The Tenths Scale

"Tenths" refer to how fast a car is driven around a race track relative to the car's potential.

Figuring out the potential itself is tricky. At amateur level, there are three main approaches:

  1. Given two drivers in similar or identical cars, or driving the same car, we can look at overall lap time to determine which driver is faster, and therefore is using their car more. With a data acquisition system we can look at multiple data points in a lap, such as speed while exiting each corner, or minimum speed in each corner, for a more detailed picture.
  2. Experienced drivers can use traction sensing to estimate whether the car is using all of the available grip as it brakes, corners and accelerates. Frequently the experienced driver will be an instructor or a coach riding in the passenger seat and evaluating the person driving the car at the time.
  3. Self-evaluation techniques can be used to spot situations matching common patterns where time is being left on the table.


Potential roughly includes:

  • Lack of coasting. The car should be accelerating, cornering or braking at all times. This is easy enough to achieve on the straights, but much more difficult to pull off in long sweepers or decreasing radius corners for instance.
  • Maximum acceleration. For example, whenever the car is braking, it should generally achieve the maximum braking deceleration as quickly as possible and hold the maximum deceleration for as long as necessary to achieve the desired speed reduction.
  • Maximum combined acceleration in transitions. The easiest example is transitioning from braking to cornering - assuming the car can brake at 1g and corner at 1g, the combined forward and lateral forces should add up to 1g from the braking zone until track out. As the driver is releasing the brakes, for example braking at 0.75g, the car should have 0.25g of lateral acceleration turning in.
  • Using all of the track: generally speaking, the entire pavement width should be used for the best lap times. Inside and outside curbs offer additional opportunities for driving faster, but their use is not as straightforward. At the absolute limit, the car should use apex, exit and entrance curbs as needed to achieve the best lap time.
  • Slip angle: contemporary radial tires develop the most grip at small, but non-zero, slip angles, about 5 degrees. This means that when a car is driven at the limit, it is sliding on the tires slightly.

Potential is also affected by driver factors. The driver factors are indirect in that they affect what the car does, which in turn shows in lap time.

  • Correctness: driving at the limit requires that the driver not make any major mistakes. As we are all human, completely eliminating mistakes is not generally possible; the goal is for mistakes to be as minor as possible.
  • Precision: when driving at the limit, the driver positions the car within inches, if not less, of the intended turn in, apex and track out points.
  • Consistency: as the driver approaches the limit, the difference between what they are asking the car to do and what the car is physically capable of doing becomes progressively smaller. It is important to be consistent to avoid going over the limit.
  • Aggression: to drive at the limit, you need to actively put in the effort to do so, which in turn requires desiring to be at the limit.

Last but not least, potential is always dependent on the person measuring it. Because potential is relative to the data that the measuring person possesses, or is a function on said person's traction sensing abilities and experience with the type of car being evaluated, one person's 9/10ths might be another person's 8/10ths.

The subjective component of potential measurement makes it even more difficult to classify rain driving into "tenths", as typically drivers have less experience driving in the rain than in the dry, and the amount of grip actually available in rain varies enormously based on precise conditions.

Overdriving By Inexperienced Drivers

A particular case needing to be addressed is that of an inexperienced driver with excessive ambition and aggression. Such drivers may be sliding the car due to missing braking points and subsequently performing emergency-style braking in the braking zones, turning in with excessive speed which results in the front end of the car plowing, abrupt transitions from braking to throttle and vice versa unsettling the car and so on. Such driving does not fit into any of the categories described below which assume that the driver at the minimum either intends or is trying to drive the car around the track correctly.


At levels below 7/10 the driver does not have lap time as their objective. They may be learning the track or lack confidence necessary for driving faster.

The distinguishing feature of 6/10 and slower levels is lack of consistent execution of the school line. This could be due to the driver not knowing what the school line is, lacking reference points to drive it consistently, not looking at reference points early enough, or a number of other reasons.

Novices with no high performance driving experience ordinarily start at this level.


This is a "ballpark level", meaning the driver is within the ballpark of ideal line, braking power and throttle applications but visibly not close to the limit. At this level:

  • The line is within 1-2 feet of the ideal line through the entire course;
  • The car decelerates at 50-75% of its maximum forward deceleration in the braking zones;
  • Power is applied before track out points but often past apexes;
  • Around 75% of the track width is used or needed. ("Needed" means the car may be technically driving through all of the track width, but at the speed it is traveling it can fit into the corners while using 75% of the track width.)

The driver may be committing small mistakes in more than one corner per lap.

Unlike 6/10 and lower levels, the driver is fairly consistent at 7/10. Adjustments to line and speed tend to converge the car on driving the right line at the right speed rather than simply oscillate.

At 7/10 the driver is not worried about beating lap records but recognizes the importance of correct driving technique and is working on improving their skills.

Novice drivers who are steadily improving throughout the event can drive at this level by the end of the event. Intermediate drivers start their events roughly at this level.


This is a level at which the driver drives the lap without mistakes, consistently, briskly, but without sliding the car.

The distinguishing feature of 8/10 level is lack of mistakes, and the higher degree of consistency that follows. A driver that is able to hit their reference points within half a foot lap after lap is likely running at 8/10 or better.

At 8/10 there is little to no coasting anywhere on the track. However, the car is not accelerating at the maximum longitudinal and lateral g's.

The driver should be using most of the track width in all corners. This may include apex curbs where appropriate but usually not exit curbs. In high risk corners the driver can be leaving additional margin for error.

The car has zero slip angle in all corners.

Intermediate drivers who start an event at 7/10 and have instructors frequently finish the event at 8/10. Intermediate solo drivers may run an entire event at 8/10.


This is a level at which the driver starts to apply effort to "go fast", but still does not slide the car in corners.

At 8.5/10 the line is optimized for the car being driven. Power is applied from just before apexes, and with confidence. Most of the track width is used in all corners. Apex curbs should be used where appropriate. The driver is hard on brakes at least in those braking zones which are straight.

Unlike higher levels, the car still has zero slip angle in all or most corners. There is no or minimal trailbraking being performed.

Exit curbs are generally still not used.

Upper intermediate drivers can drive at 8.5/10 once they warm up. Advanced drivers with a lot of experience who for whatever reason do not want to slide their car may drive an entire event at 8.5/10.


9/10 is a level at which the driver begins to intentionally slide the car to achieve lower lap times.

By "sliding" I mean having around 5 degrees of slip angle, which is the amount at which modern tires develop maximum friction.

Practically speaking, "sliding" means the driver can:

  • Enter a corner a bit faster than at 8.5/10, and bleed off excessive speed by understeering ever so slightly between turn in point and apex rather than disposing of that speed in the braking zone and correspondingly driving slower through the rest of the turn.
  • Trailbrake a car until the rear tires lose grip.
  • Apply throttle a bit earlier than at 8.5/10, and understeer slightly through corner exit, if the car's power level allows this. A driver may similarly oversteer the car slightly but this is much more rare as more than tiny amounts of power oversteer tend to slow the car down.
  • Combine entry and exit slides and slide the car through an entire corner.

At 9/10, the car has nonzero slip angle through important corners.

The driver may start to use exit curbs and should be using all apex curbs that reduce lap time.

Advanced drivers who are going after low lap times are frequently driving at 9/10 or above, and may spend several laps at this level in each session while they are "in the zone".


At 9.5/10, the driver tries to slide the car everywhere on the track. This means not only cornering aggressively but also doing threshold braking in non-ABS cars and allowing the driving wheels to slip under power in corner exits.

The car has nonzero slip angle through most corners. The car has nonzero slip angle into, through and out of important corners. The car has close to optimal slip angle through important corners.

"Optimal slip angle" means that if the front wheels are sliding, and the car is understeering, that the line through the corner is not compromised and exit speed is maintained despite the sliding. If the rear tires are sliding, throttle input is maintained without the car going sideways necessitating a lift.

The big difference between 9/10 and 9.5/10 is aggression level and the driver concentration. Being close to the limit at 9/10 may happen in a single corner, whereas at 9.5/10 it may happen in multiple corners for multiple laps.

At 9.5/10, all curbs that result in lower lap times should be used, including entry and exit curbs in addition to apex curbs.

Advanced drivers going aggressively after low lap times may be found driving at 9.5/10.

9.5/10 driving requires significant concentration at advanced driver level and is mentally tiring. It takes practice driving 9.5/10 to be able to do it for extended periods of time, i.e., actually drive at this level for several consecutive laps.


10/10 level, in a typical HPDE environment, occurs when a driver who is mentally prepared to drive at 9.5/10 drives a car which is perfectly set up for the track being driven and the driver's driving style. Where at 9.5/10 the driver may be fighting unwanted understeer or oversteer as the car approaches the limit of grip, at 10/10 the car is perfectly configured and does everything the driver wants it to do.

10/10 assumes that everything in the car that is adjustable is adjusted to just the right settings.

Because HPDE drivers do not normally have pit crews responsible for keeping their cars in perfect setup for each session, 10/10 driving in HPDEs is very rare.

Advanced drivers going aggressively after lap records may occasionally be found driving at 10/10, and this is when lap records get broken.


This is a "recoverable overdriving" level. At 10.5/10, the car exceeds available grip on the track but not to such an extent as to cause damage.

Examples of recoverable overdriving are:

  • Unintentional drifting.
  • Low speed spin with the car remaining on paved track surface.
  • Two wheels off track.

The first two of these are a result of oversteer, while the last one is a result of understeer.

To know what 10/10 is, going above 10/10 is normally required. In practical terms this means pushing the car ever so slightly over the traction limit until it cannot execute the driver's intentions any longer. Because there is a finite and limited amount of track time available at most events, drivers with enough skill and aggression will aim for 10/10 and eventually hit 10.5/10. When they hit 10.5/10 they will back off a little to be in the ballpark of 10/10.

Recoverable overdriving events are typically acceptable to most competitive drivers. Cars with splitters and such obviously have a lower tolerance for offs, which is why many drivers stick to cheaper cars which incidentally have fewer fragile parts hanging low off the ground.


A more serious overdriving situation involving going way off track. Depending on track design and conditions 11/10 may result in damage to the car. A high speed spin followed by the car going off track would be an example of an 11/10 situation.

At advanced skill level, 11/10 typically requires something like heavy rain, mechanical failure or a serious lapse in concentration.

At lower skill levels, drivers who push too hard without being aware of the limits, or correct technique, may find themselves in 11/10 territory. Examples are dropping loaded tires off pavement with significant steering angle. I have also seen cars spinning, going off track and then spinning off track still under power - the driver was clearly unaware of what they were doing.

Equipment Wear

The faster and more aggressively you drive, assuming correct technique, the more wear you put on the car. This applies at each level - driving at 7/10 wears the car more than driving at 6/10. With that, tire life generally gets significantly shorter at around 9/10. Brakes start to get a good workout at 8.5/10. 9.5/10 is the level at which wheel bearings, bodywork and suspension parts start to need more attention, primarily due to using curbs and going off track.

Driving faster with correct technique can be less stressful on equipment than driving slower with incorrect technique.

Concentration And Sustainability

The more experience a driver has, the higher level of concentration they are able to maintain for extended periods of time. Generally speaking, high intermediate and up drivers can run a 20-30 minute session at 8/10 level. Drivers in advanced run groups can typically run at 8.5/10 for an entire 30 minute session. Drivers with competitive driving experience or who specifically practice endurance can sustain 9/10 and up for anywhere between 30 minutes to hours at a time.

Higher levels demand more concentration from the driver and are more tiring. Intermediate drivers typically do not have enough mental resolution in their reference points and driver inputs to drive at 9/10 or above. Many intermediate drivers cannot run a full day at 8.5/10 - they would get mentally exhausted. Driving at 9/10 and above requires practice at 9/10 or above, which in turn requires desiring to drive at these levels. There are instructors, I suspect quite a few in fact, who do not drive at 9/10, much less 9.5/10, on a regular basis.

Successful racers are likely maintaining 9/10 or higher level for the duration of race sessions, and hit 9.5/10 in most practice and qualifying sessions.

Tagged: intermediate, advanced