Speedometer Tech
 

Since speedometer questions come up allot with regard to 1st Gen RX7's I thought I would post some tech information.

Basic Information

The speedometers in 79-85 Rx7's are mechanical - driven by a speedometer cable. The speedometer cable is driven by two gears in the transmisson. The speedometer gear that the speedometer cable attaches to is called the driven gear. The speedometer gear that is on the transmission output shaft is called the drive gear. Speedometers in later RX7's are electro-mechanical and are not driven by a speedometer cable.

Basic Maintenance

The only thing you can service is the speedometer cable. Is should be lubed periodically with a light grease. NEVER use graphite - it will ruin the bearing surfaces in the speedometer. The cable pulls out of speedometer cable housing from the bottom - at the transmission.

The speedometer itself is not servicable with new parts. A professional may be able to lube the speedometer that is starting to act up. You, on the other hand, should not attempt to lube or use compressed air to blow out the speedometer. It is not possible for you to place lubricant in the areas that may need it and the speedometer is sealed up real well in the instrument cluster and it is unlikely that it will fouled by dust or dirt. DO NOT REMOVE or MOVE THE POINTER....once you take this off the speedometer will have to be recalibrated by a pro using a machine designed for this purpose.

How do they work?

Mechanical speedometers use magnetisim. The speedometer cable spins a magnet inside of the speedometer. The magnet spins in a copper "speed cup" that is attached to the speed indicator - the pointer. The faster the magnet spins the more the speed cup moves and, as a result, the faster the indicated speed. The odometer is also driven by the speedometer cable but is gear driven.

Calibration

All speedometers in 79-85 RX7's have the same calibration. This means that you can swap an 85MPH speedometer for a 120MPH speedometer without the need to make any changes. The calibration spec is 1024 Cable RPM = 60MPH indicated.

Speedometer Accuracy

Speedometer accuracy - speed indication and mileage - can be affected by changes you make to the car. Keep in mind that the percentage change in speedometer accuracy will be consistent with the percentage change that you make. So if you make a 5% change in tire diameter you will cause a 5% change in speedometer accuracy.

Changes in Tire Diameter

Note: This change deals with the diameter of the tire - not the wheel. It is possible to get 15" and 13" tires that have the same diameter. Always read the diameter specs on the tires you are buying if you are not sure.

Tires with a larger tire diameter (than stock) will cause the speedometer to indicate low - you will be going faster than what the speedo indicates

Tires with a smaller diameter have the opposite effect - the speedometer will read faster than your actual velocity.

Changes in Rearend Gear

Decreasing gear ratio (going from a 4.44 to a 3.90) will cause the speedometer to indicate low - you will be going faster than what the speedo indicates

Increasing gear ratio (going from a 3.90 to a 4.44) will have the opposite effect - the speedometer will read faster than your actual velocity

Transmission Changes

Installing a transmission that came from a car with a 3.90 rearend in a car with a 4.10 rearend will cause the speedometer to read faster than you are actually going.

How do I know my Speedometer is Bad?

If the speedometer pointer moves but the odometer doesn't the speedometer is bad

If the odometer works but the speedometer pointer doesn't move the speedometer is bad

If the speedometer pointer "pegs out" at low speeds the speedometer is bad.

It may be possible to "save" a speedometer that is starting to fail. Find a speedometer shop - this is a job for a professional.

Cable Problems

If the speedometer pointer and the odometer are not working make sure that the speedometer cable is connected at the instrument cluster and at the transmission. They clip on to the instrument cluster and can pop off.

If the speedometer cable is attached at both ends and the speedometer/odometer are still in operative - the cable could be broken. In this case, buy a new one or have a speedometer shop make you a new one.

Speedometer Gear problems

The speedometer gears in the transmission are plastic. They can wear out. Visually inspect the driven gear for wear. To "slip test" the gears, raise both rear wheels off of the ground (use jackstands please). Disconnect the speedometer at the instument. Start the car and put it in gear. If you can stop the speedometer cable with your fingers the gears are slipping.

Some pictures:

http://inlinethumb28.webshots.com/46...600x600Q85.jpg

http://inlinethumb16.webshots.com/23...600x600Q85.jpg

http://inlinethumb44.webshots.com/38...600x600Q85.jpg

there are speedo gear charts at www.solomiata.com

also there is actually a seal in the speedo gear housing, if it leaks the cable wicks up oil and it'll actually leak transmission fluid from the dash. mazda pn# 9958-60-8166

and mazda used a speedo cable on the rx7 until the FD in 92, and on the miata until 99 (i think, fer shure up to 97)

great reference for people who are not aware of the components of the speedometer and the principals behind it. we can update the thread with pics of the drive gear and perhaps the different drive gears used throughout the years.

Excellent data. This is worth printing and saving. I've installed several speedos and cables that I've purchased from this forum. But it's been trial and error depending upon the condition of the part.

Unfortunately, I have had to pull the pin and faceplate to make it match my instrument cluster color scheme. I may have to re-adjust if the speedo wasn't used for a while before I got it. It'll loosen up and settle in once you remove the gunk and operate it for a while.

It's hit and miss depending upon the speedo, but I can usually get it within a 2 mph. I figure that's close enough because tire wear and other factors affect accuracy also. So I often use a GPS to help calibrate the accuracy of my Speedo. I also use the RPM/Speed charts to help calibrate my speedo.

Anyway, superb info. Thanks for the expertise.

By "Speedometers in later RX7's are electro-mechanical and are not driven by a speedometer cable." which RX-7's are you refering to, FDs? RX-8's. Curious, because the FC's are cable driven, but I really do not know about FD's but some FD guys run FC transmissions which would seem like a lot of work to change over the cable type.

How would graphite ruin the bearing surfaces? I would think it'd be fine as a lubricant...

Oops, my bad. Yeah, I see you can get speedo cables for FC's. Sorry about that.

Another cause for trans grease/fluid in a speedometer cable housing is blocked transmission vents. In fact this is what causes the seal in the speedo gear ferrule to start leaking.

It works like valve grinding compound on the bearings in the speedometer head. At the Speedometer shop I managed we had several examples of speedometers that were completely ruined by Graphite. So in my opinion you shouldn't use it.

FD's and RX8's.

To do a swap like you are describing you would need to install the speed sensor from the later trans in the early trans. Not an issue if the sensor is driven by the same type of gears that drove the speedometer cable.

When I was in the speedometer repair business one of the last uses of a speedometer cable in the 95 Taurus/Sable/Windstar.

subscribed

the FD uses a signal generator that goes into the same spot as the cable speedo drives.

the Rx-8 being modern uses the ABS wheel speed sensors.

the tricky part is figuring out what speedo gear to use once you swap rear ends.

Great info! Thanks for posting. I had a basic knowledge of speedo function already, this fills in the blanks. ;)

This can be tricky but here are some basic considerations.

To decrease cable speed, increase the gear tooth count on the driven gear. For example if you decrease the diameter of your rear tires or go from a 3.90 to 4.44 rear end. These changes will result in the speedometer reading high because the cable will be turning too fast at a given speed.

To increase cable speed decrease the gear tooth count on the driven gear. For example if you increase the diameter of your rear tires or go from a 4.44 to a 3.90 rear end. Changes like these will result in a speedometer that reads low because the cable will be turning too slow at a given speed.

I am not aware of drive and driven speedometer gear options that are available for RX7's. However, what you need to do is make a change that matches the percentage change that was made to the rearend gear and or rear tires. If it is not possible to correct error using speedometer gears, a speedometer shop can build you a ratio adapter. Go to http://www.gaugeguys.com/speedcorrect.htm to see what a ratio adapter looks like. They come in metric applications so one can be made to fix any amount of error.

If you are dealing with variables such as a transmission swap and or a rear end ratio that is not known the best way to determine error is to use the odometer. As was illustrated in the pictures in my original post the odometer is gear driven. As a result it is very accurate. So to determine speedometer error drive your car over a measured distance. A distance like 5 miles on a highway works well and you can use the mile posts or a GPS to determine that actual distance traveled. To determine error, compare actual distance covered against mileage recorded on the odometer. The percentage difference in mileage is the percentage of speedometer error you have.

For example if your speedometer is recording mileage 10% faster than it should your speedometer will register 77MPH when you are actually traveling at 70MPH. This won't get you a speeding ticket but the oppossite will - traveling at 77MPH when your speedometer shows 70MPH.

Hope this helps

wait so if the calibration spec is 1024 cable rpm = 60mph for a first gen then is the FC calibration spec any different?

nvm that last post found all of the information here

http://www.tamparacing.com/forums/ar.../t-442996.html

A bit of a thread resurrection, but it's pertinent to the posts. My question to mustanghammer is if FC speedos operate in the same way as FBs, how were the aftermarket clusters from RE-Amemiya, FEED, etc able to accomplish higher top speed speedos without changing gears at the transmission?

I gather one could under drive the speedo cable such that the speed would be lower and then with some careful marking/measuring you could create a rescaled speedometer face? If this is true I'm curious what the JDM companies did as it looks like they'd modify a cluster and charge $500.00. OR would these have required modification of the transmission speed cable gears after the fact? Not sure if I'm missing something. Thanks...

Mechanical speedometers are calibrated by adjusting the magnetism. When the cable is turning at 1024 on the OE speedometer the indicator needle is set to point to 60MPH. This is accomplished using a speedometer calibration machine that adds/subtracts magnetism to the magnet that spins in the speedometer.

If the speeds indicated on the instrument face are relocated to accomodate a higher max speed, the speedometer would be recalibrated using changes in magetism. In this way the needle will point to 60MPH at 1024RPM's where ever it is located on the instrument face

Because the cable also drives the odometer, cable rpm calibration is not used to in the manner what you are describing.

how hard is it to change those plastic gears ? just got my gsl se and ofcourse the speedometer is dead. Havent checked the cable yet but with my luck its probably the gears.

Only trouble is reaching them. There's one 10mm bolt securing them in, once that's off you can pull it straight out. Some oil may come out as well.

Sorry for the resurrection post but I wasn't sure if this (mazda speedo gear interchange reference) link/page was already referenced.

I have a question on tachometers. Ever since I swapped to 2nd gen coils my tach hasnt worked properly. I have it wired up to the trailing coil. My tach needle broke and I replaced it with another one.

By touching/replacing the needle would that throw off the calibration or something? Would I need to take it to a shop to get it right or could it be bad connection? I never had this sorted on my last setup which I didn't like.

I know your a speedo tech, but figured you've worked with both.. Thanks.

Sorry for resurrecting, but was looking for some 1st gen speedo information...

I may have problems with my speed sensor and wanted to know:

1) what it looks like and how it's attached to the speedo?
2) can it be removed/replaced?
3) principle of operation? I.e., how is it measuring speed, and what are the potential failure modes?

Pictures would be much appreciated...

The speed sensor is a gear and the speedo uses magnetism and the spin of the cable to move
the needle. If you read the first post all of this would be explained. Go read.

Actually I read the whole thing quite closely.

The speed sensor is not a gear; the description provided is quite detailed in its explanation of how the speedometer mechanism itself works, but does not explain all the components contained in the device.

Speedometer <> speed sensor.

So again: if someone with more specific knowledge of the speed sensor component of the speedometer could enlighten me, it would be much appreciated.