Top Speed in cars.

[Peter3hg]

This might be a tupid question but how do manufacturers calculate their top speeds?
I always thought it was arefully calculated but I was out in my parents Polo and reached an indictaed 110mph. As this is always pessimistic I looked at the TomTom and it read 119mph. I thought this sounded a bit high so I looked in the handbook and it claims the top speed to be 109mph. This to me makes it look like VW state their top speed as what the speedo says not the actual speed. I was on a very slight uphill with no tail wind when I got the speed so that wont be why it was higher.

[Blind_Io]

Don't trust a TomTom. I have a Garmin and it once said my top speed was 123 mph. Although I have had my car up to 100 mph, I certainly didn't do it in Bay Area traffic. Consumer grade GPS navigation units use a kind of logic to estimate your location on the map based on the direction of travel and the roads on the ground. Sometimes they make mistakes. Every time I leave my condo I end up driving parallel to the road in my parking lot - the GPS thinks I'm on the road. When I turn left to actually get to the road it thinks I'm turning into the neighborhood across the street, when I turn right again on to the road that parallels the parking lot it places me two blocks away.

Go with the spedo, no the GPS on this one.

[SL65]

Not sure how manufacturers calculate it but here's a top speed calculator I got off this site I download WRC off:
http://www.enormouslysideways.net/calc.html

Not sure how good/bad it is. Manufacturers probably calculate if using something similar to that but more advanced.

[andyhui01]

Originally Posted by Blind_Io

Don't trust a TomTom. I have a Garmin and it once said my top speed was 123 mph. Although I have had my car up to 100 mph, I certainly didn't do it in Bay Area traffic. Consumer grade GPS navigation units use a kind of logic to estimate your location on the map based on the direction of travel and the roads on the ground. Sometimes they make mistakes. Every time I leave my condo I end up driving parallel to the road in my parking lot - the GPS thinks I'm on the road. When I turn left to actually get to the road it thinks I'm turning into the neighborhood across the street, when I turn right again on to the road that parallels the parking lot it places me two blocks away.

Go with the spedo, no the GPS on this one.

I think it depends on the quality and the number of satellites the GPS uses to get your coordinates, I know in cases of Yatchs, a GPS speed is way more accurate than the one displayed in the dash... if you're GPS is a good enough one, it should display speed very accurately.

and Blind_Io, I experienced something opposite, the GPS my dad uses in Hong Kong can even tell which lane we're driving on in the car, we found out because there are roads in Hong Kong where different lanes lead to different locations and are all double lined... the GPS just said, please switch 1 lane to the right.

[Peter3hg]

Originally Posted by SL65AMG~V12~612BHP!!!!!!!

Not sure how manufacturers calculate it but here's a top speed calculator I got off this site I download WRC off:
http://www.enormouslysideways.net/calc.html

Not sure how good/bad it is. Manufacturers probably calculate if using something similar to that but more advanced.

Cant find the drive ratios anywhere, not even in the manual, so that isnt any use.

[fbc]

Originally Posted by SL65AMG~V12~612BHP!!!!!!!

Not sure how manufacturers calculate it but here's a top speed calculator I got off this site I download WRC off:
http://www.enormouslysideways.net/calc.html

Not sure how good/bad it is. Manufacturers probably calculate if using something similar to that but more advanced.

As the site states it's purely theoretical and ignores engine power, aero drag, and rolling resistance, so it's not really all that useful - all it's doing is calculating the road speed at a given rpm based on overall gearing, which is only relevant if your car can reach that rpm in top gear.

[klorphil]

Here's what you need to know to calculate the theoritical top speed.

Max torque in final gear.
Drivetrain loss factor.
Final gear ratio.
Final drive ratio.
Aerodynamic drag coefficient.
Tyres drag coefficient.

I'll find my engineering books and find the correct formula.Remember this is all theoritical it doesnt take into account the wind and road surface conditions.

[epp_b]

This is just a guess, but they might possibly just actually test them on a long track or speed bowl

[Greatgraddage]

Originally Posted by andyhui01

I think it depends on the quality and the number of satellites the GPS uses to get your coordinates, I know in cases of Yatchs, a GPS speed is way more accurate than the one displayed in the dash... if you're GPS is a good enough one, it should display speed very accurately.

Assuming you mean yachts; the gps speed and the one dislayed in the "dash" are two different things. The gps shows your speed over ground, the other shows your speed through the water, you use either for different circumstances.

[andyhui01]

Originally Posted by Greatgraddage

Assuming you mean yachts; the gps speed and the one dislayed in the "dash" are two different things. The gps shows your speed over ground, the other shows your speed through the water, you use either for different circumstances.

I don't know what the 2 circumstances are but I remember being on my mom's company Yacht and in Hong Kong, we have a speed limit in the harbour of 15knots, we were travelling at 15knots according to the gps and the "dash" one was only 16-17... according to the captain, he just said there is a diff. because the sensors/propellor that measure speed is not accurate because of the water current, whether it has another use is beyond my knowledge.... but just out of curiousity, what is the one in the dash used for?