Replacing Larger Car Tires With Smaller Ones

With smaller tires, the car may be lower. You will hit more speed bumps with the body, and the suspension will suffer, because the tires are not absorbing as much shock as before. The car may not do well on wet roads if the footprint is smaller. Since the motor will do more revolutions per mile than before, you may not get better gas mileage. With a lower ratio, you will have to give the car more throttle, or gas, to go the same speed as before.

I am actually looking for taller tires for my wife's car to keep it from hitting so many bumps. The noise bothers her. I tell her it's just metal, but I know something down there is getting beat up.

Yes you are right with regards to lesser road clearance hence bad for bumpy roads. on the other hand smaller tyres have to make more revolutions but efforts required to make a revolution shall be less as such will not it be that the product of revolution and effort / revolution would remain same and hence mileage man not have any significant effect ? - either way good or bad ? may be there are studies made and data available to confirm may be even.

I can put up a hypothesis

1) I drive mainly on city roads where speed is never more than 50 / 60 KMPH.

2) At same throttle position Higher RPM would be a result of lesser load on engine.

3) Slightly higher engine RPM would be better for mileage as RPM engine efficiencies would be better at higher speeds.

3) I would be moving in higher gear at lower car speeds than earlier.

Mileage is a total effect of all these

A lot of modern IC engines are most efficient at high loads and relatively low rpm. Check the driving technique of some of the high mpg boys.

GA to you Sir.

Unless you significantly reduce tyre size the difference in torque will not be noticeable to you.

Modern IC engines mix the proper amount of fuel and air regardless of engine RPM. Your fuel mileage will be reduced. Significantly? Perhaps, depending on your driving habits.

Smaller tyres cost less. So, your overall operating costs may be slightly reduced.

You appear to have contracted a tenatous infestation of faulty logic. If you give me a few minutes and some cooperation I will try to shake the infestation from your head:

Begin by taking a deep breath. Now imagine you are in your car with the engine running and the transmission is in neutral.

You decide you want to rev the engine higher to impress the person idling beside you. Ask yourself what should you do?

Hopefully self answered correctlly, telling you to increase throttle, i.e. give it more gas.

Now, imagine, still in your idling vehicle you look back to check out the idealic boat on a trailer attached to your idling vehicle. In your mind, turn the engine off and then shift through your gears (you may need to imagine you have a manual transmission). While you are doing this, have self go back and look at the scales under each of the trailer wheels. Ask self if the weight scales indicated any change while you shifted...

Hopefully, self responds truthfully by saying that no change is indicated.

When you read the number that normally follows the number '2' (when counting up integers starting from 2 or lower), you will return free of any faulty logic.

1. If you have to increase throttle to increase RPM when in neutral, is more fuel consumed at higher RPM?

2. If the weight of the load did not change (self should volunteer any other obvious changes that might have occurred when shifting, such as parachute deployment) during shifting, is the load likely to remain the same irrespective of the gear (all other things being equal)?

3. Do you feel better?

The dominant factors in your scenario will be:

1. Effective gearing

2. Rolling Resistance

Because you have specified city driving speeds and because the difference in tire size is likely to be small compared to frontal area, aerodynamic effects will be minor.

Because you have not specified racing, the effects of rotational inertia are likely to be minor. taller tires may result in slower acceleration, but unless you are Mario Andretti doing residential mail deliveries, the increase rotational inertia to overcome will not be dominant.

Rolling resistance for larger tires tends to be lower. The additional savings in the reduced RPM along your entire drivetrain in combonation with lower rolling resistance will typically result in better economy with taller tires of similar width, pressure and construction.

Benbenben

If you are able to move your engine's average RPM closer to the maximum torque speed of the engine, and if your engine is a smallish engine that has a pointier torque curve, you might be able to save some gas.

You could also buy higher load range tires that can be inflater to higher pressures SAFELY, and gain some mileage. You can also purchase tires with a lower aspect ratio. 185-78 x14, instead of 185-70 x14, 0r 185-60 x 14. The narrower tire width will lower rolling resistance.

How far do you want to go?

Remove your windshield wiper arms. Remove your radio antenna. remove everything that adds weight to the car. Remove the heavy bumpers. Empty the trunk. Remove the back seats. Smooth over anyplace that can catches the airflow around the car with foil duct tape.

Just how far do you want to go?

You can also purchase tires with a lower aspect ratio. 185-78 x14, instead of 185-70 x14, 0r 185-60 x 14. The narrower tire width will lower rolling resistance.

I think you meant to say purchase tires with a higher aspect ratio: i.e 80 instead of 70.

However, this is generally not true, with the lowest rolling resistance tires having an aspect ratio of .65. A wider contact patch means a shorter contact patch (given the same tire pressure) and that means less angular tread flex. Unfortunately, it also means a wider tire for aero drag -- so it is a tricky trade-off. But the .65 series tires on the Prius are about as low rr as you can get, and the aero compromise is a reasonable one.

Another one of my long held truths has been shattered. What's next? You going to take a shot at the Easter Bunny? What about the Tooth Fairy? Is nothing sacred?

Your speedometer is calibrated to the stock tire size. If you change the size of your tires you will experience inaccurate readings from your speedometer, which could result in a speeding ticket.

With smaller tires, their actual speed will be less than what the speedometer shows, provided the speed pickup is coming from a wheel or driveline RPM. As long as they don't drive over what the speedo says, they shouldn't see cherries in the rear view mirror.

for a start your speedo will be altered

the ride will be harsher.

While the lighter wheels (lower unsuspended weight) will be suitable for stop & go and maneuverability by reducing low-end torque requirement, such reduction of tyre (wheel) circumference would also tend to alter the vehicle speed sensed by the engine control computer. You would want to make sure that the replacement tyres were within the calibration range factory set in the engine control computer. Otherwise, you've possibly defeated your purpose.... of having a car that runs correctly at all. Recalibration is something which, where possible, you would want to discuss with manufacturer/dealer/.... probably not worth the bother or expense in any case.