Tire Size

From what I remember gas mileage has more to do with the inflation of the tire than the size. The size just makes you more prone to blow outs and makes you more susceptible to road hazards. Next time you see an SUV with 22 inch rims think about what happens when they hit a 4 inch deep pothole.

BRhodd; Yes, I understand the importance of the inflation; I was just wondering about the radius with ration to what would be more fuel efficient a smaller tire or larger one? Or is this just the wrong question all around...maybe I should be asking about rear-end gear differential ratios? To which one would give better gas milage?

I think it has the answers your looking for.

it all depends on useage and inflation... I keep the tire pressure on ALL my cars at the max permitted pressure. It really makes a difference. Less friction means better mileage. Low rolling resistance will contribute to the difference also, but it all depends what you are replacing.

as a safety note, ***DO NOT EXCEED MAXIMUM PRESSURE!*** tires are designed for a little above the maximum pressure, but not much! this design allows variences in altitude, heat, and when at maximum pressure, the additional impact of potholes and road debris.

if a tire is at max pressure and hits a golf ball, (and for examples sake, entirely envelopes the ball) the displacement of air is between ~6in^3 and ~9in^3 (depending on treadwear and thickness of belting material) on a aftermarket 215/50R17 tire at 55psi the volume of air calculated equals an increase of almost 40psi! that same tire overinflated at just 10 psi over max has an increase of almost 70 psi! this means when you hit that golf ball in the road, for that fraction of a moment, the tire has an internal pressure of 130psi! ) just a small golf ball... a 4" pothole would displace far more area, depending on speed, so once again
***DO NOT EXCEED MAXIMUM PRESSURE!***

I drive a big work truck with big soft squishy off road tires. They work wonderful off-road and at the job sites, and construction areas, They are designed for it. They are not designed for gas mileage. This is not my commute vehicle.

LRR tires will benefit my work truck more than my wifes car with low profile racecar tires (SAME SIZE BUT NOT STOCK TIRES).

We replaced the stock 35 PSI tires with same size Dunlop tires, max 55psi. On a full tank of flat freeway driving the mileage improved by 5mpg! on the hills it is about the same. If we would have gone larger, the mileage would decrease in the hills.

The us department of energy website

http://www.eere.energy.gov/fleetguide/low-roll.htm l

says it better than I can explain it,

Generally, it has been my experience that if you change tire size, it will decrease mileage in the long run, I have tried it. (unless you also replace the gears) because larger tires will be beneficial on the flats but once you climb a hill, the mileage will decrease far below if you have just kept the stock tire diameter.

If you change your wheels, then change the profile of the tires, but keep the same outside diameter.

You need to consider tire friction relative to the intended road surface (asphalt is worse than concrete, for example).

Other factors include heating of the tire compund, compound type, tire patch area, among many factors.

I think rim size is part of the equation, but I don't know where.

Then there is alignment geometry, which is another can of worms!

So, there is no simple answer.

This will answer most of your questions about tires and rear end gearing as well. Let me know if you have any other questions

Click Me

If the new tires and rims cost $500.00, and you save $.01 per mile, you would need to drive 50,000 miles to brake even. Use your old tires.

Use the size your OEM had on the vehicle. you won't screw up your speedometer by having it run slower w/ larger tires or faster w/ smaller tires. If the change isn't to great, that is for example, going from a 225R60x15 to a 235R60x15, your speedometer will run a little slower, you will run the engine at a lower rpm for a given speed then before, slightly increasing gas milage...BUT, your acceleration will suffer just a little. From the size example, you've increased the diameter of the tire. So your engine doesn't have the mechanical advantage it had before. But it would be slight. Find out what your dealer or mechanic recommends. Most cars can stand one or two size increase's, but not any more. There are to many other factors you'd have to contend w/ like adjusting the wheel alignment to accomodate the wider tire, or tire/fender clearence issues as the tire is steered through it arc. Also clearance issues relating to the steering gear, the touching of the tire w/ the steering arm at full lock.

What gives the best mileage would not please you as you ride down the road. Check out what they use when someone tries to set a mileage record - skinny tall hard tires - not a very pleasant ride - also very unsafe handling. Some minor tire changes can help gas mileage - I increased my mileage (about 1 mpg) on a pickup truck by removing the ridiculous "off road" tires ( big knobby ugly things made for chewing up mud)and replacing with tires made for a two wheel drive delivery truck. In this case the OD was very similar so speedometer changes were not necessary. Aslo, being a former racer and having a son who is a mechanic made the also necessary in this case wheel change rather easy and relatively low cost. However, if I ever have to navigate on a muddy dirt road, I now am in deep trouble. This is also a case where road handling actually improved. Why do auto manufacturers assume they have to put on off road tires on pickup trucks?

take a look at the tires on a honda insight or the first generation prius in america. They both, I believe, have low rolling resistance tires. They are slightly harder and noisier, but have less resistance to movement, one of the three main factors against a car when it is moving.

Just having to buy new tires, I have a comment on the effect of tire size on MPG. As has been stated already, the inflation pressure is key. Then there is tire construction. The old bias ply tires of 30 years ago has worse gas mileage than radials because of less rolling resistance. The only thing left is the "apparent" gain or loss of mileage caused by the differences in tire diameter that are not accounted for in the speedometer calibration. You normally notice this as the tire wear down (or not). With the worn tire diameter less than new diameter, the tire has to rotate more times to go a mile. This registers on the speedometer as higher MPH and more miles on the odometer. (If your warranty is based on miles, you could argue for an extension of the warranty.) Therefore, the worn tires give an apparent MPG that is inflated by the difference in new tire diameter vs. old worn diameter. Putting smaller tires than stock without speedometer correction will give you great mileage per gallon! However, it is only a calibration error. Reminds me of the story about a guy who purchased a fuel efficient car. His endless bragging on his MPG got on his co-workers nerves. This was in the days before locked gas caps. The co-workers bought gas and added a gallon or two to the braggert's tank. As the apparent MPG got higher and higher, the co-workers were laughing up their sleeves! Then they stopped adding the gas. After many trips to the dealer over "bad" gas mileage, the guy got rid of the car.

going to a taller, not necessarily bigger will reduce the rpms, causing a little savings in fuel if all other things stayed the same. you diddnt say what kind of vehicle you have . on a lot ov new vehicles are very aero dynamic dependent on ride hight. taller tires would of course raise the vehicle up and might have an adverse effect on the air flow effectively canceling out any gain from the lower rpm and could actually lower the mileage....

My $0.02 worth.
the small the tyre the faster you get to go quicker the more gas you use , the bigger the tyre the faster you can get to slowerly they less gas you use unless your needing more toqure then less tyre makes higher gears less gas you use or less torque the bigger tyre the higher gear the less gas you use you deside?
My ad-vice use torque quickly change gear stopp gassin it use less gas in the long run or go slowly using less torque get to the next set of lights just as they turn green and keep driving in top gear for conditions and smile at car standing still at lights when you pass `em cos they gasses quicker and had to wait. :-)

1-Use Mobile 1 5w15 or5w20 -Maybe any full synthethic, those weights. 2- Use those 3 part "emission cleaning" products. Those that specifically have friction modifiyers for the oil portion. skip the brands that are to cheap to give you that advantage. Sold at many lube places. 3-Up the tire size one above what's called for - but not wider. Add just to the drive wheels - if your cheap like me. Add 4 more pounds air above limits. 4- Avoid warmups and over one minute idles. 5- Buy a 40-50MPG vehicle - forget 1-4 suggestions.

listen up ya thong wearing fatty, your stupid.