Part of that is a no brainier, the latter half is just here say at this point and contrary to automotive wisdom.

 

I and a customer both have tried larger tires. He has a manual and I an automatic. Both cars are 08's with 195/55x15 stock tires. We're always doing this kind of stuff; guess it's the engineer in us. Here's my logbook note conclusions.

Size tried: 195/60x15: both cars dropped 3-4 mpg by corrected mileage
165/70x15: both cars lost 1 to 2 mpg by corrected milea.
These tires are close to the 09 stock 185/55x15 tire diameter but are a trifle heavier by our scales..
The last requires some explanation, They were leftover tires from the VW bug days so they are old tires but still in good shape. We used only one set to compare a weeks mileage - a tank's worth to usual mpg's (which neither of us gets the mpg of some posters. My auto 30 mpg and the manusal 33). The 165/70x15's were used to minimize the weight increase when increasing the tire diameter. The less weight of the 165/70x15's did help compared to the 195/60's which weighed a lot more than the 195/55x15's.
My 175/65x15 winter tires are a bit larger tires than stock and weigh about the same and they get about 1 to 2 mpg better than the stock tires on corrected mileage (844 revs/mi vs 873 revs/mi) so a slight increase in mpg is possible if you lighten the tires enough and don't decrease tire revs per mile more than 5%. However it was a short test so statistical comparison isn't possible. And since neither of us slow for corners 175/65x15 is as narrow as we are comfortable with. The 165's were a bit antsy in corners though it could be their age but no sidewall cracks and tread appeared soft enough.
I think Honda engineers did a good job of choosing the correct size though I have doubts about choosing those Dunslops; I think the accountants overuled. Shades of Detroit.

 

Lots of details.. On the CeanMPG web site most of the originals members including Wayne are telling anyone that will listen that the high rev gearing is costing a great deal in MPG. And Honda boned us. It would be interesting to see/read if you whet over there and post your results. I'd like to see how they react. Maybe Shawn can.

They go as far as saying lower RPM's is always better.

I have always said I don't know, but have never had the resources to try and make a change. Often wanted to try

PS Can you post the calculations (if you still have them) with the odometer correction for the changes in the tire RPM with the tires you used? Also, did you ever try some real A-B-A testing on this using P&G and other methods that get the high FE results?

I looked up the 2008 Fit tires as well as the one you tested.

Do you have a base or Sport. The tire you listed is not shown
Maybe you did not see they are 14""

08 Sport 195/55 -15 73.7"
08 Base 175/65 14 72.1"
195/60-15 76.7" 4% more then base Sport

 

The traveled mileage was corrected using the rolling radius, aka the revs of the tire per actual mile. Its incorrect to use diameters or circumferences. And we both have sports models. Gas was #3.87 per US gal (OK I've drivein in Europe so I know gas there is double that due to taxes)
Using the standard revs per mile for the 'test' tire divided by the revs per mile per standard tire and that multiplied by the odometer reading.
For example here are the tire revs per mile for the three tires:
195/55x15: 890 revs per mile
175/65x15: 844 We did this one later because it was our choice for winter tire and because I found it first in the logbook. We're rolling on them now
165/70x15: 873* This rolling radius measured by taking the radius of the hub center to the ground, multiplying by 2 pi R and converting to ft and dividing into 5280 to get revs per mile.

According to my logbook a comparison of Fits on 175/65x15 tires showed a difference of 1.4 and 1.8 mpg on the odo mileage corrected back to the 'actual' mileage as both cars showed less miles than actually traveled. And in truth thats within a 3 sigma spread.
Its pretty straight forward. As the 175/65x15's weigh about 2 lb less than the 195/55x15's we thought they would do better but it looks as though just moving the same weight further out on radius just took too much power.
We did our best to fill the tanks to a known height using dip sticks but that could be an error of 1% on the fill quanitity. That is less than the other potential errors.
We were just proving what we thought we already knew: adding weight at greater radiuii will require more torque energy from the engine and hence more gas.
We were a trifle surprised at the 165/70x15's not being better 'but we decided it was due to the old tires increasing rolling resistance
possibly due to greater energy needed to deform the tread as it rolled around
In theory, the 165/70x15's should have gotten better mpg when pressured to 40 psig than the stock 195/55x15's. Not even sure they are available anymore. Maybe lightweight 155/80x15's? Those aren't performance tires by any stretch of the imagination being designed for an 1800 lb VW compared to a 2600 lb Fit; not a good idea..
Was that a help?

 

yes thanks.

 

UPDATE:
I got the new tires on. It looks like they actually make a negligible difference in gearing (1% or so) but they do add an inch of ground clearance. I drive my fit like a truck (bumpy dirt roads, hopping curbs) so the ground clearance does come in handy. (In my opinion, if you have to slow down for speed bumps, you don't have a performance car. Driving slow does not equal performance.)
Prior to getting the tall tires on, I ripped off the front undertray while off-roading. They would have come in handy there.
Also, the taller tires make attacking mountains roads a little more... exciting. During S-turns and decreasing radius turns, the car can rotate a little bit before the contact patches react. It takes some getting used to, but I still drive it like a race car.
Also, a week after I got the tires on, I got a scanguage, which has made a huge difference in mileage. I noticed my mileage was in the mid-30's. I started pulse-and-gliding on the freeway, and now get mid-50's. Driving habits make way more difference than mods.

 

True but the engine has to work harder; like climbing a hill. Harder work the greaterfuel injection time.

 

The simple thought in my mind is that if a car needs a certain torque to stay moving forward at a steady speed, and that torque could be achieved with different throttle positions at two different RPMs, the lower RPM would likely give better mileage.

At the higher RPM, your throttle is almost closed as the load on the engine is low to achieve the needed torque, and that creates a big pumping loss since the cylinders are sucking air in against a big restriction.

At the lower RPM, you need a little more load to get the same torque, so your throttle needs to be opened a little bit more, and that will lower pumping losses.

If you were going 55, and you downshifted into 4th, wouldn't you expect your mileage to be a little bit worse than if you were in 5th?

 

The torque curve of an engine gives the engine thrust over its entire working rpm range. The torque (thrust) at a given rpm can be multiplied by the gearing so that forward thrust can be equalized in two different gears. However,
The hp curve says that less hp is available at lower rpm usually (unless at the very top of the hp curve) so the work available is not the same; the higher rpm has more capability to do work, not just thrust the wheels around. So if a given speed has two rpm choices there are two different fuel consumptions involved and that is where lower rpm is not necessarily the most economical
An engine turning fewer rpm but working harder will consume more fuel than that engine turning more rpm but working much easier can yield lesser fuel consumption. since both are traveling the same mph the highher rpm engine will have better mpg.
All engine manufacturers run what's called a specific fuel consumption curve on their engines; all I've seen for street engines are an upsiode down curve, meaning that somewhere in the middle the curve will be lower than the upper or lower ends of the rpm range. That minimum point defines the best specific power measured in ounces of fuel per horsepower-hour. (Porsche used to present that curve in their owner manuals til they found not even 1% or owners knew what it meant)
Engineers use that information to gear the vehicle for best performance fuel consumption and still do with the caveat that emissions must be factored into the decisions.
So the best mpg doesn't have to be at the lower rpm but where the engine is more efficient.
PS at the upper end of the rpm curve your throttle better be almost wide open, not closed (idle) and its not so much that the throttle is more open but how much fuel you're injecting at that throttle opening.When a certain power is needed the fuekl injected depends on how efficient the engine uses it thus why lower rpm can consume more fuel than higher rpm.

 

Looking at a BSFC map though, you almost always find that there is a decrease in BSFC as you move toward the upper left portion of the plot--i.e. a higher IMEP (load) and a lower RPM. If you can find one in which the horsepower curves are overlaid on the BSFC plot, you see that as RPM increases, the load needed to maintain a stead HP decreases (common sense).

Thus, if you need a cetain HP to maintain a speed, if you lower the RPM at which that HP is achieved, you must increase load; those two actions move you upward and left on the BSFC map into the region of lower BSFC, do they not?

 

You lost me; what's a BSFC curve?.
Ah, you're speaking of engine load compared to the horsepower required to move the Fit which must consider the gearing. Because of increased gearing the load on the engine is reduced, however, more rpm are required and that may or may not make up the difference.
It is entirely possible that the decrease in rpm will require so much more load on the engine (gearing works against you)that because it gets further from the sweet spot the fuel required increases.
But yes, once you rise in rpm above the minimum point on the specifgic fuel consumption curve lower rpm always wins; just as as long as you are below minimum the higher rpm wins. And yes, the minimum point is typically about 1/3 of the way up the rpm scale. In most driving thats the area you are in - just below the minimum or around it. Gearing designed that way.
Unless you are running the autobahn you need to make sure your engine is running at the sweet spot rather than maximum. Course if you are running that fast mpg is meaningless anyway. unless its Talledega.

Generally, lower rpm is better but not always, depending on the rpm change. When the rpm change is more than 15% lower rpm is probably 90% chance of being more economical.
cheers.

 

If you were going 55, and you downshifted into 4th, wouldn't you expect your mileage to be a little bit worse than if you were in 5th?[/quote]


Sure. At 55 mph my Fit is turning 2000 rpm in 5th and 2800 in 4th. The change in rpm is too much and overcomes the potential savings in fuel consumption. The specific fuel consumption may be lower at 2800 rpm but not that much lower so when the lopss in rpm is considered there are so many fewer injection pulses that 55 in 5th is more economical than 4th. The load on the engine, thanks to the lower gearing in 4th, just doesn't change that much to keep the same hp required.