Gear max speed calculations

[Craig3Q]

I'm setting up the parameters for my car (max torque, power, gear ratios, etc) and it looks like the calculated max speed in each gear is different from what I expected. It seems too low, especially in the lower gears.

My car is a 2009 Corvette Z06. Here is one page that I am using to check the calculated results:

http://forums.corvetteforum.com/c6-z06- ... -gear.html

Here are the tech specs I'm using in the app:

http://www.z06zone.com/c6z06-techspecs.php

The results shown in the app:
1. 2.66 (55 mph)
2. 1.78 (82 mph)
3. 1.30 (113 mph)
4. 1.00 (146 mph)
5. 0.74 (198 mph)
6...

It turns out that the rated max speed of the car is 198 mph, but that takes into account drag, weight, and other factors. How are the max speed numbers in Lap Timer Pro calculated?

(I have an OBD PLX KiWi WiFi on order, but it might not arrive in time for the next track day.)

[Harry]

Hi Craig,

FIrst of all, here is the formula used for speed / gear / rpm calculation:

speed = circumference of tire [axis driven] * drive ratio * gear ratio * engine rpm

I'm not sure if the words used are the right ones (translation gets more difficult for me when working with this technical words), but it should give you the principle.

As stated in the linked thread, max speeds depend not only on the transmission, but on aerodynamics etc. So everything I calculate is a model taking some assumptions into account. In LapTimer 13.4 the assumption is, that max speed is reached at the rpm the engine is developing max power. This is a good approximation for cars with gear ratios optimized for sports. This is a bad approximation for cars developing max power at low rpms and keep nearly this power at higher rpms (probably your category) - numbers will be too low. It is a bad approximation for cars with fuel consumption optimized high gears (as your Z06 is) too - numbers in high gears will be too high.

So although the 13.4 assumptions are pretty good in average, they produce values you will not see in any data sheet.

This is the reason I've added another parameter in v14.0 (not yet available): max engine rpm. Using this value, you receive the pure transmission based max speeds. These are almost always theoretical (especially in high gears) but at least the max speeds are exactly those achieved when running into your engine speed limiter on lower gears.

- Harry

P.S. I have a compete set of formulas implemented to derive max speeds from engine power using all the parameters one needs here (e. g. cw value / car's front surface, transmission losses etc), but did not let this go life as it addresses a very small user group and will bring up hundreds of question marks). It is pretty good and calculates my car with +- 2km/h accuracy.

[Craig3Q]

Thanks for the explanation, Harry. The interesting thing about the values calculated by LapTimer for my car is that the factory specified top speed is actually 198 mph, attained in 5th gear. The first few gears calculated are slightly low, as you predicted. At max rpm, 3rd will be about 122 or 125 mph.

I guess this deviation between calculated and actual performance will be eliminated when I'm using OBD II (assuming that my car reports sufficient data on the interface).

[Harry]

Just try setting the rpm for max power to the max rpm your car allows before running into the rev limiter. This will calculate what v14.0 will calculate.

- Harry

[KrisS]

I have a similar question. I have focus RS MK2 and its 6th gear has ratio of 0,7, Final Drive ratio is 4 and it uses 235/35/R19 wheels. The calculated max speed for 6th gear is 216 km/h and real max speed is 263 (as provoided at ford). The max RPM is 7050 but it works for few seconds only the it falls to 6500 RPM. Should I change the drive ratio to 3.3 or similar to have max speeds closer to reality and have right shifting info concerning the gears ?

regards

K

[Harry]

Hi,

This should be a standard one. I believe one of your values is wrong, please check:



Please check your technical data. It says that the engine develops 225 PS at 6000 rpm. This means that the power decreases at higher rpms due to reduced torque. The 7050 is a max rpm the engine can reach, however, it is better to shift earlier.

- Harry

[KrisS]

Harry

Thx for fast answer. 225 PS and 320 Nm torque is Focus ST mk2 not RS MK2.

For my car standard values are 305 PS and 440 Nm.

As mine car is slightly modified the real (rolling road measured values are):

Max torque 457 Nm @ 3131
Max Power 308 PS @ 4974
Max RPM and gear ratios are provided as on your screenshots.

I believe the issue is that for calculation RPM for Max power is taken (the calculated max speed changes as I change RPM for Max Power).

So should the calculation formula be :

- changed and based on max RPM ?
- should I change the RPM for Max Power to have more realistic max speed reading for gears (for example 6500 RPM) ?
- should I leave the real RPM Max Power values and live with it ?

[Harry]

Speeds in the Gear Ratio rows are the speeds achieved at max power. You can read out all variations from the diagram above. Calculating speed at max rpm is possible but is again a calculation that will not fit every situation (e.g. in 6th gear). There are all kinds of max power / max torque / ratio / gear combinations in the market, you always need to read the story behind the numbers. With a OBD connected, max power rpm is were green changes to red. Red stops at max rpm. So while driving fast, you will be sometimes below this green / red area and sometimes above (up to the end of red).

- Harry

[KrisS]

OK thx

[Atre777]

Hello Harry,
so here's my first post.
Thank you for creating this outstanding app!
Unfortunately my car (VW Golf 6 GTI) has two different axle ratios.
Gear 1-4 -> AR: 3,944
Gear 5-6 -> AR: 3,087

What should I do to get the correct graph and calculated data results?