Here's a template for spray painting the letters debossed on the "dogdish" style hubcaps. You can print this page and use it to cut out a mask to repaint your letters. I bought some clear, low-tack, adhesive-backed sheets from an art store. I then placed this sheet over the printout and used a very sharp Exacto knife to cut out the letters. You may also be able to find a small pack of these sheets at a Staples, Office Depot, or computer supply store that you can print on directly with your inkjet or laser printer. Each letter will have to be cut apart from the others and placed on the hubcap in position. Then mask the rest of the hubcap with tape and paper. Make sure the hubcap is very clean before applying the template. I used "SEM" brand, self-etching trim paint. Although this takes some time and patience, it will give much better results that most of us can do freehand with a brush.
Note: Due to a number of variables that may affect the size of these letters after printing, you may want to cut one out and try it on your hubcap before spending time on the rest. You may find that if you cut to the inside or the outside that it works better for you.
For those who are familiar with graphics and have a program like Photoshop, you may find it better to download this GIF (hold-click for Macs, right-click for Windows) and adjust the size to your application. The GIF is 72 dpi with a width of 1500 pixels. If you reduce it by 25%, this should get you pretty close. If you don't have the "Resample Image" box checked, it should change the resolution to 288 dpi which should give you a smooth edge on most inkjet printers.