You have described a scenario where it would be feasible for the two of you not to know who was driving on a given occasion. We have heard the same scenario hundreds of times, as will the bench.
You make no mention of any attempt to determine who was driving on that occasion, other than attempting to put the onus on the police to help you.
If you are unable to determine who was driving, there is a defence available if you can persuade the court (on the balance of probabilities) both that you do not know who was driving and that you could not with reasonable diligence determine the identity of the driver. From what you have told us so far, you would struggle somewhat with at least the second limb.
The evidence of the alleged speeding offence is all but irrelevant unless you are able to name the driver. The "all but" is that any credibility that you might have had for the s. 172 defence would likely disappear.
The "pragmatic" approach recommended by the forum (which I do not favour on principle), is that if you are genuinely unable to determine who was driving after exhausting reasonable diligence to try to find out, to name the most likely candidate.