Which affects MPG more? Tire weight or tire size?

I went from 32 to 33 and notice mileage decrease. However the weight increased about 15 lbs per tire. Which is the bigger culprit for the mileage decrease and why?

 

Mass moment of inertia is what is going to have the biggest effect. That is a function of the weight but also the distribution of the weight. 15 lb heavier tires are going to have a much larger impact than 15 lb heavier wheels because the mass is further from the rotational axis.

In this case, the increase in diameter is pretty minor (1/2 inch from the hub) but the weight increase is significant. In fact, it seems like you've probably gone up in load rating or at least gone from a road tire to an AT/MT.

If you'd like, I built a little calculator that tries to compute just this metric. It's not a perfect model but it tries to account for wheel size and weight, tire size and weight, and the relative distribution of tire weight in the sidewall vs tread. Tell me what you had before and after and I can try to calculate the increase in rotational inertia.

I should mention that other than an increase in inertia that must be overcome to spin the free weight of the wheel and tire, the truck must be moved an extra three inches forward with every rotation of the tires. This represents an effective loss of torque which can have a performance/efficiency loss all its own, though I don't believe it accounts for as much as the increased rotational inertia of the tires.

Last thing is the rolling friction. Presumably, you gained some width with the diameter increase. The wider tire has more rolling friction because of the larger contact patch. If you didn't get any wider, ignore this.

In short, and in your situation, the weight increase is going to account more for the mpg decrease than the diameter. If you were to get a set of light weight 285s in a P metric that were as light as your old 32's, you wouldn't notice too much of a change. On the other hand, if you were to switch to a 32" MT in a 10 ply, you would experience a similar decrease in fuel economy despite not changing your size.

 

Is this true in town vs on the highway?

 

Based on my research here it seems that results vary WILDLY. There are lots of people talking about noticing a difference (and I don't doubt them) but in my experience there was little to no difference. My truck came with 285-70-17 and I was getting bad gas mileage (compared to the EPA ratings - this was my first Tacoma so I had no frame of reference). I assumed this was due to the big and heavy wheels/tires. I downsized to 265-75-16 (1" smaller) and went from 90lbs per corner to 68lbs per corner. Now, given I did put very few miles on the old wheels/tires but based on my driving with the new setup there was little to no noticeable difference.


Overall, I'm pretty disappointed with the mpg of this Tacoma - my 1999 V8 F-150 got better gas mileage.

 

Also, don't forget that by increasing the diameter you're also making your speedo (and in turn your trip odometer) incorrect. So, unless you're doing the math to factor that in the mileage decrease you're seeing may not be as big as you're calculating.

 

Yep... ^^^ What he said.

 

When I upsized the tires on my 08, my speedo actually got more correct. According to the speed trap radars on the side of the road, the OEM size tires were about 3 MPH less than what was showing on my speedo. When I upsized, the MPH on the speedo and they radar system matched.

 

I'm glad you brought this up. I may have experienced the same on my truck but don't have really good proof. With my 1" larger tires (265-75-16) my speedo seems to be PERFECT according to the GPS speed app I use on my phone. Unfortunately, I'm just not sure how accurate that app is.

 

The most likely culprit in your MPG decrease is driving style.

The next is the Odometer will under report both the speed and distance with a larger diameter tire. Sure you can calibrate the Odo for the larger tire, but most forget about this step.

The under reporting of distance will produce less miles driven per tank of fuel. Directly impacting the "MPG number".

It has been mentioned that tire/wheel inertia plays a big part in this mpg loss. I'm not so sure. The increase in the tire/wheel inertia is true, but in comparison to the Truck's inertia.......the increase is minor.

 

I assume it was a +1 diameter increase?
265/70r16 to 265/75r16 or
265/65r17 to 265/70r17

 

I went from E load to C load, same tires and size. Gained around 1 to 1.5mpg. Barely noticeable considering a 8lb difference between tires.

 

Same thing happened to me. 265/75R16 made my speedo dead on when checked with the GPS app on my phone. It's slightly off now that I've moved up to 255/85R16. IIRC now I'm off by about 2mph @ 40mph.

 

Weight, everything else being the same, has virtually no effect on fuel mileage. I've had my Tacoma for 14 years and 219,000 miles. I've have had tires that weighed 38 lbs each and tires that weighed 55 lbs each. Fuel mileage has never changed in any measurable way. Maybe with some high tech measuring equipment and adding a few number to the right of the decimal point. I've had heavy E rated tires and standard load tires on multiple other vehicles since getting my drivers license in 1974, never noted any difference in any of those vehicles either.

The key is that I've never varied tire size by more than 1" in either height or width and I always stayed with a moderately aggressive All Terrain tire. When you go to significantly larger and wider tires that is where you see dramatic fuel mileage loss. Especially width. The tire tread matters too, aggressive mud tires suck gas. When you go to larger, more aggressive tires they are naturally heavier. Many guys mistakenly believe it is the weight that is making the difference.

Weight will have an effect on acceleration. It does take more engine power to get the heavier tires moving. When accelerating from a stop, or when I needed to accelerate onto an on ramp while interstate driving there is a noticeable lag in speed. But once up to speed it doesn't take any more power, or fuel to keep them moving. Wider, more aggressive tires have much greater rolling resistance meaning your truck won't coast nearly as far when you ease up on the gas. You have to keep your foot on the gas pedal more just to keep the truck moving.

 

I’m looking to replace my 3 year old GY Duratrac tires on my ‘15 DCSB TRD O/R with 2.5” lift front and rear.

Trying to decide between
265/70r17 “C” BFG AT KO2,
285/70r17 “C” BFG AT KO2,
or
285/70r17 “D” GY Duratracs.

Each are white letter tires which is what I want. My concern isn’t so much MPG’s. It’s drivability I’m concerned with. I know these trucks aren’t necessarily fast but I like how my truck drives now and I don’t want to feel like I’m forcing the truck to move with 285’s. The weight and tire dimensions aren’t such a huge difference in these 3. Anyone here gone similar route can comment their experience or opinions?

Refer to the pics to see the tire details.

 

Three thoughts….1) why ask about tire experience while not caring about mpg in a thread about mpg. Consider starting a new thread or a more relevant thread. 2) if you go up in tire size the drivability won’t change drastically except the the truck will feel a little underpowered. You’ll get used to everything else super quick. 3) Why go up in tire size at all? To look cool? Then you lose mpg, power, cause more wear on your truck. Why not just stick with the smaller size. Save yourself the trouble.

 

1). I’ve considered it, and done it in the past and people always gravitate to MPG’s, which is why I wanted to clarify that MPG’s aren’t a concern for me.

2)
That makes sense.

3) more contact patch when aired down but I guess not a big enough difference to warrant the negatives.

 

Correct. Don't recall the actual stock size tires, but it was whatever size they put on teh 2008 Sport 4x4. I went up to 265/70R17.

 

They came with 265/65r17 so you definitely did the +1 fitment.

 

I went from 285/75/16 tires to 305/65/17 on 1/2" wider wheels and my mpg stayed pretty much the same. So the 305's on wider wheels were definitely heavier but did not affect the mpg at all. They also had a wider footprint so that argument can also be tossed I guess. The height is about the same so no correction was needed. Going to taller tires will result in slightly lower miles if you don't correct the speedo for it and that could throw off your calculation a little making it look worse. There is definitely a point where taller tires will hurt mpg regardless of speedo correction or not. It's usually the truck being lifted with taller tires that creates more wind resistance and hurts mpg.