I couldn't help but add a comment, I am from the Willamette Valley Soaring Club in Oregon. Once a year we do a Safari to the Alvord Desert which is a dry lake 17 miles long and 7 miles wide, that's right there is no end to the runway so to speak.

We have been doing a weeks worth of auto tows since the 1950's and I can say while I have only been a part of this Safari since 2000 I am hooked on auto towing at this location.

All of the variables that have been mentioned exist such as air density, horse power of the car, car weight, length of wire, etc.

Up until this last year we have always used a length of 12 gauge agricultural wire (about $125.00 for 4,000'), we take a new wire every few years to replace the old one. You can tell when it is time to replace the old because the number of wire breaks. Wire breaks are a real pain as it takes a half hour to get the two ends nicro pressed back together and walk the wire to check for kinks. We have used a number of old cars, the 1960's Buick's were the best as we just left them at a farmers ranch and came out, fueled them up, reinstalled the battery and fired them off.
The last care we used as a 92 Cadillac which only lasted about 5 years before the transmission went.
This year we took both a new wire and a take up reel I built. the reel had 4,000' of spectra and was used much as the wire was in that it was run out full length before the tow was started so the only variables in tow were the car speed and the pilots control. ( I had a few very uneasy flights years ago on a pay out winch where the car speed, winch pay out and pilot were all variable).

On the wire we use a tow release on the car with a release cable run to the spotter. On the spectra cable we have a weak link at the reel and the spotter is in the back of the pickup ready to sever the weak link if required.
With our launch system we get a launch of every 10-15 minutes, the launch itself takes 3 minutes and the release altitude is 2,000-2,500' AGL.

Steve R.