Printable Version of Topic

Posted by: JFG Sep 23 2018, 02:32 PM

on todays drive out i noticed that mileometer doesn't increase and i'm not certain that the speed is accurate (kept leaving others behind) so i'm wondering are there other gauges that swap in?

Posted by: Steve Sep 23 2018, 02:35 PM

Do a search, others have installed GPS speedo’s.

Posted by: JFG Sep 23 2018, 02:44 PM

i'm not a fan of them. i know of a spitfire with one and it's just not my thing

Posted by: mepstein Sep 23 2018, 03:41 PM

You can have yours fixed and recalibrated.

Posted by: ClayPerrine Sep 24 2018, 05:40 AM

You can use an Electronic 911 speedo driven by GPS. Matches the rest of the gauges, and it is accurate with any sized tires.

Posted by: mepstein Sep 24 2018, 06:21 AM

I didn't know this product existed but what a great idea. It would make the use of a later 915 trans much easier.

Posted by: GeorgeRud Sep 24 2018, 10:22 AM

When I converted to a 915 trans, I had to switch to an electronic speedometer with the attendant sensors on the ring gear. The GPS solution would have been much easier.

Posted by: 914forme Sep 24 2018, 10:53 AM

Yes they exists, https://www.amazon.com/VDO-340-786-Speed-Sender/dp/B00T9LX0R6. And they are not cheap. Then you need to find a working 9xx electrical speedo. If not working it will need to be repaired.

The mileage part will require the speedo to be taken apart, find the gear that is bad or slipping on the shaft. Measure it, and get the proper gear or on close to fit the application. It is not har to do, unless you need to modify the gear to fit into the odd stack. That takes a bit of work on a piece of flat sand paper, a file, or the easies way, a lathe. Most people do not have access to the last one.

As far as speed goes, they can be off, and, or recalibrated. Mine is dead nuts on, which means I do leave people behind, manufactures always make the read lower than the real speed. Do not want to get sued, some are almost 5 MPH off at 70 MPH. So you know what happens when you get the correct, your passing lots of people, and not exceeding the speed limit. Or the lader gun for that matter.

Clibration of the mechanical speedo is not as easy as it is with a electric. Electric you can use a low frequency that matches to the calibration marks on the face of the speedo. Mechanical you will need a setup that measure the speed of the unit driving the speedo cable. then you have to do a bit of math to get this figured out or find the calibration chart for your specific speedo. All the above is a lot easier.

If it was me, I would get the speedo fixed by a pro. If you want to do it yourself, you can, but getting the needle in the proper spot means you have to lift it over the stop, it is spring loaded. Take a bit of painters tap and mark the location of the resting needle. This way you can get it close.

Dissemble the unit
Fix the mechanical issues
Assemble the unit, leave the glass face off.
Install the needle on the resting mark.
Add some tape around the 60 MPH mark or where ever you want to be accurate.
Route a speed o cable into the passenger seat area
Get som one to drive or be the instrument person.
Get up to speed and hold stead at the speed the GPS says.
Mark the face, where the needle is.
Pull over calculate the difference for where the needle was (marked) and the Tick mark on the gauge face.
Mark the new needle rest position based off this difference. Calipers can do this for you, measure the distance between the two lines then transfer it to the resting spot.
Move the needle back to the resting spot
pull the needle
move it to the new location
Retest with new tape
Got the result close to what your looking for
Reassemble the gauge
Reinstall into the car

If you need to check you accuracy the state ( at least Ohio ) have sections of the roads marked for measured miles. Accurately marked, not the highway markings. Do a quick search of your area for one. If you need to see if it is accurate and only have the highway to do it on. Find a section to go 60 MPH on and drive an hour and see where you got. Easy math, 60 minutes should = 60 Miles. If your impatient, you can do the math and go like six miles in 6 minutes. Or if you have issues with attention you can do like I do, pull up WAZE or similar app, that gives you the GPS speed. Waze it is in the lower left corner on my iPhone or iPad.

You could go old school and just draw a tick mark on your bevel that represents the MPH you travel at and where it is.

All instrumentation is reliant only in the perceived condition in which your viewing it at. I will tell you they are all highly subjective and by no means very accurate. When you do things like timed events we do not rely on a clock in the car, we setup measured devices using (IR) most of the time to get the exact time. No the exact length of the corse as long as straight line you can calculate speed of the object in motion. Even when measuring the speed of a munitions round we shot them between two measuring devices a measured distance apart. You get your speed as long as the ordinance pierces both plains of the measuring device. Unless calibrated their is no way to confirm accuracy of the gauge. And in the case of the speedo you might get an accurate reading if all the stars aligned at one location on the face. If someone had a bad day and got the design off by .1 of a degree this could be a mess.

I will correct my self your speedo should be accurate at two locations. One where where the given speed lines up with the actual speed, only know with calibration or verification form another device, and at rest. Hopefully when going zero miles an hour it reads your going zero miles an hour.

What are your objections to GPS, it is not like we are trying to put your car into a vent pipe on a bunker. That is where the GPS variance would be ill suited for the mission. Your trying to get an accurate speed, a couple feet off in the bigger picture means nothing in that situation.

Posted by: 914forme Sep 24 2018, 11:05 AM

The gears or a close match can be found http://www.odometergears.comgood guys to work with, and they have the most common gears. Early 914 instruments you will need to machine the gears as thy do not have an exact fit. You have to measure your original. Then make the new one the same thickness. Not a fun process, rather tedious if you ask me.

Posted by: 76-914 Sep 24 2018, 11:10 AM

SpeedHut! I love mine and as others mentioned, no calibration required, ever!