modems autonegotiate things like that, and the only way to know for sure, is to force the modem to connect at a specific speed, and none other. with gkrellm, you get a download bar and rate, and if you are downloading uncompressable data, you can figure out your
current connect speed. most importantly, you can tune your modem init string to try to connect to the highest speed where you can usually connect and not get errors. you can also spot if something has gone wrong with your connection, by the bar, and reconnect.
my bar is constant right now at about 4.6k per second with few peaks and spikes. if something goes wrong, it often jumps to something liike 3.2, and 1.6 spiking up and down between those 2. luckily having gkrellm, i know to disconnect and retry when i see that.