We went to the Turkmenistan Embassy in Tashkent on a holiday (end of October 2013), but fortunately the consul just walked out and told us to come at 13:00 the same day with two color copies of our passports (including the Iran visa) (there is a photo shop a few minutes away in the middle of the apartement complex behind the embassy). We waited there at one o'clock for a while, but nobody opened and the Uzbek police wouldn't do anything. So we went to the other (car) entry around the embassy, where a very helpful employee of the embassy told us he would let us enter at the original gate. Once inside the consul himself and the employee helped us to fill out the russian forms (although they speak little english themselves). We handed over two passport photos and the photocopies, and had to fix the entry and exit points, and entry/exit date (five days). No problem concerning the car.
He asked us wether we want to pick up the visa at the embassy in Tashkent or directly at the border. The border-pick-up is only possible at certain border crossings (we choose Dashoguz, some friends picked Farap – Alat, the consul said Konye-Urgench is not possible) - you get a code (two letters and three numbers). Exit point for us was given through the Iranian visa application as Sarhaks and the consul didn't challenge that. We didn't have do pay anything - the visa is supposed to be paid for at the border.
At the border (mid-November 2013) We were one day early (Uzbek visa 30 days expired, but valid period for two more days). From the Uzbek border gate you cross a bridge - there the first Turkmen guard allowed us into a 1km border corridor - we had also play taxi - usually everybody has to walk there - even German diplomats. At the Turkmen border post, the employee asked us friendly to sit, only to tell us that our "Invitation" only starts from the next day - but everything else should be o.k. We tried, but were not allowed to camp in the no-mans-land, so back the Uzbek border invalidated the exit stamp and we were back for one more night in Uzbekistan.
The next day at the Turkmen border we had to wait for app. one hour for the visa to be processed. The route was again extensively discussed - a side trip to Konye-Urgench is forbidden, there is a direct road via the Dashoguz airport bypassing this city. We had to specify for every night where we intended to stay and argue, so we could use the full 5 days. We had to pay the fee for the visa (55$) + 10$ entry fee + 2$ transfer fee at the "bank" next to the visa counter. For the car: filling out forms, define the route, pay 125$ (desinfection, entry & transit, compensation of fuel cost (depends on your route), insurance, processing) +2$ transfer fee - take care that no mistakes are made ... And then we got the best inventory of our car so far by Turkmen customs
In the customs form we were not "allowed" to put money, laptops etc. (as you should do in Uzbekistan) - but everything was ok.
Diesel is 0.58 Manat per liter, there is a gas station in Dashoguz in the direction of / near the airport. We were first not allowed to fill canisters, but after a while the guy put one canister into our car to hide and filled it there. There is another gas station near the crossing with the road from Konye Urgench, one somewhere between Derweeze and Ashgabat (more south, didn't note the coordinates) and several around Ashgabat.
Exit at Sarhaks: Bypassing a long queue of trucks. Exit form filled out for us by a border guard. Only marginal check by the custom guys (no cameras & sd-cards checked - although our cameras were inspected in Ashgabat by policeman).