I feel I need to be more clear/detailed about this point.
it's quite common that decks usually can be a hair faster at the beginning of the side and a hair slower at the end of the side... easy to check this while playing a proper speed test tape (with "proper" i mean a true one where the recorded tone is at constant speed throuout the whole side of the tape) and it's the normal behaviour of so many decks out there.
But, when going to record a homemade speed tape on a deck which has some speed differences throughout the whole tape side (usually just at the beginning and at the end zones of the side) , it's just the (homemade) test tape which has some inherent speed differences and while mixing this with the speed difference throughout the tape side of the deck under test, all bets are off because you cannot know what is due to the (homemade) test tape and what is due to the deck under test.
So, to try to minimize such variables a bit, it's better to stay around the center zone of the tape.
But, if you have a proper speed test tape, you will know that any differences throughout the tape side are due just to the deck under test (and quite normal that there are some)... another nice thing related to a proper speed test tape is that it's also good to measure Wow and Flutter in a reliable way while the "homemade" ones would have too high inherent W&F just recorded on the tape so that they aren't suitable to measure W&F properly, and especially on those decks with very low W&F which is most likely lower than the inherent W&F of the (homemade) test tape.
So, it's OK for you to use that speed tape from ebay (which is of "unknown" quality and precision, after all) but it's wise to avoid relying on it just blindly, unless you also have a proper one to compare it against.
In other words... people like, i.e., our Alex/A.N.T. are perfectly able to check the actual quality of such cheap test tapes available on ebay... while the rest of us (myself included) are not able to check and know it so, our best bet is to try to keep our hands on a few proper test tapes which can be taken as a reliable reference without worrying.
(of course, the proper ones are noticeably more expensive... and for a few good reasons)
I hope it helps.