billsfan

When I bought my car, the speedo wasn't working. I bought a new cable, but the speedo works but reads slow now. My speedo and Odo both read about 15% slow. I replaced the speedo gear. I used the 18 tooth brown gear, for my 336 rearend gear. There already was a brown gear in the tranny. It's an all original '76 vette, and I physically checked the rearend gear. The speedo needle jumps around about 5 mph at steady speed. Could there be a problem with the drive gear? I lubed the new cable when I installed it.

MelWff

What kind of lube did you use? If it gets into the speedometer head it can lead to problems. Are you using the original size tires?

billsfan

It was a graphite type lube that the parts store recommended. Just a little bit along the cable. The tires are 235-60-15, so it would not cause such a large error. The problem started as soon as I installed the cable, so there was not enough time for the grease to impact the speedo head.

MelWff

according to this site the tire size reduction would cause the speedo to read higher than actual road speed which is the opposite of what you are experiencing, correct? Was the original brown gear damaged? You are positive about the rear ratio?

http://www.roadkillcustoms.com/hot-r...Calculator.asp

billsfan

The original brown gear was not damaged, I thought thathat the square drive end might be worn, so I replaced it. I physically checked the rear gear by rotating the tires and counting revolutions on the driveshaft. I thought about checking the speedo by driving it with a variable speed drill. If so, forward or reverse? I know that 1000 rpm should be 60 mph.

MelWff

I'm not sure where you got that value 1000rpm=60mph but you can try either direction slowly, the drive gear is always turning even in reverse gear. Based on this link I would be tempted to try the 19 or 20 tooth gear to correct your speedo inaccuracy.

http://www.thegearbox.org/catalog/it...00/3553330.htm

Lil" Red Corvette

In regards to the needle jumping 5 MPH at steady speed. That is usually caused by a wore out cable. What's happening is the cable is slightly binding and twisting inside the cable housing. You may be able to remove the inner cable, clean and lube it and put it back in. That might fix it.

gcusmano74

How are you checking the speedo calibration?

billsfan

Thanks for the ideas and questions guys. First, the 1000 rpm of cable speed is equal to 60 mph of speedo readout is apparently a calibration standard. Next, if the speedo reads low, you use a driven gear with fewer teeth, not more teeth. I could try the 17 tooth purple gear. I will pull the cable, clean it, and re-lube it. I'll also check the speedo head for signs of grease from too much lube. Finally, it is exactly 5 miles to my mom's house, according to my truck, which is fairly new. With the vette, it's only 4.2 miles! I also occasionally drive by a portable speed readout, and my car always reads 5 to 10 mph slow.

gcusmano74

Check your speedo against the readout of a GPS. At a constant velocity, that will be quite accurate. Use a straight and level interstate highway, if you can.

mrvette

I can guaranTEE you that the speed CAN be dead on up to ~80 mph.....


when checked with a GPS/Garmin.....IF you have the correct driven and maybe even drive gears....for the tire diameter and rear gear ratio....

Do NOT take those tire diameter/gear ratio sites to mean anything....

you simply need a GPS and so to note what your car does in comparison....

AFTER THAT, you will note that your dash is accurate to maybe 80 mph and after that will significantly over indicate.....

so will the tach....

billsfan

My nephew has a gps I can borrow. I will also clean my cable. I may also check the drive gear, it could have a few stripped teeth, which would also account for the needle jumping around. Right now, I know it's way off, just got to figure out why. By the way, I have 235-70 tires, not 235-60 like I mentioned before. So tire diameter is close to stock. Rear gear is stock, tranny is original. The only thing I haven't looked at is the drive gear.

mrvette

I have a LITTLE of needle jump at really lo speeds....talking 25 mph or so, smoothes out at higher speeds, the cable is the first suspect, and after that you gotta do the nasty, pull the speedo assy out and almost totally apart.....werd of advice, IF you can get under there, and pull the speedo cable off to lube/replace it...hit the back *** of the speedo with a little WD40, don't drown the bitch, just use common sense....

over the years, I have sprayed the cable housing and replaced the cable itself....and found a years long fair amount of success....

TimAT

^^^^^
This guys speedo can make some noise too


Billsfan- Is your speedo making any noise at all? I had a 77 that the needle started jumping a bit- did that for a while and then had the screaming fits just before it quit. The jewel bearings were just plain worn out. $125 an it was good as new.

billsfan

No speedo or cable noise at all. I was also careful to install the new cable with no tight bends. I'm pulling the interior out this winter to install hushmat. Not too much more work to pull the dash apart and look at the speedo. That's why I'm getting this info now, I only want to do this once, and get it right.

Lil" Red Corvette

I watched a speedometer shop calibrate my spedo years ago. What I saw and he explained to me is our speedometers are not perfectly linear. It can be dead on at 30 MPH and off a little at 40 then dead on again and 50 and so on. I had him calibrate it to work best in the 50-70 MPH range. When I did my calibration in the car I did it to the odometer. The speedo is within a couple miles per hour at 60 checking with either a GPS or the stop watch method on the interstate.

billsfan

I don't expect it to be dead on. But 10mph off at 45 mph is way off. Plus I still need to fix the jumping needle.

gcusmano74

A 235-70R15 is an oversize tire. Depending on your year, that will make a slight or significant speedometer error. The early C3's used an F7015 tire. Probably close to a 215-70R15 today. Chevrolet changed to radials in 1973, using a GR70-15. Most tire outfits today say that is about a 225-70R15. By the late 70's Chevrolet was installing the 225-70R15 as the standard tire. The 255-60R15 tire was an extra cost option.
PS- I'm using Firestone 235-60R15 tires now on my '74, and the speedo is either spot on or within 5% of correct up until about 70 MPH. When I used the 225-70R15, it was as much as 10% pessimistic most of the time.

gcusmano74

Yes. Think of your dashboard gauges as indicators. Not instruments.

racerx1

Sorry to highjack the thread, but I'm about to replace the speedo cable in my '74 and I wanted to know what is the recommended lube?

The prior owner gave me a GM NOS cable and one of those 90' degree speedo cable adapters that has a grease fitting. It looks like a quality built item. Are these adapters any good....is it worth me installing it?