I was out shopping for tires and at the local Kaufman Tire they have a set of 205's/70/13 tires on clearance for $100 and I'm wondering how much of a negative effect the 205's will have on the performance (if there is any) and fuel efficiency? I gotta admit, $100 for a set of brand new tires is tempting, but at what price?
I doubt the difference in MPG would be much more than 5 mpg, unless you drive at high speed much of the time.
When I bought my VX, it had 185/70R13s without much tread and I put on a set of lower rolling resistance 175/70/13s. I noticed a 4 mpg difference with the same driving. Note that the older tires were smaller, so that would make up for part or all of that difference.
What worries me is that 205/70R13 tires would have a LOT of sidewall. The tires would be much taller than the ones specified. If you switched to a 205 width with a lower aspect ratio (maybe 50), it would be closer to the same rolling circumference.
I would also get an idea of the rolling resistance of the tire. A good indicator is the mileage warranty. The general rule is: The longer the warranty, the harder the tire. The harder the tire, the less rolling resistance. An aggressive tread pattern also adds rolling resistance, and in turn, hurts FE.
It's impossible to compare Rolling Resistance when changing tire model AND size at the same time. Load, pressure, size (width, height, wheel size), wear, model, and probably something else are all variables that affect RR and changing more than one at a time makes it very hard to compare.
I'm not convinced that narrower is more efficient. With tube type bias ply tires (as found on old cars and on most bicycles), there is a direct relationship between width and RR -- wider has less RR, because at a given pressure and load you're going to have the same size contact patch, but a wider tire won't have to deform as much of its sidewall to make that contact patch. I used to be very confident that the same concept applies to tubeless radials, but the small amount of data I've been able to find is questionable.
The height (sidewall, circumference, diameter, however you want to look at it) is guaranteed to have a significant effect on FE. I don't know how the VX is affected by changing circumference, though any car will have a different effect based on a given driver's style/route/traffic conditions. When changing height, you cannot compare FE unless you calculate in the difference in mileage before calculating your FE (or adjust your odometer for the new tire size).
I think Fetch is right about rubber compound. I think softer tires probably have less RR. Another thing to consider is tread depth; it is known that a worn tire will have less rolling resistance than a brand new tire of the same exact model.
IMO, there's just not enough data to make an informed decision. The strategy I recommend is:
Change size only if you want the change in gearing or if you can't get the right size at a decent price
Choose long-wearing tires (unless you expect to junk the car before the tires wear out)
Choose tires that can handle higher pressure (unless you've already found the maximum comfortable pressure)
Choose tires that are cheap
Obviously, you have to do that within your own comfort level of quality, traction, etc.
Doing it that way, you know you will be saving money. So, addressing the tires you're specifically looking at...do you think your FE would benefit from taller gearing? Do you drive on the highway or at high speeds a lot, or do you do more city driving at low speeds?
I have Michelin Destiny tires on mine size 175/70R13 on the original VX rims. I paid $52 or $56 per tire at Tire Barn. They have an 80,000-mile warranty. They're quiet, smooth (even when overinflated), and I cannot make them squeal even when I try. They were outstanding in the snow, too (the outer tread blocks have no connecting band of rubber. This makes them slightly louder, but pays huge dividends in the snow).