Changing wheels/tires any guess on the mpg gains?

[therealtime]

I am currently running stock del Sol Si wheels with 195/60/14 tires.
Average weight of a 195/60/14 tire is 18.3lb (calculated from tire rack tire weights). The del Sol wheel weighs 15lb.

Total unsprung rotating weight per wheel is 33.3lb.

I am hoping to snag a set of Civic VX wheels with 175/70/13 tires.
Average weight of a 175/70/13 tire is 14.8lb and the VX wheel weighs in at 9.7lb.

Total unsprung rotating weight per wheel would be 24.5lb (reduction of 26.4% or 8.8lb per wheel, 35.2 total)

The vehicle velocity would also change by reducing tire circumference by about 2.5% (my speedometer is currently 2.1% too slow because my tires are larger than stock and the vx tires would make it 0.4% too fast)

The frontal area would be reduced because the car would sit 0.3 inches lower and each tire is 0.8 inches narrower.

seems pretty dramatic to me when you combine the reduced area, reduced resistance to rotation and reduced total mass. the only drawback is the smaller circumference which would make the engine run at higher rpms (81 more rpms to maintain 70mph).

any educated guesses as to the mpg gains to be found?

thanks,
-tony

[theholycow]

Those tires will need more pressure than the existing tires. Also, don't forget that the odometer will suffer from the same change in accuracy that the speedometer does, so calculate that in to your FE calculations.

I am skeptical about the rotating weight issue. Based on the science that I know about it and the words of experts in other contexts, I believe that it's a myth.

[bobc455]

Originally Posted by theholycow

I am skeptical about the rotating weight issue. Based on the science that I know about it and the words of experts in other contexts, I believe that it's a myth.

Decreasing rotating weight won't help much, unless you are drag racing where increasing rotational inertia sucks energy away from increasing your forward speed.

However reciprocating mass (pistons, valves, etc.) do "suck" energy to move.

-BC

[therealtime]

Originally Posted by bobc455

However reciprocating mass (pistons, valves, etc.) do "suck" energy to move.

-BC

I was thinking about lightweight spring retainers... i wonder what the improvement might be? I know they wouldn't pay for themselves with improvement but that doesn't matter when you go all out like I have a habit of doing.

[Ford Man]

Going to a smaller wheel and tire is going to hurt you because of the difference it will make in gearing. The engine will be turning more RPM's to do the same amount of work.

[therealtime]

Originally Posted by theholycow

Those tires will need more pressure than the existing tires.

Why? I am running my existing tires over the recommended pressure anyway. i plan to do the same with the new ones.

Originally Posted by theholycow

Also, don't forget that the odometer will suffer from the same change in accuracy that the speedometer does, so calculate that in to your FE calculations.

I mentioned in my post that the vehicles velocity would change. I am currently adding 2.1% to my mileage because the OE tire for my car would have been a 185/60/14. For the new tires I will be subtracting 0.4%

Originally Posted by theholycow

I am skeptical about the rotating weight issue. Based on the science that I know about it and the words of experts in other contexts, I believe that it's a myth.

From an engineers perspective, in a vacuum, it would take no extra energy to maintain the velocity of a larger mass than a smaller one. But accelerating the mass is where energy can be saved. It would consume less energy to accelerate to speed from every stop.

So if you did an A-B-A with a scangauge and started the measurements when you were allready at cruising speed then you would see no improvement. BUT, if you consider the full mileage covered by a full tank (like I do) then you should see more of a difference. That is why racers use lighter wheels, because less energy goes to accelerating the rotating mass and more energy goes into accelerating the vehicle.

If less energy is needed to drive the same way that I allready do, then less throttle would be applied and less fuel burned.

Originally Posted by bobc455

Decreasing rotating weight won't help much, unless you are drag racing where increasing rotational inertia sucks energy away from increasing your forward speed.

However reciprocating mass (pistons, valves, etc.) do "suck" energy to move.

That seems to contradict itself and confirmmy argument. If less energy is lost to rotational inertia then less energy is required to accelerate the vehicle (assuming you are not trying to accelerate harder like a drag racer would be). So for stop and go driving, a vehicle with less rotational mass would be more efficient right?

I contend that if every other parameter was maintained equal, and a vehicles rotating parts were replaced with lighter weight materials (from the crank pulley, through the drivetrain, all the way to the tires) that the vehicle would be more efficient.

I am assuming of course that techniques like P&G are not used because that in fact requires rotational inertia to extract energy back out of on the glide. My assumption is for the standard American driver or for people like myself who accelerate normally through the gears and use cruise control often.

And Ford Man, I mentioned in my orginal post that to maintain the same vehicle speed (69.03mph, my car would change from 3200rpm now to 3281rpm with smaller tires) but I don't think 81rpm will consume significantly more fuel. i am considering a transmission change in the future however that would bring the rpms below stock at cruising speed.

Sorry for the long post, and I am very open to criticism or correction because that seems to be the best way to learn.

-Tony

[theholycow]

Originally Posted by therealtime

Why? I am running my existing tires over the recommended pressure anyway. i plan to do the same with the new ones.

The new tires are smaller and rated for less load at a given inflation. While I'm not suggesting that you'll be overloading them, your overinflation will be less with smaller tires at the same pressure.

From an engineers perspective, in a vacuum, it would take no extra energy to maintain the velocity of a larger mass than a smaller one. But accelerating the mass is where energy can be saved. It would consume less energy to accelerate to speed from every stop.

You make it up on the other end, unless your driving style is the polar opposite of basic efficient driving. Dead weight is ballast that stores kinetic energy reasonably well, and rotating weight is a flywheel, a concept that is often considered very effective (but not practical on a large scale). Basic efficient driving includes not accelerating all the way to your next stop, instead accelerating the minimum required amount and trying to roll through green lights and coast up to stops.

That is why racers use lighter wheels, because less energy goes to accelerating the rotating mass and more energy goes into accelerating the vehicle.

Drag racers use lighter wheels because they need to squeeze out every bit of acceleration possible and will continue accelerating until they stomp on the brakes/deploy parachutes. Circle/road racers use them because unsprung weight affects handling.

So for stop and go driving, a vehicle with less rotational mass would be more efficient right?

If it's going to make a measurable difference, that difference will be found in extreme stop-and-go traffic.

My assumption is for the standard American driver or for people like myself who accelerate normally through the gears and use cruise control often.

For that type of driving, it will have no measurable effect. The little bit of extra fuel would be smoothed out in the average.

I say go ahead, and we'll see what the long-term effects are...though there are too many variables to say for sure which are responsible for any measured effects. It's my prediction that there won't be much difference. There's one way to learn that's better than being corrected: Finding real world data.

[suspendedhatch]

I have found on my Hondas over the years that reducing wheel weight improves ride quality, nothing more.

However, switching to smaller wheels means switching to thinner tires, which does in fact improve mileage significantly. It's nearly impossible for me to predict how it would affect your fuel economy with so many real world variables to contend with, but I wouldn't expect more than a couple miles per gallon.

[Mayhim]

After having had four sets of tires of varying size on the CRX, I would say that any improvement would be so small as to be lost in the background noise. How long you idle at the Burger King drive-thru twice a week, or a small change in PSI would make as much of a change.

IMO.

[therealtime]

Well, the concensus on this forum seems to be that there will be negligible gains from the change.

But, I need tires anyway so I will be picking up and installing the VX wheels today after work. I start back to school next week (125 mile a day commute) so I will be updating my gaslog soon and often (fillup about once a week). I will post back here when I have results.