MPG vs. Tire Size

As I was laying in bed this morning contemplating whether to have oatmeal or eggs for breakfast, my thoughts drifted to the several different threads I have been reading relating to the “fantastic” MPG that 4Rs get.

There seem to be two different types of threads. The first are by the new owners who is shocked by the fact that their new 4R gets less than 1/3 the MPG that their Prius does. Ok, if it looks like truck, drives like a truck, and climbs like a truck, it’s going to get the MPG of a truck.

The second type are the seasoned 4R owners who have embarked on their first mods which usually involve new tires and suspension mods so that their new “Bad Boy Grippers” will fit without rubbing. These MPG conversation generally revolve around the weight of the tires being the culprit. Granted, weight probably figures into the issue. However, there may be another aspect of larger tires that figures into the the more diminutive MPG numbers that occur post upgrade: the circumference of the tires.

Back in the day (‘50s, ‘60s, ‘70s), when gas was $0.35 a gallon and a car getting 10 MPG was no big deal), it wasn’t uncommon to “supe” up your car by putting on some bitchin’ rims and skins. Often it was smaller tires and rims in front, larger in back (of course with the requisite high lift shackles in back). It wasn’t uncommon to get pulled over by the “fuzz” for doing 45 mph in a 30 mph zone (no radar gun, they just followed you and looked at their speedometer). Your speedo was reading exactly 30 mph so you were sure the officer’s speedometer was out of whack so you decided to fight the ticket. The Auto Club would regularly take their portable speedometer testing rig around to local AAA offices and you could have them give you a certified form showing your speedometer’s indicated speed vs the actual speed (usually in 10 mph increments). Sure enough, your speedo indicated 30 mph while your actual speed was 45 mph (time to pay the fine).

The reason for the speed discrepancy was that the speedometer was calibrated by the factory to be accurate with the stock tire size. The speedo got its speed info from a spinning cable that was driven by the driveshaft through a fixed ratio reduction gear. If the motor is turning 2000 rpm, therefore the rear wheel was spinning at, let’s say, 200 RPM (engine rpm divided by transmission gear ratio divided by rear end ratio), and the circumference of a stock tire was 75 inches (diameter times pi), the factory calculated that that stock tire would roll 30 miles in one hour (speedometer shows 30 mph, your map says you’ve traveled 30 miles, and the odometer racks up 30 miles, the birds are singing and all is good). You fill up your gas tank and discover that, during that 30 mile drive, your car has burned 3 gallons of gas (you take out your slide rule and figure out you’re getting 10 mpg).

Ok. Now you put on your new Cragar rims and “Big Meat” tires and drive the same 30 miles at the same 2000 engine rpm. Since the same speedo is still reading the same driveshaft driving the rear wheels at the same 200 rpm, it still indicates the same 30 mph. However, according to the hands on your wristwatch, the same 30 mile trip has only taken 50 minutes, the odometer indicates you’ve only gone 25 miles, and there is a gentleman at your window asking to see your driver’s license and registration. After unenthusiastically signing your speed violation, you go to the gas station and top off with the same 3 gallons of gas. You again pull out your trusty slide rule and figure out that you only got 8 mpg for that 30(?) mile 25(?) mile trip. What gives?

The larger tires have covered the same actual 30 miles as before, but, because of their greater circumference, they have covered it in fewer revolutions (hence the odometer only indicates 25 miles). The preventive fix to get the mpg back to 10 and lessen the chance of getting another speeding ticket, would be to have the gearing in the speedometer changed to match the larger diameter tires. This would be the same for the 4R. The odometer (and subsequently the mpg calculations) are based on the circumference of the factory tire size. You will also find a slight variation in mpg between worn tires (shorter circumference) and new tires (greater circumference) of the same size (even more pronounced with aggressive AT or off-road tires since their new tread is much deeper than that of street tires). Unless there is a way to reprogram the computer to account for the different tire sizes, the only way to accurately get the actual miles for the mpg calculation, is from a source that is not dependent on the vehicle power train. GPS driven navigation (for realtime readings), or the calculated distance provided by mapping software are the only ones I can think of.

 

You are correct in that larger or smaller tires than stock will change your speedometer, tac, and mileage. And a lot of people seem to calculate their mileage with gps and hand calculations as well, which I actually think is more important with stock tires. Here's why:

Something I've heard from quite a few different sources along the way is that our 4Runners speedo's are actually calibrated for a 285 tire and that the stock 265's will show false speeds. I can corroborate this when going by those speed reader things that municipalities put up to slow you down. With 265's I would often read 2mph faster on the sign than what my dash was reading. Now with 285's, they both sync perfectly.

One thing to note is if you're going 30mph with a small tire vs 30mph with a larger tire (lets say you've calibrated both perfectly), the larger tire will have lower rpms.......potentially saving some gas. However if everything else is equal, the taller tire has more rubber and weighs more, which is why I think weight is more of an issue when it comes to mpg loss than the actual size of the tire.

 

True, a bigger tire resulting in lower engine RPM should help a little with MPG, however the added mass and bigger tread patch of that bigger tire I would think would offset some or all of the MPG gain. With my 2003 Tacoma, I found a 0.5 to 1 MPG loss when I bought new tires (same make, model, and stock size as the worn out ones).

 

When I had my 2nd gen Tacoma, I would hand calculate my mpg’s. I did a little test when I had it. The first tire I used for comparison was a Nitto Terra Grappler P265/65R17 which weighted 41 pounds. The second tire I used was a Michelin LTX MS 2 P265/70R17 which weighted 39 pounds. I pretty much drove the exact same routes and calculated from new. Additionally at the time it was my primary road trip vehicle so 85% of my driving was cruise control highway driving. Even though the Michelin was a highway tire and was 3 pounds lighter, it really accounted for nothing. I was NEVER able to match the factory size tire mpg’s. The Michelin’s came close but could not beat the Nitto’s. For me personally, larger tires saving gas is a myth...
Maybe if you drove at 65 mph for an entire tank of gas and made zero stops that would hold true. In the real world I don’t believe it’s an accurate debate. I love larger tires for the looks other than that I don’t see any benefits UNLESS you are doing serious off roading.

 

Factory size tire was more likely smaller than the new tire you put on. Being smaller, it rotated more times per actual mile than the larger tire. The odometer (where you get your distance traveled data for your hand MPG calculations or the the computer gets its distance traveled data for its calculated avg mpg) interprets a certain number of revolutions of a tire that has a circumference of so many inches (stock tire size) as 1 mile. A larger tire will rotate fewer times than a smaller tire for the same actual distance traveled so the odometer provides a smaller number (fewer mile) that gets used in the MPG calculation, resulting in a lower MPG. Thinking in extremes helps to understand. How about if factory tires were the size of skateboard wheels and the new tires were the size of 18-wheeler tires?

A math guru could probably add a correction factor in the hand MPG calculation that would compensate for the difference in distance traveled per revolution (indicated mile on the odometer) between the smaller factory tire and the larger aftermarket tire. That would make an “apples to apples” MPG comparison possible between the tires.

 

Prior to entering the mileage I did use a calculation involving the 3.3 % difference in revolutions. I don’t recall the exact formula but I definitely remember revolutions. It was not a drastic difference but a difference. Some where in the neighborhood of .5 to .75 a gallon.

 

On my 2013 Tacoma, I found that the oem P265/70R16 BFG Rugged Fail tires it came with never gave me an accurate speedometer/odometer reading. I read several posts that owners were getting a more accurate reading with the next bigger size tire. I proceeded to get Hankook Dynapro LT265/75R16 tires, which were 1.04" taller than the stock tires. That did rectify the speedometer discrepancy.

One mystery is - although the LT Hankook tires were heavier and taller, I never saw any mpg difference after changing tire size. Just a more accurate speedometer & odometer. Go figure.

 

Interesting about the factory speedo calculations... I was pretty surprised when I installed my 285/75 to see that my speedo was not really off that much at 70 mph. It's not till you reach 80mph+ that it's off 4 or 5 mph.

Also, I remember getting pretty high mpg on factory tire size, according to the dash display. It was pretty common to see 22-24 mpg on the highway. I didn't hand calculate it too often, but it seems like the few times I did, I was actually getting pretty close to the display, maybe slightly under.

 

I drive through a toll booth every day that has two signs that tell you your speed. My speedometer and those signs are always exactly the same, even though I have 285's which leads me to believe the speedo was off from the factory with the 265's, or those signs are not calibrated.

 

The MPH isn’t the issue with MPG (although added wind resistance at higher speeds does have a negative effect on MPG, but that’s another thread). It’s the number of miles the odometer is counting. Smaller circumference tires = more odometer indicated miles for map distance traveled (therefore more MPG), and higher speedometer MPH because the wheels is spinning faster. Greater circumference tires = fewer odometer indicated miles for map distance traveled (therefore less MPG) and lower speedometer MPH because the wheels are spinning slower.

The actual MPH or MPG difference isn’t as significant as the percentage difference.