Linksys Wireless Router :: Wrt54gs Wifi Download Speed 1/3 Of Wired Speed

I just discovered that my wireless download speeds are 5Mbps, compared to 16Mbps if my laptop is connected directly to the modem or wired through the router.  Uploads are 2Mbps either way.  I have a WRT54GS v.6 with firmware version v1.52.8 which I updated today, completed by setting factory default through the router webpage after I discovered this discrepancy.  The firmware update did not improve the download speed appreciably, if at all.Should the wireless connection be that much slower than the wired connection through the router?  If not, what sort of issues could I be having to account for this?
 
I did just replace my modem when I discovered I was having erratic speedtests and that corrected that problem.  That is when I became aware of the wireless issue, so not sure how long that has been the case.  I have Charter's 16/2 HSI, living in a sparsely populated area with solid internet service.  There is no external nor internal interference on this 2.4Ghz band.