You don't realize it until it's time to trade or sell your used car, but depreciation is your single biggest auto expense. A lot of things affect how much your new vehicle's value drops, but its mileage rating is becoming a much bigger factor for some vehicle categories.

So Drive On welcomes new projections by a major auto market crystal-ball outfit to tell you just how much a $1 change in gas prices will hurt or help the "residual value."

If gasoline goes up $1, your big SUV loses 13% in resale value when it's 3 years old, forecasts ALG, a.k.a. Automotive Lease Guide, the leading reference for residual values on which leases and other auto transactions are based.

At the other end of the scale, ALG says a $1 gas rise will boost the value of your cheapo econobox by 10% at age 3.

The gas effect is mainly at the edges: a $1 rise will have no effect at all on your vehicle's value if you own a full-size car or compact SUV, ALG forecasts.

As for gas prices, ALG expects the average to hover around $3.65 per gallon this year, rise a dime next year and edge up to $4.06 on average in 2016.

That's when federal regulations require the auto industry to average 35.5 miles per gallon across all the vehicles sold. (Each vehicle doesn't have to hit that.) That's also when new mpg regs now under furious discussion will kick in. The Obama administration floated a number of 56.2 mpg by 2025, then seemed to back away slightly.

Now, there's talk that the regs, to be officially proposed this fall, could go easier on pickups and SUVs -- see earlier post here -- because part of the appeal of those vehicles is antithetical to high fuel economy: Their size and weight and the engines they need to do the work buyers expect, such as tow 8,000 pounds, haul six or eight people or carry a ton in the cargo bed.

Here's a chart of how ALG figures a $1 gas price rise effects 3-year trade-in values by product category: