Speedometer Accurancy

[Keith Lyles]

Ok, I got a ticket the other day for doing 78 MPH in a 60 MPH zone. Because my speedo tends to bounce between 60-66 MPH I rely more on my tach reading. I believe between 2000-2100 RPM puts me in that 60-63MPH range in 5th gear. I never got above 2100 RPM's and now I have to wonder if I have grossly miscalculated the gearing for the car. The WSP sure in the hell wasn't buying it and right now I am afraid to go over 1500 RPM with the way my speedometer and tach are behaving. Is there a way to calibrate the speedo or tach without having to send them off somewhere to be bench tested and re-instaled later or do I have to resort to having one of Washington's States finest check for me? I plan on going to court on this one because I honestly did not believe I was going that fast. In addition, I want to make sure I understand the technical factors which could have caused or contribute to this gross mismatch and make durn sure I get it fixed promptly. Any jail house lawyers out there?

[RedBarchetta]

Tough break.....

I had the dumb luck fortune of driving by one of those "Your speed is ___ " automated signs during my test drive. Learned right then and there that the speedo was off by 10mph (to the high side, so now I just subtract 10 from the speedo and know I'm in the safe zone. One of these days I'll crawl underneath and install the right speedo gear (probably after I change to 17's).

[anchor2]

Possible a quick fix for you. Call Dynamic Motorsport (SPF dealer) in Ohio and ask for Mark. He sold me a new spedo cable that cost about $15. It was a great improvement. The spedo dose not bounce much at all anymore.

[SCOBRAC]

Here ya go...

http://www.corral.net/tech/gearcalc.html

[DonC]

Keith:
You can run out the theoretical number using any of several gearing/speed calculators. Here's one that I've found to be pretty easy to use, just plug in the numbers for gear ratios, rear end ratio and tire size.
This should at least put you in the ball park.
http://www.368s.com/tools_rpm.phtml
DonC

[Keith Lyles]

Thanks, I'll give Mark a call. I'll use the gearing calculator and see what I come up with. Meanwhile I need all the help I can muster for a Perry Mason defense strategy. Funny thing is the Officer claimed I was being tracked at 78 MPH on mobile and aircraft radar. I didn't see no stinkin plane. I hope the traffic judge is a Cobra fan.

[G.R.]

Keith
Sorry about the WSP encounter. My SPF speedo was almost 10mph off---tested it alongside a friend in his MB--I too always watched the tach and the speedo--I never did anything about it just started driving a little slower after the test--kept it under 2000 rpm on the highway.


Wasn't on 3--the WSP Ticket Zone was it?

[strictlypersonl]

If you have a measured mile and a stopwatch (or even a willing passenger with a watch), drive at a constant speed and measure the time for a mile. Divide that time into 3600 and you will have your actual speed.

Alternately, find a friend with a GPS. They're surprisingly accurate. Or at least mine is.

[HI Cobra]

Here on Oahu our Honolulu Traffic Division
(I think) are frequently setting up radar signs
on various freeways which at times are fairly
accurate. They helped me when I was calibrating
my speedo on my Cobra and on the 3rd try
found the right gear. Way cheaper than paying
someone $100+.

Flip

[Roscoe]

My electronic Speedo by Autometer is right on all the time. Easy to calibrate and you can re-calibrate if you change rear end gears or wheel size.

Roscoe

[jwoodard]

Nisonger at: http://www.nisonger.com/

They will help you calibrate your speedo and send the right gear.

[Zoom This]

Under the assumption you have a 3.73 axle, a 5 speed Tremec with .68 OD and standard tires (275/60/15) this is the following speed calcs in 5th on the freeway.

63 mph = 2000 rpm.
71 mph = 2250 rpm
79 mph = 2500 rpm

Your speedo should not bounce. I too had a bouncing speedo until I swapped out the original installed by the engine and tranny installer for a new one from Superformance referred to as gold in color.

[Ron61]

I like the electronic speedometers also. Like Roscoe, I have one except mine is a VDO. I went to a police test and they let me run their radar setup at 50,60,70,80, and 90 MPH and the speedometer and tach were both right on after I did the math to find out the exact RPM.I can't read 2525 RPM on my tach but at 60 in 4th it was reading just above the 2500 mark and 2525 is what the math shows it should have been. It is exactly on in 5th at 70 when I tach 1900 RPM. If I change tires or gears, I just start the car while holding the button in until it scrolls to automatic, then drive a mile. Stop and push the button and it calibrates the speedometer. I tried it once by driving just past the mile marker a little ways and it put 2/10s on the tripometer and told me I had driven 1 & 2/10s miles.

Ron

[Keith Lyles]

You guys are great, thnaks for all the info.

Keith