Inaccurate Speedometer

ahetzel

ahetzel

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I have known for awhile that the speedometer is not accurate and am replacing the dash this winter so it's a good time to R&R the gauges. First let me say that the car is a 1965 MK1. The engine and rear end in the car are original and the trans is a 4 speed Toploader from a MK II (I have no idea how it got there). The speedo has never been out of the car. I did a quick test and the results are worse than I thought.

Gear Tach Real Speedo % Fast
2 3000 40 56 40%
3 2000 40 55 37%
4 1500 40 52 30%

I am hoping that I can have it calibrated to get it closer rather than changing the gear in the trans. Any experience getting the speedo refurbished and with whom? It doesn't need to be exact but close would be nice and I would rather not go electronic. Thanks
 
Cable gear

The gear on the trans end of the cable probably needs to be changed as it may not have the right # of teeth. I think that they are sold by S.S. as well as maybe on CAT parts list too and a cable replacement too. If I had to do it again , I'd go with a refaced Autometer speedo. I had my original speedo rebuilt locally and like the old sign at the lock smith that says,"The broken lock when brought to the locksmith for repair works perfectly." It works well but you must bump the dash as the needle sticks after sitting for a week.

If you search the forum you will see considerable info on the swap.

By the way Paul B has been doing tachs for some time but has just recently started doing the speedo's as I hear.
 
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First, Do No Harm

ahetzel said:
I have known for awhile that the speedometer is not accurate and am replacing the dash this winter so it's a good time to R&R the gauges. First let me say that the car is a 1965 MK1. The engine and rear end in the car are original and the trans is a 4 speed Toploader from a MK II (I have no idea how it got there). The speedo has never been out of the car. I did a quick test and the results are worse than I thought.

Gear Tach Real Speedo % Fast
2 3000 40 56 40%
3 2000 40 55 37%
4 1500 40 52 30%

I am hoping that I can have it calibrated to get it closer rather than changing the gear in the trans. Any experience getting the speedo refurbished and with whom? It doesn't need to be exact but close would be nice and I would rather not go electronic. Thanks
Click to expand...

The MK II HEH-CF tranny has a yellow 7 tooth drive gear whereas the HEHE has a green (rare) 6-tooth. You need to replace your driven gear with a 17 tooth to get the percentage error down to a minimum. Save your original driven gear, they are unobtanium now. . . . .
 
Being that far out.. and the fact the trans has been changed I would suspect you really need to change the speedo gear on the trans. Also what wheels/tyres are you running? If the over all rolling radius is different from the stock setup you are also going to get percentage errors.
 
Alpime gits

I mean guts with a new Pauls redone face. My speedo guy turned up one gear to replace my worn one, visually I could not tell the difference outside of counting teeth.
 
michael-king said:
Being that far out.. and the fact the trans has been changed I would suspect you really need to change the speedo gear on the trans. Also what wheels/tyres are you running? If the over all rolling radius is different from the stock setup you are also going to get percentage errors.
Click to expand...


Your problem is most likely in the speedo and not the gears or cable. The way the speedo works is the indicator needle is coupled to the gear drive by a magnet pulling on a cup shaped reciever ( I'm sorry that the best I could come up with) as the magnet gets weak the coupling is weaker and the indicator registers less. Your speedo just needs to be rebuilt and there are many shops out there that can do the work.

Moondoggie
 
Moondoggie said:
Your problem is most likely in the speedo and not the gears or cable. The way the speedo works is the indicator needle is coupled to the gear drive by a magnet pulling on a cup shaped reciever ( I'm sorry that the best I could come up with) as the magnet gets weak the coupling is weaker and the indicator registers less. Your speedo just needs to be rebuilt and there are many shops out there that can do the work.

Moondoggie
Click to expand...

If I read correctly, the speedo is indicating FASTER than actual speed. I am not very up on instrumentation, but it seems to me that if the magnet was weak, the speedo would be reading LOWER than actual speed, not HIGHER. I would certainly check the speedo cable gear and the gear on the tranny. Also, if the speedo is reading FASTER than actual speed, it seems to me that having a larger diameter wheel and tire combination would not be the cause, since the car would be going faster than the speedo is registering. Another potential source is the spring in the speedo that counteracts the pull of the magnet. Maybe it has weakened over time and does not resist the magnet enough.

My speedo suddenly started reading way too high just before it gave out. The cable stuck too far into the back of the speedo, causing the rotating part to contact the stationary part. At times, the speedo was reading 120 mph while I was still in first gear. But it was not consistant, such as what you are seeing with your speedo. Paul's Speedo in Redondo Beach is where I took my gauge and got it sorted out.

Good luck with yours!

David
 
HolyCat said:
If I read correctly, the speedo is indicating FASTER than actual speed. I am not very up on instrumentation, but it seems to me that if the magnet was weak, the speedo would be reading LOWER than actual speed, not HIGHER. I would certainly check the speedo cable gear and the gear on the tranny. Also, if the speedo is reading FASTER than actual speed, it seems to me that having a larger diameter wheel and tire combination would not be the cause, since the car would be going faster than the speedo is registering. Another potential source is the spring in the speedo that counteracts the pull of the magnet. Maybe it has weakened over time and does not resist the magnet enough.

My speedo suddenly started reading way too high just before it gave out. The cable stuck too far into the back of the speedo, causing the rotating part to contact the stationary part. At times, the speedo was reading 120 mph while I was still in first gear. But it was not consistant, such as what you are seeing with your speedo. Paul's Speedo in Redondo Beach is where I took my gauge and got it sorted out.

Good luck with yours!

David
Click to expand...

David,

While people often fit larger wheels their speedo still under reads. This is because the rolling radius with the lower profile tyres is often smaller than the original overall rolling radius of the original 13" wheels and tall side wall tyres. the originals had a 22.38 in dia...
 
Thanks for the posts. A couple of things. The tires on the car are 175/80R13 with a height of 24.0 inches, taller than stock. The bigger tires should I believe cause an problem opposite to what I have. I also did try another speedo graciously loaned to me by another CAT member and it also read high but not nearly as bad. True speed 40, my speedo 55, other speedo 45. 45 I could live with. I am thinking that getting the speedo rebuilt is the first thing on my list. Any recommendations?
 
I guess it depends where you are located.. David has given you a recommendation and was happy with his results.. then there is MoMa.. they might have split into 2 companies at some point recently? Some local guys should be able to get you the details.
 
Another Smith Gauge Repair Shop

I guess Redondo Beach, CA, is a little far to travel to from the Chicago area just to get your speedo fixed. Michael mentioned MoMa for repair of Smith gauges. I don't know their current status, but in the past oOthers have reported good luck with them. A few years ago, Nisonger rebuilt my clock (via mail order) and I was happy with their work. You might check some of the other threads about instruments - perhaps other postings might have something about gauge repair shops closer to you.

David
 
Once You Get Your Speedo Fixed.....

Does your car still have the original 16-tooth gear (Mk Is) at the end of your speedo cable instead of the 17-tooth version (Mk IIs)? With 16 teeth instead of 17, your speedo would read about 6% high (17/16 - 1) assuming the tranny gear is what came on the Mk II trannies. However, your taller than stock tires would make your speedo read low. Using the 22.38" diameter Michael quoted and your reported 24" actual for your car, your speedo would read 7% low (1 - 22.38/24). With a 16-tooth gear and 24” diameter tires with a Mk II tranny, your speedo should read about 1% too low [(22.38/24)*(17/16)]. That is pretty good accuracy!:D

David
 
The tranny gears come in 6 (green, MKI), 7 (yellow, MKII) and 8 (brown) teeth. The speedo cable drive gears range from 16 to 20 teeth.

For the MKI 6/16 combination:
16 17 18 19 20
6 100% 106% 113% 119% 125%
7 117% 124% 131% 139% 146%
8 133% 142% 150% 158% 167%

In mph:
16 17 18 19 20
6 40 43 45 48 50
7 47 50 53 55 58
8 53 57 60 63 67

For the MKII 7/16 combination:
16 17 18 19 20
6 86% 91% 96% 102% 107%
7 100% 106% 113% 119% 125%
8 114% 121% 129% 136% 143%

In mph:
16 17 18 19 20
6 34 36 39 41 43
7 40 43 45 48 50
8 46 49 51 54 57


So if your speedo is reading 55 at 40 mph, you are off by 38%. The ideal combination in your case would be a 8 tooth tranny gear and a 19 tooth speedo gear (36%). The problem with changing the tranny gear is you have to remove the drivetrain to take off the tranny tailshaft. The easiest change you could make is to change the cable gear from a 16 tooth to a 20 tooth for a 25% (10 mph) reduction in speedometer reading to 45 mph.

BTW, having a MKII tranny will not make a difference if you only change the cable gear. It is still only a 25% change.

Go to

http://4speedtoploaders.com/shopsite_sc/store/html/speedometergears.html

Gary
 
Ahetzel,

I have a very similar problem with my speedometer reading high. However the odometer is relativity accurate which suggests that my speedo is out rather than having incorrect drive gears fitted. How accurate is your odometer?
 
Hi Arnie,

One possibility is that you may have a non-standard 2:88 rear end ratio.

In the old CAT Shop Notes it says you need an 18 tooth speedo pickup gear to compensate for the faster turning rear wheels with a 3.54 rear end ratio. A 3.54 ratio is 1.23X faster than a 2.88 ratio. And, you now know your odometer is reading 1.25x too fast.

So, in a variation of the info Gary provided, trying an 18 tooth pickup gear would seem like a good idea. FWIW, that used yellow pickup gear I showed you back when has 18 teeth. You are welcome to give it a try is you want.

Just for grins when its cold outside and you have nothing better to do, you might put the Tiger up on your lift and see how many drive shaft rotations you get for one wheel rotation with the car in neutral.:)

Gene
 
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Another FYI

Just a note to remind all that there is a left OR right driven gear to pay attention to when swapping out the ratios. At least twice a year I deal with owners who have stripped out one or both gears by installing a mismatch. 2-1/2 cents
 
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