Drop unsprung mass, gain serious MPG

[Hohn]

I have a little IS250 as my main commuter. The stock 225/45r17 tires have been fragile and offer poor ride quality, so I went to smaller 16" wheels that allowed me to drop down to 205/60r16 tires. The weight of wheel/tire assembly is about EIGHT POUNDS LESS PER WHEEL.

And that's with steel wheels instead of aluminum.

My MPG is consistently at least 3mpg better. Before I would barely crack 29-30 on extended highway runs, now I can get 32+ even a shorter hwy runs. Around town was 22mpg and now I'm getting 26mpg in the city.

Low profile tires and large wheels are an aesthetic choice with real MPG costs to attain.

 

[b4autodark]

What, if any, is the difference in tire diameter? I would guess that may have more to do with mileage than 32 pounds of weight.

 

[nwjones18]

What, if any, is the difference in tire diameter? I would guess that may have more to do with mileage than 32 pounds of weight.

225/45-R17 vs 205/60-R16 Tire Comparison - Tire Size Calculator | Tacoma World

205/60-R16 tires are 0.71 inches (18.1 mm) larger in diameter than 225/45-R17 tires and the speedometer difference is 2.8%

www.tacomaworld.com

Looks like the new size is 2.9% larger so odometer would be showing 2.9% less miles than he's actually traveling if my brain is working right this morning..

 

[BMWTurboDzl]

I have a little IS250 as my main commuter. The stock 225/45r17 tires have been fragile and offer poor ride quality, so I went to smaller 16" wheels that allowed me to drop down to 205/60r16 tires. The weight of wheel/tire assembly is about EIGHT POUNDS LESS PER WHEEL.

And that's with steel wheels instead of aluminum.

My MPG is consistently at least 3mpg better. Before I would barely crack 29-30 on extended highway runs, now I can get 32+ even a shorter hwy runs. Around town was 22mpg and now I'm getting 26mpg in the city.

Low profile tires and large wheels are an aesthetic choice with real MPG costs to attain.

Hard to say because you went with a significantly narrower tire which has less friction. Why did you not choose say a 225/50R16?

The IS250 is a sport sedan. 205/60 16's are underwhelming

 

[Ausfahrt]

.

 

[oil pan 4]

Nope.
205/60R16 tires are about 3/4 of in inch greater in diameter than a 225/45r17.
Correct milage and recheck.
No doubt lighter wheels and thinner tires are helping, but not that much.

 

[UG_Passat]

You can pay more money to add lightness in wheels, using the same diameter where available, such as Forged wheels ($$$) or Flow-formed wheels ($$).

And/or you can get LRR tires and also pick up some mpg as well.

 

[IndyIan]

Narrower tire helps, for rolling resistance and I suppose a little bit less aero drag too. 15" is as small as will fit over the Focus brakes so I put on some just higher profile 65 series instead of the normal 60 series, and its been nothing but good. Less noise, a bit smoother on broken pavement, and the same recorded mileage but going 3% further. Even handling seems to very close really, for street use anyways.

 

[edyvw]

I have a little IS250 as my main commuter. The stock 225/45r17 tires have been fragile and offer poor ride quality, so I went to smaller 16" wheels that allowed me to drop down to 205/60r16 tires. The weight of wheel/tire assembly is about EIGHT POUNDS LESS PER WHEEL.

And that's with steel wheels instead of aluminum.

My MPG is consistently at least 3mpg better. Before I would barely crack 29-30 on extended highway runs, now I can get 32+ even a shorter hwy runs. Around town was 22mpg and now I'm getting 26mpg in the city.

Low profile tires and large wheels are an aesthetic choice with real MPG costs to attain.

225/45 R17 are not hard-riding tires unless they are some mediocre brand. Bridgestone can have poor riding quality.
Did you have RFT?

Also, why IS250 if that is a concern?

 

[MNgopher]

Recalculate after you correct for the new tire size changing the # of miles being travelled. The two tire sizes are not the same diameter/circumference.

There are a lot of other factors at play here besides just the weight. The tire section width, the type of tread and rubber compounds, etc...

 

[Danno]

I have a little IS250 as my main commuter. The stock 225/45r17 tires have been fragile and offer poor ride quality, so I went to smaller 16" wheels that allowed me to drop down to 205/60r16 tires. The weight of wheel/tire assembly is about EIGHT POUNDS LESS PER WHEEL.

And that's with steel wheels instead of aluminum.

My MPG is consistently at least 3mpg better. Before I would barely crack 29-30 on extended highway runs, now I can get 32+ even a shorter hwy runs. Around town was 22mpg and now I'm getting 26mpg in the city.

Low profile tires and large wheels are an aesthetic choice with real MPG costs to attain.

If you're using the onboard computer, the tire size has made the mpg read outs inaccurate. Even calculation by hand would be difficult to equalize to the previous sized tires.

 

[thescreensavers]

Narrower front tire also decreases drag.

You calculating mpg via miles driven and gallons added?

 

[UG_Passat]

I have a little IS250 as my main commuter. The stock 225/45r17 tires have been fragile and offer poor ride quality, so I went to smaller 16" wheels that allowed me to drop down to 205/60r16 tires. The weight of wheel/tire assembly is about EIGHT POUNDS LESS PER WHEEL.

And that's with steel wheels instead of aluminum.

My MPG is consistently at least 3mpg better. Before I would barely crack 29-30 on extended highway runs, now I can get 32+ even a shorter hwy runs. Around town was 22mpg and now I'm getting 26mpg in the city.

Low profile tires and large wheels are an aesthetic choice with real MPG costs to attain.

The stock size is 215/45r17. So the little smaller sidewall will make a difference poor ride quality.

The appropriate equivalent that toyota/lexus uses is 205/55r16.

 

[Hohn]

225/45-R17 vs 205/60-R16 Tire Comparison - Tire Size Calculator | Tacoma World

205/60-R16 tires are 0.71 inches (18.1 mm) larger in diameter than 225/45-R17 tires and the speedometer difference is 2.8%

www.tacomaworld.com

Looks like the new size is 2.9% larger so odometer would be showing 2.9% less miles than he's actually traveling if my brain is working right this morning..

That is correct, so using the onboard MPG display would cause MPG to be indicated as *less* after the tire swap. That strongly suggests the economy gains are real.

 

[Hohn]

The stock size is 215/45r17. So the little smaller sidewall will make a difference poor ride quality.

The appropriate equivalent that toyota/lexus uses is 205/55r16.

No, the stock size is indeed 225/45/17. At least, going off the bold assumption that my owners manual isn't misprinted, www.tiresize.com isn't wrong and www.wheel-size.com isn't wrong, and that they are not all wrong in precisely the same manner.

The nice thing about being the owner and an engineer is getting to decide what *I* think is appropriate.

 

[Hohn]

What, if any, is the difference in tire diameter? I would guess that may have more to do with mileage than 32 pounds of weight.

The diametrical difference is not what is important, it's the revs/mile spec that we care about because circumference and not diameter per se is what we care about for our calculations. And this varies even in comparing identical size tires across manufacturers and tire models.

The 17" wheels with 225/45 BFGs were 822 revs/mi
The 16" wheels with 205/60 Nokians are 807rev/mi

That produces a difference of -1.8%

That small difference is well within the margin of error of any measurement noise and variability.

 

[kschachn]

That small difference is well within the margin of error of any measurement noise and variability.

And at the same time there are many other variables that affect fuel economy in everyday driving. Even the energy content of the fuel changes, I once linked an article here that showed that it can vary up to 4% even at the same gas station. It's not really possible to correlate a fuel economy change to one isolated variable in normal driving without standardized test fuel and a repeatable test track. Even then it's difficult.

 

[Hohn]

Narrower front tire also decreases drag.

You calculating mpg via miles driven and gallons added?

I don't bother hand calculation with this car anymore because the onboard computer was consistent. It was consistently wrong of course, but the offset was repeatably ~-1.2mpg (i.e. it was a bit optimistic vs true) over 3 years and I stopped bothering with hand tracking (well, using an app, but whatever).

Also, hand calculation has variables that people often fail to account for-- like the degree to which the tank is filled. Unless you fill up at the same pump at the same station facing the same direction at the same time of day every time-- your hand calculations are also prone to error because of the invalid assumption that the definition of "full" is always the same for each tank. It isn't. Even if you top off and "fill" your tank, the amount of air in retained in the tank (as it is even when you top off to spilling out the neck) will vary.

Outside of a dyno and lab conditions, the best any of us can do is come up with a useful approximation of true MPG. And the observed gains I'm seeing are well outside the measurement noise of my somewhat crude measurement system. Indeed, it's exact *same* measurement system and it's indicating gains despite the minor size difference which would cause error in the opposite direction (tend to show MPG loss).

 

[Hohn]

And at the same time there are many other variables that affect fuel economy in everyday driving. Even the energy content of the fuel changes, I once linked an article here that showed that it can vary up to 4% even at the same gas station. It's not really possible to correlate a fuel economy change to one isolated variable in normal driving without standardized test fuel and a repeatable test track. Even then it's difficult.

Exactly. Which is why my initial observations took time to trust. Only after it was consistent across fuel stations, driving routes, and cold/hot engine type conditions did I come to the conclusion that the phenomenon was real.

 

[Hohn]

225/45 R17 are not hard-riding tires unless they are some mediocre brand. Bridgestone can have poor riding quality.
Did you have RFT?

Also, why IS250 if that is a concern?

I'm not sure I'd want to be contentious over something as subjective as ride quality. I'll be my own counsel as to what constitutes hard riding and you can be yours.

These are not run flat tires, which past experience soured me on.