Using tire/wheel upgrades for economy?

All: I intend to upgrade my standard 195/14-60 wheel/tire combo(97 Passat). However, I realized that(for example) a 205/60-15 mag/tire upgrade will increase distance traveled by 6%. Since these are low rpm, torque rich engines, they can handle SOME(in effect) numerical decrease in their final drive ratio-but what is the practical limit? There will be some loss of acceleration in the lower gears. Also, what is the max width and height for clearence of the wheel wells/suspension? C'mon TDI's, I need to hear from you!

I don't know about wider tires but there was a time when I was thinking of getting a set of narrower, slightly taller tires for my car. Something like 185/65-14 or even 175/70-14. The narrower tires will have less wind resistance and a slightly taller tire will effectively raise the final drive ratio improving mileage. Going with a wider and taller tire will cancel out this effect.

Keep in mind that although a taller tire/wheel combination will increase fuel efficiency (by lowering the highway cruise rpms down to the efficient 1500 to 2000 rpm range), it will also decrease your measured fuel efficiency.

Your speedometer will show a lower speed than reality, and your odometer will show fewer miles travelled than reality. You will have a mpg measurability problem.

Changing the final drive ratio gears will not have this effect on measurability.

Larger diameter tires (within reason) will improve your highway mileage, but will probably decrease your in-town mileage due to the added power needed to accelerate the vehicle. "Minus sizing" your tires (going to a taller profile and narrower tread while maintaining the same overall diameter) will help because the skinny tires will reduce drag and rolling resistance while the taller profile means you've got "more tire and less wheel." Rubber and air weigh less than metal, and light weight is better if you are going for max mileage. Skinny tires are also better in the snow. The handling/braking downside on dry or wet pavement are obvious, but at least the tires will be cheaper! I'd look at Minus-sizing before I'd tinker with the rolling radius of the tires.

-mickey

No such penalty on snow. Skinny is good.

Increasing the diameter will decrease the rolling resistance of the tire itself. Ask any cyclist.

Going with a thinner but slightly taller tire, IMHO, is the best compromise (the compromise being the trade-off of handling versus mileage). This combo gives less wind resistance at speed and less rolling resistance. Sure, the larger diameter will effect the performance at around town speeds but if you're concerned with mileage, that's not an issue, isn't it?

There's a miniscule improvement in longevity of the vehicle since the actual miles travelled per revolution of the tires is slightly less. Sure the odometer won't be accurate, but the speedometer will be (they're slightly optimistic to begin with). Thc clutch is about the only thing I can think of that will suffer.

[This message has been edited by Peter Cheuk (edited February 06, 2000).]

Something like 20 years ago, Renault developed a micro van and one of the things they did was find the best optimization of tire size for least rolling resistance and best drag profile. The size they came up with was in the vicinity of 11 inches.

Smaller is better to a point which is why the cyclists in timed trials and velodromes use smaller front wheels. There's a few other considerations for the rear wheels, like rider comfort and drive mechanics. And since it is in the turbulence generated by the frame, its tall size isn't the problem that a large wheel up front is.

Our abilities in trying to optimize wheel/tire sizes are going to be severely limited due to the chassis design and transaxle gearing. I'd vote a try for minus sizing like mickey suggests.

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Always interested in steep & deep.
Just wishing for the deep part right now!!!

You have to figure out how much your speedo is out of whack and then apply that factor when calculating mpg. In fact, everybody who's into mpg should do this. I will try to clock mine at some time soon.

To bad we have full size spares. The super economy tire is the micro spare found on hondas.

It would lower the ride several inches and has a very very narrow profile.

Rated for 50 mph or less and 50 miles total (I've heard of little old ladies that had a flat and somebody changed it for them - they run on the micro spare for years).

Speed loss from the smaller diameter would be OK, the loss of ratio just means you drive slower, which is more efficient anyway.

Powder Hound, the reason that they use smaller front tires is because the trade-off of rolling resistance and aerodynamics favor aerodynamics. If the smaller tire had less rolling resistance, they would use smaller rear tires because rider comfort is secondary to speed in these trials of speed. Since the rear tire is buried in the framework of the bicycle aerodynamics is not a consideration and they can get away with a larger rear tire to decrease rolling resistance.

I still feel that a taller, thinner tire section will give the best results as far as economy is concerned.

Well, there's also the problem of gearing and the mechanics of the human body: you can only make a chain ring so large before it would hit the ground. You'd need a much larger one to drive the rear cogs on a smaller rear wheel.

In that environment, all kinds of things make a difference, like 200 psi helium filled tires. I'm glad things here aren't that crazy!

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Always interested in steep & deep.
Just wishing for the deep part right now!!!

Karl, et.al. Not only your speedo, but your odometer as well. If you have a car that comes with an optional wheel and tire package from the FACTORY, they often do not recalibrate the odometer. I took a long trip where I was able to compare mile markers with the odometer of my second car (unmetionable Japanese brand). I found that I was actually getting 3% BETTER fuel economy than I thought. I don't think this is much of a problem with VW's since most models (TDI, GTI, GL, etc.) seem to come with one optimal wheel/tire package for that application

Peter is right ...going to a 70 series (185/70/14 perhaps) will decrease rolling resistance and improve mileage by a few percent. I too am into bicycles ...my current ride is a "LOOK" composite (monocoque graphite/epoxy - no lugs), triathalon bike with Shimano Ultegra components and a 7075-T6 Al fork - it's a material engineers' wet dream!

BB: any trucker will tell you that to save fuel, you want to cruise at the torque peak for the engine, in this case 1900 rpms. Anything you can do to effectively change the gearing to reduce highway rpms is going to get you better mileage with a diesel. I always run my gas engine Jetta @ 3000 rpm if I possibly can (about 72 mph) ...it does better than at 65 rpm with regards to fuel efficiency ....My Audi 100 has a peak torque of 3500 rpms (3 speed auto) ...again, using the trip computer and observing mileage it does the best at about 70 mph. At 60 mph the mileage actually drops off. I have varied speeds on a flat highway using the cruise control, while observing the mpg readout and also done 50 mile stretches of highway driving at various speeds to test out this theory (my wife thinks I'm nuts - some people just don't appreciate engineers!!!)

TooSlick

[This message has been edited by TooSlick (edited February 19, 2000).]