last three cars came with tires that were within one size, as large as you could fit. None of these had gas mileage you would write home about.
Large tires and the best gas mileage just don’t seem to go together.