Wiper fluid is typically a non-corrosive mixture of alcohol and water. Has to be non-corrosive because of the paint on your vehicle. The -20F freeze feature is a function of the alcohol content. But alcohol evaporates faster than water. And it's a proven fact that air slowly leaks through rubber. Even if your valve stems and rims have good seals, air still works its way through the sidewall. If air can, it stands to reason that evaporating alcohol molecules ca as well. In a windshield reservoir evaporation is not a big deal because you top it up occasionally. Not so inside a tire. I wouldn't be surprised if some of those using wiper fluid are unknowingly driving around with slush inside their tires on a freezing morning. That said, the black rubber absorbs heat from the sun, which helps resist that solidification - especially as the day wears on. Until all the alcohol is gone that is.
Since alcohol has a lower specific gravity than water - and the alcohol content displaces water content - wiper fluid is actually lighter than plain water. Rimguard on the other hand is approximately 1.375 times heavier than water. And unlike other liquid ballast, it's biodegradable. Environmentalists love that, because - if/when you suffer a tire puncture - any subsequent spillage doesn't threaten the ground water supply.
That said, you don't need much RimGuard for a 75% fill on your 4300. Took 88 gallons to do all four R4s on my 3720. $238 for 88 gallons = $2.70/gal (installed). Since then I've talked to folks who've paid as low as $2.27/gal. Seems that location drives the price.
Now there are a lot of folks out there that are satisfied with wiper fluid in their tires, so this is not a condemnation of the practice. My intent is only to apply a little science that helps tell "the rest of the story".
//greg//
