speedometer error

[Barber Dave]

Yesterday while on the e-wayat 70 mph everyone was passing me. This got me thinking is my speedometer accurate? Is there some formula to check this? At 70 mph engine is turning 2,900 rpm,I have 185-65-15r tires. Does this sound right rpm, tire size,= 70 mph. Is there is a formula to check it? Thanks Dave

[Dave Bell]

Stock was 165/78/15 I belive

Yours at 185/65/15 is about 2.6% smaller rolling circumference than stock.... result, when your speedo reads 70mph you are only traveling 68.2 mph.

But there is so much variations in tire makes and mounting and inflation that the only real way to check the net effect on your speedo is to do it against some calibrated reference. Easiest way I have found is to borrow someones hand held GPS and find a nice straight stretch of highway. Compare GPS speed to Speedo.

- Dave

[CoryB]

I had the same problem. What you could do is find one of those police roadside speed readers(the ones that tell you how fast you are going) And compare it with what your speedo says

[brain009]

I also have this same problem. I went by one of the police roadside speed readers, in a 30 mph zone, and with the speedo reading 30, it displayed a speed of 24 mph. Anyone have any ideas on how to fix it?

TIA

[John Rogers]

The most accurate way to check the true speed is with a GPS unit. I have a Street Pilot III and it showed that the speedo in our stock 914 was 10 MPH too high at 70, I.E. we had to show 80 to go 70. You gould down load my GEARS.EXE program from PP here and it is within 1 MPH if you measure the tires correctly. Good luck.

[rockaria]

Or you can use my perfected way...

------------------------------------------------------------------------
Huge Texas Police Officer: "Do you know how fast you were going?"

Me " Not really, I was hoping you could tell me."

PO: "Funny Guy Eh, Out of the car!"

Me: "But I really need to know"

PO: "You guys in your fancy sports cars, burns me up!"

Me: "But sir I really..."

PO: "Hands on the car spread em!"

Me: "But, I.. Er..."

PO: "you have the right to remain silent..."

Later in the day after 5 hours in the pokey!

Judge: "Do you know how fast you wre going?"

Me: "That is the whole reason I am here..."

Judge: "oh I see why you were arrested. Trying to make fun of us eh?"

Me: " No sir, I really want to know how fast I was going."

Judge: " Oh you know how fast you were going, Balif impound the car. Thats 7 days suspended, $600 fine. Pay the clerk on the way out!"

As I looked at the final paper work it seems I was doing 130 and my spedo was on 140.

Cool! Simple solution! And there is a good chance I will be on COPS!

[Dave at Pelican Parts]

There are a few ways you can correct the speedometer error, once you have figured out what it is. (Remember, the error may not be linearly dependent on speed; I know one guy whose speedo seems to read 5 MPH high at all speeds, and another who has a speedo that read high at one point and low at another!)

First, you can try disassembling the speedo and mucking with the spring that returns the needle to 0. Guess, mess, and try--until you get results you can live with or break something.

Or, you can try messing with the magnet-and-cup arragement that pulls the needle against the spring. Guess, mess, and try again.

(Note that both of those have no effect at all on the odometer reading! But the odometer might not be as far off as the speedo.)

You can send your speedo to North Hollywood Speedometer (http://www.nhspeedo.com) or Palo Alto Speedometer (http://www.paspeedo.com) to have it recalibrated. You'll need to tell them how you want the readings changed. They may be able to change the odometer as well--not sure. You can also try contacting Pelican; I'm not sure if the people we use for gauge rebuilding will do the recalibration or not.

Finally, you can look for old-school rally equipment. There used to be a little "gear box" that you could plug into the speedo cable, then plug the speedo itself into. You could swap gears inside this box to change what the speedometer and odometer "saw". This will change both speedo and odo, but will do so in a linear fashion only. (E.g., if you make it read 3 MPH higher at 30 MPH it will read 6 MPH higher at 60 MPH.) This will probably not be that easy to find.

...Or you could just live with it. That's what most people do.

--DD

[Paul_Heery]

Try this calculator. http://www.csgnetwork.com/speedocalibcalc.html

After you put in the stock tire size and your new tire size, you can try different relative speeds and see what the difference is.

[rfoulds]

After measuring against a GPS, I put tiny, red line decal (pinstriping) on my speedo where 35 and 65 REALLY are. Then, I just estimate in between.

It helps calm my wife down when she thinks I am 15 mph over the limit.

[John Rogers]

To find out the real expected speed vs RPM you need to do several things: first mark one of the tires at the edge of the tread pointing down, mark the ground and then roll the car and tire one revolution then mark the ground. Measure the distance between the marks to 0.1 inch. Then find each gear ratio and the final drive ratio from the owners manual or on the PP site here. Then download my GEARS.EXE program from the tech section and install it. This program has the necessary formulas to give you the theoritical speed vs RPM as you move the slider on the screen. It shows what is happening in each gear and is good for up to 6 speeds. It is pretty accurate and is used by repair shops and others to see what happens with tire changes. Or you can spens $$$$ and buy a GPS system. Good luck.

[Joe Ricard]

Well, is is pretty fast. Ran 50 miles on the freeway yesterday. RPMs were around 3800 to 4000. Speedo showed 90 / 95. I was pacing with traffice that normaly runs 75 / 80. So the speedo is off. I guess I will really be hauling the mail when the speedo shows 120.
P.S. just add that GPS to the things I want in the ulitmate 914.

[red-beard]

I looked at the GPS units around and found I didn't need the MAP stuff. I did need a Radar Detector!

Uniden makes a Combination Radar Detector and GPS unit. The cool part is that is can remember locations even if a radar signal isn't being transmitted (Radar instant on traps). And it can learn false signals and locations and ignore them.

AND it functions as a speedometer. The speedometer and the Radar detector are my primary functions for this unit.

UNIDEN GPS-RD

James