3rd Gen Speedometer Accuracy?
 

How well is the stock 3rd Gen Speedo calibrated at high speeds. Does anybody know what the variance is at say 155 MPH. I was just wondering and I know that top speed runs are very dangerous and not very smart but it is the morbid curiosity in me that wants to be able to say just how fast I have really been.

Just an FYI: The Mclaren F1's speedometer is calibrated within a mile per hour or so throughout the entire speedo. What an amazing feat that must have been.

if you really want accurate go and get a GPS unit that shows your speed. that's as good as it gets. i'm sure all speedometers have a degree of variance so you never can be sure.

either that or you can just find a road with lots of cops.

Actually, there IS a way of calculating it using mile markers on a flat hwy, but I forgot how to. I'll keep an eye out for it again.

Re: 3rd Gen Speedometer Accuracy?
 

[QUOTE]Originally posted by SpeedracerRX7
[B]How well is the stock 3rd Gen Speedo calibrated at high speeds. Does anybody know what the variance is at say 155 MPH. I was just wondering and I know that top speed runs are very dangerous and not very smart but it is the morbid curiosity in me that wants to be able to say just how fast I have really been.

Naw - nothing morbid about that! Just do it...... (Just make sure you sweep the road first, literally and by sight, and do it at dawn - there's this great little section of about 14 miles just west of Yuma, Arizona on I-10 in California that's as straight as an arrow with no side roads and.......no cops!) You can use a stopwatch between mile markers with your sweetie beside you to do et's between the posts! I came up with 155 indicated and actual at 144 - I was too chicken shit to go any faster and that was all she had on a stock 93, with a light headwind!!
Fun? Scary? Nuts? Hell yeah - that's why you drive a sports car in the first place, No?!!!

Thanks man I really appreciate the strait forward answer. I also have another question for all you PFC experts out there. Do you think that the speed on the Commander is any more accurate than the indicated speed on the speedometer?

Just to add to what I was saying before, I did the 155 MPH run on a private airstrip that is about 30 min from my house. The owner was a nice guy when I asked him if he would let me do it. The only condition was that he wanted to be there when I did it. This was not at all the top speed of the car, but I ran out of runway. The only mods I had at the time was a downpipe and a PFS CB exhaust. I have an intake and a PFC now and I want to give it a go again.

P.S.-->By the way DeNguyen I just wanted to let you know that I have tried that and I got incredibly lucky. I got caught at 124 MPH in a 50 MPH zone last Oct. The cop said that I was lucky his shift was over soon because he didn't want to hassle with arresting me, impounding the car, and having to fill out all the paperwork. This was even before I got the RX-7. Remarkable it was in a 95 Probe GT.

Hey Speed: That's good thinking man - doin' it on a runway -cool.
There's way too many idiots like me doing it on a public highway.
I wonder if we put a canard wing on the front and a big freakin'
spoiler on the rear tipped UP a little if we could get it to FLY?
There's a runway here in Tucson at the Air Force Base that's FOUR MILES LONG, dood. C'mon out and we'll sneak in at night and....... Oh, ........I guess that's not very smart right now.:ak47: Damn, I love the smell of Napalm in the morning. :D

124 and he let you go? You must have had some fresh donuts in the back seat.

The speedo is right on the money at hiway speeds when I compare it to my GPS. I haven't done any 120+ MPH runs since there are too many cars where I live in SF. I doubt if the accuracy would be off by more than a couple miles at high speed since it's within a MPH at normal highway speed.

I would be inclined to GUESS at 155...I'd almost say you where probably doing around 150...

FWIW, no speedometer can be accurate all the time. As tires wear out, circumference decrease, thus so does the actual speed vs indicated.

Even tire pressure make a difference. That's how the oval boys use stagger. A couple of psi more in the outside tires make em turn in better. Any good old-fashioned racer measure tire circumference instead of pressure.

Hell, even speed affect circumference. Tires are heavy - spin em fast enough and they get taller and narrower due to the centrifugal force. This is easy to see on dragsters - I know those are soft-wall units but the academic point remains.

Anyhow... you're lucky if your speedometer is within 3% any given month of the year.

--
cheers & peace.
Edwin.