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G-Tech Pro accelerometer


The G-tech Pro is a small device (smaller than a packet of cigarettes) that attaches to the windscreen using rubber cups and plugs into the cigarette lighter. With no other connections this little gadget will give you 0-60, 1/4 mile time and speed, 60-0 braking in feet, horsepower at the wheels and G-force measurements. Sounds too good to be true ?, read on....

The G-tech uses an accelerometer, a timer and a computer to calculate the above stats. The accelerometer measures the G-force and the computer samples this 400 times a second and calculates the distance covered and the speed that you are travelling at. There is one multi-function switch/button on the device that is used to select the desired operating mode. All tests have to start from a stationary position as it is the only reference point available to the unit.

Acceleration and braking.

Having put the device into acceleration mode you press the button while stationary, wait a second or two while the device zeros out (to compensate if the unit is not mounted completely hoizontal), then go for it. Once you reach 60mph your time is displayed. If you carry on accelerating the G-tech will go on to record your 1/4 mile time and speed. If you start to slow down it will wait until you are doing 60mph again and record how long it takes to brake to a standstill (in feet).

Power measurement

After selecting power measurement you have to hold in the button and tilt the unit until it displays the weight of the car in pounds. Reattach to the windscreen, press the button, wait for it to zero out, then accelerate all the way through 1st or 2nd gear. When you come to a halt the G-tech will display the horsepower at the wheels. This is NET of aerodynamic losses, transmission losses and inertial losses. More about this later.

G-force measurements

The unit displays the current G-force measurement while you are driving. It stores the maximum instantaneous G or the maximum sustained G (maximum value held for at least 3 seconds) according to your choice. The maximum can be viewed by pressing the multi-function button.

Does it work ?

In short the answer is yes. I have tried the device out in a number of cars apart from the Westfield and it is very consistent and fairly accurate. In general it reads slightly optimistic. This happens because a car pitches under acceleration and braking and the G-tech has no means of compensating for this. The degree of error is dependant on the suspension rate, vehicle weight and centre of gravity height. I determined that the error is approximately 2.5% for a very stiff car like my Westfield, 5% for a GTi type car and 7% for a family saloon.

Acceleration and braking

I used 2 methods to check the accuracy of the G-tech. The first was to run my Westfield at the drag strip with the G-tech installed and compare times and speeds between the track and the G-tech. When all 5 runs were averaged the G-tech read less than 0.1 seconds faster than the track and 4mph faster. If you bear in mind that a strip reads about 0.3 seconds faster than real life (due to the roll-out on the timing equipment) and about 1-2mph less than real life (at the strip the speed is measured over the final 66ft, not right at the finish line) then the G-tech was 2.5% fast.

The second method was to do some 0-60s and standing quarters on the road and note the reading from the car's speedometer when the G-tech flashed up the 0-60 and 1/4 times. By calibrating the car speedo against the 100m posts on the British motorways I could see what percentage error I was getting.

I had no means of checking the braking distances but theory says that the degree of error should be about the same as for acceleration and the tests I did certainly came up with numbers that looked right.


This was the main reason I wanted the G-tech. You don't need to do a dumped clutch start or anything, just drive off normally. Horsepower is speed*acceleration*mass. The G-tech handles the first 2 and you have to enter the third. This was easy for me as my car has been corner-weighted and I know the exact weight. With other cars I looked up the kerb weight in the owners manual and added some more for driver and luggage. The G-tech delivers fairly consistent results, usually within about +-4% each time. Consistency is not as good as for acceleration runs, probably because for this function the unit uses instantaneous readings which are more likely to be subject to spurious readings e.g. hitting bumps in the road &c. The number produced is useful for tuning purposes, but is not comparable to a chassis dyno because it is NET of aero, transmission and inertial losses. It is possible to compensate for these losses. Aero losses are easy to calculate (approx 5HP for a saloon at 40mph and 10HP for the Westfield at 50mph). Transmission losses are about 17% for most cars and 20% for the beefy 5 speed used in the SEiGHT. Inertial losses are the hard part. You can calculate these if you know the weight of the rotating components (mainly clutch/flywheel) and the rate of acceleration in your test gear. I did the maths for a few cars and came up with 17-18% for an ordinary car in 1st gear and slightly more for the Westfield in 2nd gear. On top of all this you need to take the inherent optimism of the G-tech (see above) into account. Because the G-tech multiplies speed by G-force this error becomes squared i.e. 5% for the Westie, 10% for GTis and 14% for family saloons. Putting all this together I came up with the following formulae for 'correcting' the HP reading into something that could be compared to a chassis dyno:

SEiGHT (HP/1.05 + 10) * 1.22 = HP * 1.16 + 12
GTi (HP/1.1 + 5) * 1.2 = HP * 1.09 + 6
Saloon (HP/1.14 + 5) * 1.2 = HP * 1.05 + 6

These formulae give wheel horsepower. To get flywheel horsepower you have to multiply the SEiGHT reading by 1.25 and the others by 1.2.

In order to test this I used a Peugeot 1.9 205, a Toyota Celica GT, a Vauxhall Astra 1.6 16V, a Ford Mondeo 2.0 16V, a Ford Escort Si and a Jaguar XJ6 4.0. The 205 had been measured on a rolling road at 95HP at the wheels which was exactly what it tested at with the G-Tech after applying the above formula. The Astra tested at 101HP at the flywheel compared to Vauxhall's claim of 99HP. The Celica tested at 169HP at the flywheel compared to Toyota's claim of 171HP. The Mondeo tested at 128HP at the flywheel compared to Ford's claim of 130HP. The Escort tested at 92HP compared to Ford's claim of 90HP. Just to prove that I didn't fiddle the figures the Jaguar came out at 213HP at the flywheel compared to the manufacturer's claim of 235HP, however seeing as it was an automatic and had a long 1st gear this might explain the discrepancy.

My Westfield produced 175HP at the wheels on a rolling road and the average of 4 G-Tech runs was 174HP with a minimum of 169HP and a maximum of 181HP. Unfortunately this sort of variation on power readings means that the G-Tech can't really be used to tune the engine but it does seem to give a fairly accurate indication of how much power you have. If you have ever seen the results of a rolling road shoot-out then you'll know that most people vastly over-estimate the amount of power that their engine is producing so this is a useful measurement.

G-force measurements

I had no way to test these but I am pretty sceptical about the unit's ability to accurately measure cornering forces. There must be some amount of noise during the readings and there is no explanation of how this is filtered out. The problem is that you are looking at a very small range of values between a humdrum car and a car that corners very well. Body roll also affects the result.


As you can see the G-tech seems to give reasonably accurate and repeatable results. During all of this testing I NEVER had an obviously spurious result like a 0-60 that was 2 seconds slower or faster than it ought to be. As long as you can find a smooth, level road to do your testing on and you mount the unit properly I think this device is a good buy. It has to be pointed out that it is difficult to find a quiet stretch of road that is level for a whole 1/4 of a mile, especially if you want to be able to turn around quickly and do back to back runs over the same stretch of road. You also need to use some ingenuity to use the G-tech in a Westfield. You have no cigarette lighter and the supplied mounting bracket is no good for a windscreen that is almost vertical. I bought a cigarette lighter socket and connected it to some crocodile clips that can be connected straight to the battery. The G-tech was taped to the tranmission tunnel with a piece of card under the leading edge to get it exactly level (the trans tunnel is almost level anyway).

The G-Tech was tested by Cars and Car Conversions in the December 1996 issue. They compared it to a Datron and came up with similar conclusions to me, although they only tested acceleration. T3 magazine tested it in the July 1997 magazine and gave it a favourable review. I understand that Max Power tested it in February 1997 (compared to Datron timing gear) but I haven't seen that.

There are other similar devices on the market. The most notable of these is the Vericom VC2000 which works on exactly the same principles. The Vericom boasts a lot more features and costs about 4 times the price of the G-Tech. Fast Car tested the Vericom in June 1988 (when they were actually a good magazine...) by comparing it to a Datron, and the increased price doesn't seem to buy you any better accuracy, although some American car magazines disagree.

© Ian Crocker
Last updated on Sep 12th 1997