My speedo is way off (see previous posts). I want to check the speedo calibration to see if the speedo is the problem. I plan to disconnect the cable at the tranny, connect my 2 speed drill to the cable, and have someone check the speed at the gauge. My drill has settings for 350 and 1200 rpm. Does anyone know what these cable speeds translate to in MPH? Also, should the drill run in forward or reverse? I don't want to pull the dash apart and send the speedo out to be fixed if it is not broke.

 

I believe 1000 RPM = 60 mph indication.

 

Thanks Mike.

 

 

This might help too. Harbor Freight and probably others have a rpm indicator, some photo and some other that you could use to check your drill rpm to get the right rpm's.

 

Why go thru all that trouble, just use a gps to check your speed.
I believe that the tach and speedo use the same internals so in 4th gear (engine rpm matches tranny output rpm) the mph and tach needles move the same amount.

Drill runs backwards.

A #2 Robinson screw driver bit (square tip) fits into the speedo to drive it

Steve L
73 coupe since new.

 

I tried the Harbor freight counter for a different idea I had.. it was about as accurate as counting the rpm's your self... but it's china made so I wasn't disappointed.

GPS I've had great luck with and there is an app for your phone if you use a smart phone.. works pretty darn good too.

IMHO,

Willcox

 

Thanks Steve. I know that the speed is off. I need to find out if the problem is the speedo or the tranny drive. It's easier to use the drill then to pull apart my dash. It' s reading 50% low, so I'm just trying to find out if it's close. Thanks for the info on speed direction.

 

Steve-

I'm not sure they've got Robertson screws in the US. A Canadian invention you know. , like basketball.

 

non-variable 60 cycle ac motors will run at 1800 or 3600 rpm.

 

I was wondering that as well. But just recently I have seen some Robertson screws at a FL home depot. I think some of the screw bit kits may have them.

Steve L
73 coupe since new

 

Don't know what year you have, but check the plastic gear in the adapter at the tranny. On my 73, the gear sheared off every 25k miles since it was new and the speedo would be low for a short while before it finally gave up completely. I kept replacing the gear. Yrs later I found out there was a TSB for the right angle adapter that slowly siezed up causing the gear teeth to disintegrate.

BTW, the whole IP is only held in by only a handful of trim screws. You can take off the IP including speedo in 10 minutes. The cable for both speedo and tach comes off be pressing a tab and pulling the cable off. Takes a few seconds.

Steve L
73 coupe since new

 

Thanks Steve. I've already replaced the driven gear, the old one didn't look bad. It might be a problem with the drive gear slipping or worn. Also, the odometer also reads 50% less, could the odometer gears be the problem?

 

I don't know if you have an auto or manual, but the manual the gear on the tail shaft is a worm gear that is pressed onto the shaft so I would doubt that this gear is worn or slipping.

If your plastic gear is OK and your adapter that fits on the tranny is ok then it has to be your speedo head or there is slippage between the cable and the adapter or speedo head. As Mike said previously, something must be slipping since your odo is reading low as well.

If your connection at the tranny adapter is tight and the end of your speedo cable is not mangled then it must be at your speedo head.

You can try driving your speedo cable at the tranny end with a known drill speed but how do you know what speed the drill is really running at without some sort of tach.


I think your gonna have to get behind the IP to sort this out.

Check the cable end at the speedo head if it looks OK then
try swapping the tach cable and speedo cable behind the dash and go for a drive. I'm pretty sure that both tach and speedo are the same other than the faceplate. Now see if the tach is reading the right speed by mentally superimposing the speedo markings onto the tach. I'm assuming you have a mechanical tach. What yr is your car and is it an auto or manual- should have asked this first.

 

Steve, it's a manual tranny. I installed a new speedo cable when I bought the car, the old one was broken. It has always read too slow. Started out at 20% slow, now it's 50%. I'm using the process of elimination to find the cause, starting with the drill to check the response at various speeds to see if the speedo responds reasonably. Not too concerned if the drill speed is off a bit, I'm not using it to calibrate the speedo, just to check the response. Then I'll at least know where the problem is.

 

Let us know how you made out,

 

wilcox, do you guys rebuild speedometers? if not anybody have suggestions?

 

Here's an update. I have a 2 speed drill, 0-315 rpm and 0-1200 rpm. I disconnected the speedo cable at the tranny, and hooked up the drill to the cable, with a buddy checking the speedo reading. I calculated that at low speed, the reading should be 19 mph, and 72 mph at high speed. (based on 1000 rpm=60 mph). Running in reverse, I ran the drill at low speed. The speedo read a steady 19 mph. At high speed it ran a steady 65 mph. I'm sure that the drill is probably not exactly 1200 rpm, but it's reasonably close. Now I have narrowed the problem to the tranny output. I have already replaced the driven gear, which didn't fix the problem. (18 tooth brown). It's time to at least check the drive gear.