Firstly, i should explain that the machine i am referring to here belongs to me and is a lock out hand crank machine, thus a continuous pull machine (probably electronic) that also has the ability to prestretch strings (so they stretch less after being pulled) tends to maintain a better tension, and this seems obvious in my opinion!

It is my experience (as I string my own rackets and some for my friends)  that the slightly thinner strings like BG80 and BG85 retain a little more tension in my rackets than slightly thicker strings. I feel the thinner ones (or maybe THESE thinner ones) stretch less and hence retain their tension better.

In most instances, I find that if i string a racket at a certain tension e.g. 24/26, then after the strings have settled (24 hours) they tend to have lost 1 lb more than what i did on the main strings – i.e. before playing but after stringing, the stringbed tension is about 23 lbs. This is because, as Paul says, the string stretches. After you start hitting, the tension will go down a little more as the strings continue to stretch. I find they will lose a further 1 lb, finishing at 22 lbs string bed tension. This tends to be true up to about 29/31 lbs when i string my rackets. However, using a thicker string, the string tends to lose another 0.5 – 1 lbs in tension.

For example: Stringing my carlton Vapour Trail Tour racket 25/27 lbs with BG80 will result in a final tension in the string bed after a few hours of play at about 23 lbs. The Same job done with BG65/BG65Ti would finish slightly softer at 22 – 22.5 lbs.

Fortunately i tend to find my rackets (all carlton) tend to hold their tension after these initial losses very well, until they are nearly about to break, when there is a noticeable loss of tension.

For BG 80 (given I play on average about 10 hours a week – 15 when training) the above stringing job would last me about 4 weeks before I decided i needed to change the strings (but they wouldn't BREAK for another 2/3 weeks). However, when I am using the higher tensions (27 upwards) I tend to find i need to change the string every 2-3 weeks to maintain a good tension – and NOT changing can result in noticeable losses in accuracy and power, immediately rectified by restringing. However, having improved my technique a lot recently, I now find the lower tensions (about 23/24 lbs string bed tension) are all i need, as accuracy is coming a result of better technique.

This is just some information about my experiences as a stringer/player and hope it helps some people when considering string tensions. I would not recommend stringing a racket at too high a tension unless you can afford to keep restringing and the higher tension IMPROVES your game, which it does not usually do beyond a certain point.