This may spark some debate, but the short answer is: yes — and here’s why.

While the AMG GTR is quicker in acceleration due to its lighter weight and slightly higher power output, both the GTR and GTC share the same engine, same larger turbos, same transaxle, same gear ratios, and same final drive. The key mechanical difference that influences straight-line top speed is actually the rear tire size.

• AMG GTC uses 305/30ZR20 tires, which rotate 764 times per mile
• AMG GTR uses 325/30ZR20 tires, rotating 750 times per mile

(Reference: Michelin Pilot Super Sport and Pilot Sport Cup 2 specs)

That’s a +1.87% difference in tire rotation on the GTC. This means, purely from a gearing standpoint, if you fit the GTC with the same 325-size tires as the GTR, the theoretical top speed increases from the factory-rated 196 mph to around 199.7 mph (196 × 1.0187).

So why is the GTR’s top speed officially rated at 198 mph instead of 199+? The answer lies in aerodynamics. The GTR has significantly more downforce-generating components — including a large fixed rear wing, an active front splitter system, and a deeper front splitter — all of which increase drag at high speed and act as aerodynamic “brakes.”

To test this theory, I conducted real-world top-speed runs:

• In a friend's stock GTR with the rear wing adjusted to 0 degrees, I recorded a top speed of 197 mph over three runs (I only had the car for couple of hours)
• In my GTC, although I didn’t have 325-size tires, I ran tests with even larger rear tires:
  • Michelin Pilot Super Sport 335/30ZR20 (744 revs/mile, +2.69%)
  • Michelin Pilot Sport Cup 2 345/30ZR20 (738 revs/mile, +3.52%)

With the GTR wing set to 0 degrees:

• 335 tires: 199 mph
• 345 tires: 200 mph

Then with the OEM GTC retractable wing:

• 335 tires: 200 mph
• 345 tires: peaked at 201 mph (only once)

At these speeds, the car appears to be nearing its aerodynamic drag limit.

Conclusion:

If you normalize tire size and reduce aero drag, the AMG GTC can exceed the GTR’s top speed in a straight line, thanks primarily to lower aerodynamic resistance.

Of course, to be clear — the GTR remains the superior track car, with better high-speed stability, grip, and cornering performance. This comparison is strictly about top speed in a straight line.

Note: At the time of testing, the GTR had approximately 15,000 miles and the GTC about 24,000 miles. The tests were conducted on different days, so minor variations in temperature and wind may have had some effect on the results. In both cases (GTR and GTC), it was clear that the cars experienced increased aerodynamic drag when equipped with the GTR-style rear wing. All speeds were measured with 2 GPS speedometers, one of which was the Dragy and Dragy-lap running at 10Hz.

 

Interesting data. On the GTC, was the retractable wing deployed or retracted for the runs? It would be interesting to see the GTR's speed with the wing set at it's ~10 degree "high downforce" position.

Did you happen to capture the speedo reading, as well? The difference is often interesting. I know from the track data I capture that the ODB-captured wheel speed data agrees closely with the 25 Hz GPS speed data but the sync'd speedo video always shows an elevated value. Presumably the head unit does some scale-up on the speed to prevent any claims of "I got a ticket because the speedo reading was too low." In my GTR Pro, I see a consistent 5 KPH increase over real speed at about 200 KPH (about 2.5%) and, according to various road-side "your speed is this" radar displays, 53 KPH on my speedo is a real 50 KPH (about 5-6%) so it appears that the "fudge factor" is non-linear (though we have to keep in mind that any digital display is only +/- 1 accurate on the least-significant digit).

Mostly I'm just jealous (but glad for you) that you have somewhere you can run at top speed. #;-))

 

At those speeds, the GTC’s retractable rear wing was fully deployed.

I’m confident that raising the GTR’s rear wing to 10 degrees would negatively impact its top speed. While the GTR’s fixed rear wing and extended front splitter are excellent for high-speed cornering—by increasing both front and rear downforce and planting the car more firmly onto the surface—they also add significant aerodynamic drag. This added "weight" on a straight line doesn’t help; in fact, it slows the car down.

I did capture speedometer readings during several runs. When using 305-width tires (the GTC’s OEM size), the speedometer matched both GPS devices perfectly, with no discrepancies.

However, since the GTC’s speedometer is calibrated for 305/30ZR20 tires, switching to 335 or 345 series tires—which rotate fewer times per mile—will cause the speedometer to underreport the actual speed. For example, in the photos below, the speedometer shows 192 mph, while the GPS accurately reads 198 mph, running on 335/30ZR20 tires.

If you calculate speed based on approximately 6,550 RPM in 7th gear (as seen in the picture), comparing 305 tires (764 revs/mile) to 335s (744 revs/mile), the numbers align very closely.

So no—I don’t believe AMG is secretly helping us make our top speed case. 😊


Dragy-Lap app showing the actual GPS speed at the same minute 6:55.15 of the run.




Inside cam showing the car's speedometer and RPM at minute 6:55.0 of the run.

 

Interesting. Didn't know whether the GTC had the ability to turn the rear wing deployment off, or not.

Yes, I can confirm that full angle on the GTR's wing slows it down. I did a 1-session test and saw a difference on the long straight at Area 27 raceway.

That's the first time I've heard of a Mercedes speedo reading "dead on" (with the proper tire sizes). Every Mercedes I've ever owned has always shown a "protective speed skew" according to radar displays (a couple of them being of the "highly calibrated" kind <sigh> and GPS speed comparisons when on track. IIRC, when you do the math, the diameter/rotational difference due to full vs worn tread depth can yield a couple of percent difference. When you consider the diameter difference between different tire manufacturer's tires of the same size spec, you can see the problem for the car companies.

Still jealous about the high-speed run opportunities, though (OK, there are roads where I _could_ do it, but getting caught would have extreme consequences). #;-))

 

Just to clarify how the GTC's retractable rear wing works: you can manually raise or lower it when the car is stationary or at low speeds. However, once you exceed 40–50 mph, the wing automatically deploys and cannot be manually retracted until you're back below that threshold.

You're absolutely right about the general accuracy of Mercedes-Benz speedometers, but in the case of AMG models—particularly the AMG GT—I’ve found the speedometer to be surprisingly precise.

You also made a great point about how tire wear affects speed accuracy. Since I’m very particular (for safety reasons) about tire maintenance and typically replace them when tread depth reaches 4/32”, it helps keep my speed readings consistent and accurate, hence during the above tests.

I am sure you know all this, but I'll still say it for those here who do not.
Key Considerations for a Safe and Accurate Top Speed Run:
- A newly resurfaced, smooth, and even highway.
- At least 2 miles of straight roadway with minimal steering input (no more than ±20°) and no intersections or entry points.
- A connection with local law enforcement who has jurisdiction over the segment.
- Knowledge of early morning hours when police patrols are minimal (again—knowing someone helps).
- At least one (ideally two) spotters in cars ahead of you to report traffic or unexpected obstacles.
- Wait for a time that there are not other cars between you and the last spotter (don't trust other drivers as they look at you approaching via their rear view mirror and start drifting into your lane.),

Of course, tires must be in excellent condition, properly inflated and warmed up, and brakes should be fully maintained and tested.

At 200 mph, you can’t make sharp directional changes, and deceleration takes a significant amount of distance. If the road is known for frequent wildlife collisions, it’s the wrong stretch to be testing on.

 

Sidebar - What accounts for the higher HP/TRQ of the GTR? Is it simply the tune? Thanks!

 

Correct. When it comes to engine horsepower and torque, the only real differences between the models lie in the tuning maps—specifically in ignition timing, fuel delivery, and boost tables.

Based on what I’ve observed on this forum over the past few years, the GTC, GTR, and GTR Pro can reliably reach around 670 horsepower and 600 lb-ft of torque at the crank while retaining the stock engine, OEM turbos, and transaxle. This appears to be a safe performance ceiling for long-term reliability.

Of course, there are tunes that push well beyond these numbers, but those typically exceed the "safe zone" for the factory components.

 

Thank you, very much! What you can get out of these motors with a safe tune alone was also what I wanted to follow up with & you've covered it well!!