Wheel Alignment and Balancing

To the person who said MPG goes down with larger tires. That may be, but it has nothing to do with your answer, which said larger tires are heavier, and more to do with how the car is driven and engine power curves. A larger tire size is going to add at best a few ounces to the weight of the tire. In fact, the lead balancing weights might make more of a difference in tire weight. A few ounces on a 3,000 car makes no difference. The power to start a vehicle from a dead start would be affected by tire size, but not due to tire weight. This whole concept is demonstrated by the fact that vehicles need a transmission. (If the car just had one gear, 4th, the engine would not have enough power to start the car from a dead stop. That's why there is a transmission.) 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.

As for the original question, tire size is DIRECTLY related to speed and mileage readings on your speedometer/odometer. Your speedometer expects the car to go a certain distance for each revolution of the tire. Imagine: If the tire is HUGE, the car will go very far with one revolution. (Your speedometer readings will change with new tires vs old!) Almost every vehicle in the country is purposely made to READ about 5 mph HIGHER THAN actual speed. I say the manufacturers do this to make the vehicle odometer rack up the miles more quickly, which is "Gotcha Capitalism" at work. But, the manufacturers avoid massive class-action lawsuits by claiming they must do this to make absolutely sure that the car isn't going FASTER than speedometer speed so they don't get sued by people who get a ticket.

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From a purely geometric perspective, changing the tire diameter could effect the "Perceived Mileage" - assuming the odometer (and speedometer) tabulates the revolutions (RPMs for the speedometer), the traveled distance would be greater than the measured distance if the tire size was increased. (Just as someone would run slightly further if they ran around a track in lane two versus lane one.) Calculating the MPG would provide an error unless the odometer/speedometer was re-calibrated for the different size of tire.

A friend of mine was once pulled over for exceeding the speed limit shortly after putting larger tires on his vehicle. His speedometer measured his speed right at the speed limit, but his actual speed was higher.

This error does not "change your mileage", rather just gives an erroneous value if you are using your odometer. If you are using known distances in your calculation instead of your odometer, you would avoid this problem.

However, newer/luxury cars may be using GPS or other high-tech detection methods to determine distance and speed, rather than tracking the revolutions of the axle - thus eliminating the error and the need to recalibrate.

From a mechanical perspective, an increase of tire/rim diameter might effect the efficiency of the transfer of energy or torque upon the axle. However, I would speculate that the change would be negligible.

In summary, I would suspect that the change in tire size would have minimal effect on your "actual mileage", but the "measured mileage" could easily vary as a result of calibration issues.

yes rim size does affect the mileage and speed of your vehicle. as you increase the speed your traveling at the difference in the reading on your speedometer and actually traveling at. you can get some devices to recalibrating your speed whether you have a mechanical or electrical sending unit .

larger diameter heavier tires take more energy to spin and will lower your MPG Imagine a tire on spindle and you have to turn it by hand, now imagine a tire twice as heavy, you would have to put a lot more effort into keeping it spinning at the same speed. It can have a significant effect on mileage both by increased weight and the fact that on a larger diameter tire the weight is farther from the center

i have recently put a semi aggressive mud tire on my s-10 blazer and mpg dropped 6 mpg instantly so my answer is yes it does make a difference

Your new tyres are indeed 1.48% smaller (diameter reduced by 0.37ins/10mm)

In regards to the 6mpg drop after installing aggressive mud tires, the mpg dropped primarily from rolling resistance. The aggressive tread has more leading edges and more block type tread. The aggressive tread gains traction by increasing rolling resistance. The opposite of a mud tire, a full race slick, would only have the rolling resistance of the material compound acting on the contact surface, very smooth rolling and yet sticky traction. That being said, a mud tire does not increase traction on dry pavement when compared to a slick. The mud tire decreases traction by having less surface area while still having a higher rolling resistance due to the aggressive tread design. A slick on anything other than dry pavement decreases frictional traction by no longer sticking to a hard solid surface, but a slick surface such as sand or water or mud. All DOT approved tires will have a rib and/or a tread design to evacuate water or debris and are not full slicks.

In regards to inertial power required for larger tires. This is more of a micro scale inquiry and not a macro scale problem toward the tire issue. A larger diameter tire will increase torque required from the engine to accelerate. This increase in torque required for acceleration increases geometrically compared to the torque requirements to maintain a speed which is linear. The tread design is also a factor as a uniform 24lb tire will require less torque than a 24lb tire consisting of a large blocks shifting the weight to the tread surface.

If the goal is to increase mpg, go with a tire that has a higher speed rating which will have a stiffer sidewall. The trade off is expense and possibly comfort. Decrease the revolutions per mile no more than 10%. A tire with a narrower tread width, higher sidewall ratio, smoother tread design, and higher speed rating will increase mpg at the cost of acceleration, foul weather traction, and increased turning skid. If you're running a 205/50/16 and a majority of the driving is interstate roads then try a 205/55/16 tire. If you went with a 215/50/16 or 215/55/16 tire the revolutions per mile decreases and the contact patch increases from 205 to 215 which negates any mpg improvement by dropping engine speed (revolutions per minute) at highway speed. The benefit is increased bump absorption and increased stability at a minimal decrease of mpg depending on tread design because a majority of travel is at high speeds. If a majority of travel is stop and go urban travel or may include extended steep grades, then try a minimal change such as the 205/55/16 from a 205/50/16. The tread width change may also be negated depending on the section width of the wheel rim it is mounted. A five inch width rim will decrease the tread with when compared to a six inch rim even if the exact same tire is used. The best alternative is to get a better grade tire with the original equipment size. Changing tire size will not only affect the speedometer but also the ABS and traction systems if your vehicle has them.

Another approach is to decrease excess weight by reducing wheel rim size. Metal is seven times heavier than rubber. If you have a premium car with big brakes, then mpg generally isn't a priority issue. If you are trying to get the most efficient set up with unlimited expense, then get the smallest diameter lightweight forged wheel that fits around the brake caliper or drum. Then put the corresponding tire around it. The expense of this approach is generally a wash because the cost outweighs the savings even at $4 a gallon.

My suggestion would be to invest more into the vehicle maintenance and spend a little extra on a tire like the Micheline Hydro edge or Continental Extreme Contact DWS, they have excellent all weather capability, low rolling resistance, and may last two to three times longer than economy tires. To compare: mount, balance, and dispose of the the economy tire twice when comparing something with a 40,000 mile tread wear and a 90,000 mile tread wear. Another alternative is the cost of buying a set for summer condition and a set for winter conditions complete with rims. This comparison is not universal as someone in Grand Forks, ND would be better off with a dedicated winter tire or someone in Texas might require an off road tire for some excursions.

Speedometers require maintenance like anything else especially when something like tire size is modified. Speedometers do have a required tolerance for accuracy. I believe it is + or - 2 miles per hour at 75 miles per hour. As speeds increase the tolerance for variance increases. At 30 miles per hour the tolerance would be below a half of a mile per hour. I would have the speedometer inspected if your display is off by 10 miles per hour when compared to a controlled calculation. A speed trap on a closed course would be a controlled environment as other vehicles may affect radar trap readings. Most cars manufactured after 1997 have OBDII computer systems which means the vehicle will have an electronic system reading either transmission speed or axle speed. A mechanic may look at you funny when asking them to check this issue but if you're going to pay them generally they'll look into more than you ever wanted.

I don't know if ABS or traction systems can be adjusted for different tire sizes. I would guess they are capable.

A truck is designed for a specific tire size. That information is on the inside of the door. Changing the rims and/or tires such that the wheel diameter is increased introduces an error in the speedometer and odometer as they are configured for the original equipment. Example:

Stock Tire =255 / 70 R16 (Diameter of 30.1 inches)

New Tire = 265 / 75 R16 (Diameter of 31.6 inches)

-5.31% Error in speedometer and odometer readings

Ergo when the speedometer says 65mph, you are actually traveling at 68mph

If you are calculating 15mpg, you are actually getting 15.8

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I believe the question you meant to ask was, "Does increasing the tire diameter improve the mileage by in effect increasing the gear ratios?"

... Negligible

For trucks, overall fuel economy is a function of aerodynamic drag. The best fuel economy for a truck is cruising at about 40mph. If the truck is rated for 20mpg, it should be achieving it at 40mph... However, most people do a lot of highway driving at speeds in excess of 60mph. Going 55mph will cost you 1.5 mpg Bump it up to 60 and you lose another 1.5. Drive at 70mph and now you are only getting 14mpg.

CONCLUSION:

Know the effect on rim/tire sizes on your vehicle's instrumentation, and keep it under 60mph on the highway.

I have a 2001 Ford 150 (4.6 liter V8 with 5 speed manual and 4X4) with the tire scenario listed above. My actual average mileage per tank is 17mpg and I drive at 60mph on the highway.

nope.. although several have tried, and had disastrous results. 16.5 inch rims can be found cheap enough, check military surplus for hummer rims.. they are all 16.5.

good luck