SOUTH DENNIS

YOU ALREADY SUSPECTED funny arithmetic on your credit-card bill. It’s worthwhile to pick up your calculator and check the next one.

I use two credit cards. One is USAA, the best in the business (I’ll get to that later). The other is Chase, a company that I have found to use deceptive tactics.

I had a previous Chase credit card, but when they sent out a notice that they were raising rates, I canceled it. Who needed it? The credit-card companies act as if they’re doing us a favor when in fact it’s our interest payments that are doing them a favor. They should be currying our favor.

Then Chase came out with an AARP card with reasonable rates, so I got it. After all, AARP’s reputation also hangs on the same limb, so it should be okay. Right? ’Tain’t necessarily so.

My set interest rate for months on end was 7.99 percent annual percentage rate (APR), set in bold on the bill. But on my latest bill, the APR rate set in bold was 12.99 percent. Everybody knows that interest rates have been dropping, so I called Chase and spoke with a “customer-service representative.” That’s code for one who’s trained to deflect, obfuscate and lie.

I asked where the 12.99 percent suddenly came from. She said my rate was 7.99 percent (first lie), to look at the fine print, which lists, “Purchases, 7.99 percent.” I said, “I see that, but I also see 12.99 percent in bold.”

My interest amount was exactly $1, which I found odd. (I had been paying down this card, because the USAA rate is only 5 percent, bold print or fine print, so I’ve been using that one.) I asked, “Is there a minimum interest you charge, like one dollar?” She said, “It depends on your balance.” (First deflection.)

I asked again, “Is there a minimum interest you charge, like the one dollar shown on my bill?” She again did not answer the simple and direct question, but tried to obfuscate the situation. (Second deflection, first obfuscation.)

So I put it quite clearly. “Look, I’ll make this simple. If my unpaid balance was one dollar, would you charge me a minimum interest of one dollar, making that 100 percent per month or over 1,200 percent per year?” At that point I think she starting sniffing smelling salts because her repertoire of lies, deflections and obfuscations was exhausted.

I was insistent, you see, because while I was talking to her I was computing the interest rate on my calculator. It came to 13.7 percent. Given that credit-card companies use special formulae that take into account not only average daily balance, but phases of the moon and the rotation of the Earth to maximize income from their stated rates, I figured that was close enough to their stated 12.99 percent. They were charging me 12.99 percent and doing all they could to deceive me about it.

I thanked her and hung up and wrote a check for the balance in full. My revenge? Each month I’ll rent a DVD from Redbox with my Chase card for $1.05 and have them bill me. I’m sure it takes many times that amount to prepare and send a bill. There are always ways to punish transgressors.

Now back to USAA. USAA (5 percent current credit-card rate, no penalty for late payment or non-payment) is for military officers and their spouses and families. This may not apply to most of you, but there are many reservists, such as at Camp Edwards, in and those out of active duty who may not know about USAA. I didn’t until I served at Fort Monroe, Va., and a colleague told me about it.

They also have the best auto-insurance rates and home-insurance rates in America. That’s because military officers are considered a low-risk group. And yes, they insure in Massachusetts. The Web site is www.usaa.com.

One last word, when you calculate your interest rate on your next credit-card bill, use their stated “average daily balance” since it’s impossible to calculate it yourself. After you get your monthly interest rate, multiply by 12 for the yearly. It won’t give you the exact APR, but should be close enough for you to decide if you’re being robbed.

Solon Economou, a frequent contributor, is an engineer and Cape Cod-based writer ( capecodder1@hotmail.com).