A new independent report from inside Tesla's Fremont factory shows production of the  Model 3 is becoming sustainable.

The report, from automotive analysts Eversource ISI Research in London and first carried in FT Alphaville, shows Model 3 production improving, and that the new General Assembly line 4 is making a significant contribution.

Tesla invited two analysts with Eversource ISI, George Galliers and Arndt Ellinghorst, into the factory in early August to gauge progress since the company's early-July production report, in which the company said it had produced 5,037 Model 3 sedans in an exhausting, extended, round-the-clock week. In its latest earnings report, Tesla said it had produced at least 5,000 Model 3s in additional weeks, but did not indicate that it had achieved that goal consistently.

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Galliers and Ellinghorst reported that the new GA4 assembly line constructed in a tent adjacent to the main factory now accounts for about 20 percent of Model 3s produced, all of them high-end all-wheel-drive cars.

They said, "ignoring the building’s fabric, it looked very much like general assembly at other auto plants which we have visited."

The analysts said the tent assembly process involves only a quarter to a third as many steps as a traditional auto assembly line, in part because it produces only certain versions of the Model 3. They added that the line looks set to become permanent and should be able to support much higher production.

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They noted that both Model 3 assembly lines still looked to be having teething problems with frequent shutdowns, especially on the highly automated main assembly line (GA3). They said the main problems seemed to be related to equipment calibration, which would not require the kinds of large capital expenditures to fix that would concern investors.

They said they saw production rates of a car every 65 seconds or less—Tesla's reported target—on the main body assembly line over the course of some shifts, but that stoppages interrupted this rate on other shifts.

They note that conveyor belts designed to move parts from one floor of the main assembly line to another have been replaced by 120 workers unboxing the parts and hauling them manually, because the weight of the parts tripped up the conveyor belts.

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One challenge they noted on the main assembly line was the density of robots and conveyor belts which left very little room inside the factory for workers or visitors to move around.

The team also noted that quality issues such as inconsistent panel gaps seem to be improving, and said the cars they saw being produced had gaps about on par with German luxury sedans. In the factory's body stamping process, "All the observations which we were looking for were fulfilled," they said.

They noted the factory should be able to produce 7,000 to 8,000 cars per week without any reconfiguration, once "teething problems" such as calibration stoppages are ironed out.

Tesla CEO Elon Musk has set a new goal of building 10,000 Teslas a week by 2020, which they said would require some additional investment. That won't all have to happen at the Fremont factory, though, as Tesla plans to be building cars in China by then as well.