Tsuboi set the third-fastest time of the opening day of the test on Saturday and overall in the #36 TOM'S Toyota he shares with Ritomo Miyata, lapping within two tenths of the pace set by ARTA Honda's Nirei Fukuzumi.
However, during the 10 minutes of GT500-only running in which most teams performed qualifying simulations on Saturday afternoon, the top GR Supra was the SARD machine in eighth place, with the #36 car only managing 12th.
Toyota proceeded to set the pace on Sunday with Racing Project Bandoh's Sena Sakaguchi leading Miyata in the morning session.
The Okayama test marked the first time this off-season that all manufacturers have been using SUPER GT's new-for-2023 carbon-neutral fuel (CNF), and Tsuboi remarked that Toyota appears closer to rivals Honda and Nissan that it had been during manufacturer testing.
However, he believes the evidence from Saturday suggests Toyota heads into the opening race on the back foot.
"I honestly think we can’t be optimistic," Tsuboi told Motorsport.com's Japanese edition. "When you look at how the other Supras struggled, and the long runs and attack laps in the afternoon [on Saturday], it can’t be said that the Supra is doing well.
"Honda and Nissan look stronger, and this trend has been evident from February testing. I wondered how CNF would affect things, but I think we are closer than we were then. At that time, the gap was bigger and things looked terrible.
"In that sense, I think we have reached a point where we are not too far off, but that said, I think the rankings are representative and we need to do more to close the gap before the opening round, otherwise we’ll be off the pace."
Toyota's apparent lack of speed on the opening day of the test raised eyebrows elsewhere, with Nissan's Kazuki Hiramine notably saying he thinks the GR Supra contingent could be hiding their true pace.
Honda NSX-GT project leader Masahiro Saiki also told the official SUPER GT website that he thinks Toyota is furthest advanced with adapting to the CNF, noting: "Toyota has the least dirty exhaust".
Asked if Toyota handling the CNF better than its rivals was the reason for the GR Supra's increased competitiveness compared to before, Tsuboi replied: "If you just look at the times, it seems that way, but I don’t know if that's the reason.
"But as long as we see the Supras all towards the bottom of the times, I think we need to have a sense of urgency."
Besides the #36 TOM'S and Bandoh cars, the remaining four Toyotas were all at the bottom of the combined timesheets at Okayama: Rookie Racing was 12th fastest, followed by the #37 TOM'S car, SARD and Cerumo.
The chief engineer of the #37 car, Kenta Odachi, pointed out that the Toyotas set their best times in the morning when conditions were relatively more suited to the GR Supra.
"In the final attack runs [on Saturday], the #36 was unable to set a good time," said Odachi. "In the morning the track surface was in good condition, but in the afternoon when the track was more rubbered in the balance got worse. That trend was more striking than I imagined.
"Generally, the Supras were towards the bottom during the GT500-only running in the afternoon. [Ukyo] Sasahara did an attack lap but said that there was still room to improve. But in terms of reaching the top time, it’s doubtful."
For his part, Toyota newcomer Sasahara said he was relaxed about the apparent lack of speed of the car he shares with Giuliano Alesi.
"We were trying to fix our balance issues, and if we fix those it will be much easier to be quick. So I am not too worried about [the Toyotas appearing to struggle]," Sasahara told Motorsport.com.
"Maybe Toyota are struggling a bit more [than the other manufacturers]. But from our side, it’s more the balance that’s the issue."
