Well, can you guess?
The short answer is no, you can't and you shouldn't.
Want to know why?
It's because when you guess you could be right and then again, you could be wrong.
Let's say this is a car accident case.
You're asked during cross examination how fast you were going fifty feet before the impact.
You're not sure.
You think you were going slow.
You think your speed was less than 20 mph.
Instead of saying "I don't know," you say "I was going 15 mph."
It turns out that there were security cameras that recorded the intersection where this happened.
The security camera recorded every second.
An accident reconstruction expert reviewed the video.
He calculated that there was no possible way you were traveling 15 mph based on the distance you travelled in that short time frame.
Based upon known formulas for speed, time and distance, he calculated that you were traveling at 57 mph in a 20 mph zone.
What do you think your answer of 15 mph will do to your credibility in front of the jury?
It will destroy it.
Plain and simple.
If there are other ways to show that your 'guess' was incorrect, you can be sure that the opposing lawyer will do everything possible to expose your 'lie'.
On the other hand, your guess could be right.
When you don't know the answer to a question, simply say "I don't know," or "I don't remember right now."
You may have known the answer at a previous time.
You might have known it when it happened.
You might have known it a year later.
You might have known it when you were questioned at your pretrial question and answer session known as a deposition.
But for some reason, you have a brain freeze and just don't remember the answer while you're on the witness stand.
What if the opposing attorney asks you for a range of your speed?
"Were you traveling somewhere between 15 mph and 30 mph?" he asks innocently.
If you know the answer, tell him.
If you don't know, tell you you don't know and that you don't want to guess.
He may ask "Were you traveling between 30 mph and 50 mph?"
Again, he wants to see if you'll commit to a particular range.
If you do, he's going to lock you into that testimony, even though you're not exactly sure.
Trying to guess an answer does more harm than good.
Once you give a definitive answer to a question and you finish testifying, you can't later go back on the witness stand to make corrections.
You don't get a do-over.
That's not how this works.
That's not how any of this works.