Ya usually a taller tire will get you better gas mileage as long as it's not a big weight difference. More distance per revolution= more miles per tank

There are SO many variables in gas mileage that I don't even know where to start.

I guess the obvious things are first:

(1) Tire pressure. Even a couple of pounds difference can show up in your mileage figures. These days I'm running 32 pounds all 'round on a nearly stock box, and the MPG difference compared to 29 pounds is about 3-4 MPG at freeway speeds.

(2) Tires. The stock 185/60R15 Goodyears are really a poor tire IMHO, especially when it comes to wet weather performance, and considering I live in the Seattle area... I replaced the OEM tires with one-size-over BFG Traction T/As (185/65R15) and slightly wider, lighter alloy wheels. Not only do I get better wet traction and a better ride from BFGs, they also deliver slightly better mileage because of their construction.

I guess I should talk about tires sizes a little more...
(A) Skinny gives better mileage than fat.
(B) Harder tread gives better mileage than softer.
(Ç) Taller gives better mileage than shorter.
(D) Higher pressure gives better mileage than lower.
(E) Lighter gives better mileage than heavier.

All of those, of course are "within reason"...

Speaking of reason, there are reasons why each of those affect mileage, and they are straight physics.

Skinny tires give not only less air drag (minor improvement) but also less rolling resistance. A harder tread compound has less hysteresis, and converts less energy to heat. Taller tires effectively change your final drive ratio to a less aggressive one. Higher inflation pressure allows less deflection, and deflection robs energy. Lighter takes less energy to start or stop rotating, saving fuel and brakes.

Most of those are small differences (biggest is inflation pressure), but they ARE there and they DO have an effect.

First thing to do when figuring gas mileage (if you want to be accurate) is to get the numbers right.

FIRST is to know how far off your odometer reading is. Yes, your odometer IS off.

Stock-from-the-factory the box has about a three to four percent odometer error (in your favor: your warranty will last three to four percent longer.)

That means you need to "calibrate" your specific vehicle/tire combination against the Real World™ by going a known, measured distance and comparing it to what your odometer displays. (The longer the distance you do this on, the more accurate you will be. I use interstate mile-marker signs and do it for AT LEAST ten miles - 100 miles if I can.)

For MY setup if I travel 100 actual miles my odometer displays 94.1 miles which means to get the correct number of miles traveled I multiply what the odometer shows by 1.0626992561105207... Really I don't, I just use 1.06 - it is close enough.

Second, always fill the tank to the same level.

Since you don't want to overfill your tank (that can even mean needing to get a new charcoal cannister when raw fuel gets into it instead of just vapors, and your OBDII system throws a code), don't try to squeeze in fuel until you can see it in the filler neck.



Fuel expands when it gets warm, and the underground tanks are usually pretty cool, so when the pump nozzle clicks off, QUIT...

Try to fill up at the same pump at the same station until it clicks off automatically. That's usually the best you can do for consistency.

Every time you fill up, click one of your trip odometers to zero (I use "B"). Every time you fill up, jot the trip odometer reading on the fuel receipt before you reset it to zero for the next tank. (That way you have the numbers of gallons used and number of miles traveled to use for your calculations.)

My last tank used was for the Scion "Annie's Pizza Run III" cruise and my trip to Canada and back. My readings for that tank were 9.286 gallons to fill up and 354.4 miles on the trip odometer.

Figuring my mileage: 354.4 / 9.286 * 1.06 = 40.45 MPG

Not at all bad for "winter fuel mix" in rainy weather at 65-75 MPH in an automatic.

Some of you may object to running 32 PSI in the tires because of the brutal ride. That's is an excellent reason to spend the $80 for a pair of Sensatrac shocks in the rear. The shocks for the 2002 Echo slip right in, and change the buckboard ride given by the OEM shocks to something that you won't need offer your passenger a kidney belt to survive.

There are other things that seriously affect fuel mileage, including fuel quality, air cleaner, plugs, oil, vehicle load, driving style, etcetera. Some are easy to fix, some are cheap to fix, all can make a noticeable difference, and at $4 a gallon it might be time to think about it.

Right now, Vanilla could use a clean air filter and maybe new plugs. I'll do those before I drive to Montana this summer.

This post was last modified: 03-17-2008 01:34 PM by Tomas.

Nice and informative post Tomas. Thanks for the info!

just a question, i was wondering why u multiplied by 1.06??

Wow, that is an amazing write up!! you need to make that into a DIY!!!! Thanks for the info. I need to check and see if my oil filter is clean. She's getting her oil changed this weekend

He multiplied by 1.06 because of the difference his odometer was off from the actual mileage he was traveling.

That is correct.

The odometer showed I had traveled 354.4 miles, but since my odometer is six percent off from actual miles traveled (due to inherent 3% factory error and the additional 3+ percent error from having tires that are 0.7 inch larger in outside diameter than stock).

Since the odometer converts tire rotations to miles, and since my tires rotate 827 times in a mile instead of the 877 times the factory calibration thinks they do, I must multiply the odometer reading by 1.06 to get the actual mileage (375.7).

Hopefully that made sense.

i understood that, i see where u got the numbers from
just so i kno what is the stock diameter, so i can figure out how to convert mine so it is more accurate

185 - 60 - 15
width - height - diameter

Go here.
http://www.miata.net/garage/tirecalc.html

ok that makes me feel a lot better about my mpg

i also found out that intake, headers, exhaust can make ur mpg a lil worse if u dont remap the ECU bec it still thinks it cannot adjust for the better breathing
does anyone agree with this^^^???

Disconnect your Negative terminal on the battery for a few minutes. This will clear any previous memory in the computer. Most newer cars don't need this done, but it doesn't hurt.

The OEM tires are 23.7 inches (603mm) diameter. Here is a comparison chart to show how much larger or smaller other common sizes are, compared to the OEM size:



My tires are 185/65R15, which the chart shows to be 18mm (0.7 inch) larger diameter. That matches with my empirical testing by actual measurement in the driveway and on the highway.

very good info tomas thanks for chart

thats a good one,
heres one thats always been my favorite
http://www.rims-n-tires.com/rt_specs.jsp?postId=906

ok, so I just put all that info into the tire calculator....now what haha

wow, I just read the info at the bottom. It says my tires are 8.3% too slow.
When my speedo says i'm traveling 60mph, i'm actually traveling 65mpg.

So I take it all of that is not a good thing haha