About all those recent threads dealing with top speed claims. I have to wonder just how accurate they are, given all the tolerances on measuring rpm's, actual tire diameter (accounting for wear or different tire mfr even for the same size), accuracy of the speedometer itself, etc.

This isn't intended as a flame or a dispute of anybody's observations, just curiosity over how good the speedo display really is up there at the top of the dial.

Norm

 

drive around all the time at top speed.. when you get clocked by a cop, ask him how fast you were going... then you'll see if the speedo is true or not

 

Not to mention, that travelling at over two miles per minute, one second glancing down at your speedo is 200 feet... How long you wanna look down there?

Top speed runs to me means two timed runs in opposite directions, to cancel out wind, elevation change and bad mojo. Elevation and temperature corrections are okay, though.

At 140mph, if the linear error in my speedo really is linear, it'll read over 5mph high.

 

I haven't done a high speed run in my car yet and I've had it since late 1999. Half the time I have my kids in the back and I haven't had a good straight available. So far I've only gone up to 110.

 

I've got a spreadsheet (under continual development) that mathematically models acceleration. Top speed is defined as the point at which acceleration equals zero. Here's a brief summary of some of what happens:

For a car that tops out in the mid 150's, one of the things that comes out of that sheet is that the last mph will require over half a mile of road. The last 3 mph need perhaps 1.25 mile (that's 4 or 5 times the distance needed for the entire first 100 mph to get only the last 3). From rest, you're looking at over 3 miles. So even if it feels like it's still pulling pretty good at, say, 145, be careful about overestimating how much faster than that you could really go. The aerodynamic wall that you may be about hit doesn't give as much warning as you might think.

Norm

 

Lots of the newer GPS units have a speed feature built in. I think that would be the most accurate way.