Discrepancy between UI Speedometer and Dashcam SEI Data (100km/h vs 99km/h)

Drifting_Swift | 2026-03-13 13:31 | 9 views

I recently observed an interesting technical detail regarding how the car logs speed data versus how it displays it on the screen, and I'd love to hear some thoughts on the underlying mechanism. Here is the scenario (video attached): I was driving on a highway with a 100 km/h speed limit. I had Autopilot engaged and set exactly to the 100 km/h limit, and the UI speedometer consistently displayed 100 km/h. However, when reviewing the saved dashcam footage later via an iOS app, the embedded speed data on the video showed 99 km/h. To rule out any parsing anomalies from the 3rd-party app, I plugged the USB drive back into the car and watched it on the native Tesla dashcam viewer. It confirmed the 99 km/h reading. Discussion Point: There appears to be a consistent 1 km/h offset between the real-time speed displayed on the instrument cluster and the SEI (Supplemental Enhancement Information) data embedded into the dashcam video files. I'm curious about the engineering choice here. Is it safe to assume the UI speedometer acts as a traditional "optimistic speedometer" (displaying 100 instead of 99) for compliance and safety buffers, while the dashcam strictly records the absolute raw telemetry from the wheel speed sensors or GPS? I'd love to read your insights on how Tesla processes and splits this telemetry data between the UI and the video logger.

Comments (11)

collegedreads 2026-03-13 14:24

Sir this is a Wendy’s.

Ancient-Sandwich9400 2026-03-13 14:27

This has almost always been the case of optimistic speedometers. 1km/h or .62mph is nothing but a rounding error but could easily be accounted by differences in wheels or tire size differences.

asterlydian 2026-03-13 14:29

I'm far from knowledgeable in this but perhaps it's the way the UI versus the dashcam handles rounding. A value of 100 doesn't mean the car is travelling at exactly 100.00; actual speed probably fluctuates between 99.5-100.5 depending on traffic, gradient, wind etc. So if you're travelling at 99.8 for awhile maybe the UI rounds it up whereas the dashcam drops the decimals

DaSandman78 2026-03-13 14:58

Automated speed signs always show my speed 1-2 kmh less than what my car shows. I assume most (all?) cars err on the side of caution and round up instead of down, to avoid you getting speeding tickets?

Drifting_Swift 2026-03-13 15:16

That’s a really solid theory! The UI rounding up (e.g., 99.8 to 100) while the dashcam just truncates/drops the decimals to 99 would perfectly explain this constant 1km/h gap. Great thinking!

Drifting_Swift 2026-03-13 15:17

Yeah, that safety buffer seems to be the industry standard to avoid tickets. It's just fascinating that the car's own dashcam actually logs the 'unfiltered' lower number instead of what's on the screen!

Drifting_Swift 2026-03-13 15:18

😂

DaSandman78 2026-03-13 15:20

If it uses GPS to track the speed that would explain why it shows the REAL speed ?

Drifting_Swift 2026-03-13 15:20

True, tire wear and rounding definitely play a part in actual speed. But what really caught my eye is that both data points come from the car itself at the exact same time. It’s weird that the system feeds one number to the screen and a slightly lower one to the video logger.

Ancient-Sandwich9400 2026-03-13 15:32

Yeah but that’s like my odometer and the Trip B that’s never been reset showing a .1 to .2 mile difference. Marginal at best.

JtheNinja 2026-03-14 00:40

https://preview.redd.it/j7820mmupwog1.jpeg?width=900&format=pjpg&auto=webp&s=85844605ddeecc868f218a3d9a9885dd92ebc837

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