Quote:
Originally Posted by flash968
yup
speedos are usually off, and generally read high - the average 968 for example reads 150 when you are only doing 139 - it's pretty close under 100 though
GPS is usually off at high speed too - the delay time to the satellite almost always causes high readings at the upper end - it's because of the calculation that is done, and the refresh rate - i've "fooled" my GPS into reading 10mph high at about 120 - it isn't too hard - i didn't feel it was safe to play around beyond that - too many thing moving too fast to keep fiddling
radar is about the only way to measure speed, unless you can sustain it for a while, which is why the bonneville runs are conducted the way they are - i use a radar gun for my local stuff, and at the track
kids, don't try this at home - high speed runs are DANGEROUS - tires are extremely critical at high speeds
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Thats why Y rated tires are so important. GPS can only be fooled if you're bursting up to a speed that is not held. If your car is kept at speed for any period of time over 10 seconds the gps reading is dead on. Radar is very hit and miss and can, depending on the contour of your car, take multiple reading to get an accurate reading. The only sure fire 100% accurate reading is lidar. Also accurate reading can be taken from your ECU (assuming you don't have larger tires than normal) since many car manufacturers (especially German ones) build in anywhere from 5-7 mph extra between the ECU and your speedo gauge. (My 05.5 Jetta TDI was 7mph difference between the ECU and Speedo)