TR6 Speedo Experts: Troubleshoot question…

[Rocky_LC]

I have a Smith’s Speedometer in my car.

It is reading very closely to kilometers per hour, if I am doing 64 mph (measured via GPS) it reads 100.

It is clearly marked “MPH”, and the bottom of the dial says:

Smiths S/N 5324/65 1000

Any easy way to figure out if the issue is in the speedo, or the drive gear in the transmission?

Any suggestions as to how to re-calibrate or repair, other than putting a piece of masking tape marked KPH over the glass where the MPH legend is located?

Does the gear come out of the transmission easily? Is the gear marked for easy identification?

Moss used to sell a $6 speedometer gear (now out of stock) and does sell a $56 speedometer gear assembly. What’s the difference (other than $50).

Thanks -

Rocky

 

[CJD]

I would start by running a measured mile on the interstate and see what your odometer says for 1 actual mile. The speedo can be out of calibration, but the odometer is always the same. Once you have the odometer reading, we can tell if you have a gear difference or calibration difference.

 

[Sarastro]

The odometer runs directly off the cable, through gearing, so it is not subject to calibration. Since you have a GPS, you could compare the GPS distance indication to the odometer, and that should tell you if it's really set up for miles. This is something you'll want to know in any case, for maintenance intervals and such.

Also, is the indication proportional to speed? For example, if it reads 50 is the real speed now 32 MPH? Check a couple more points and see if they are proportional.

That said, I think it's unlikely that your indication would be consistently right for Km in a MPH speedometer. That's more than just calibration being a bit off. But if something were not working right, you would see more strange behavior, like it failing to return to zero when you stop (which would happen if the spring had come loose).

So, as John says, step 1 is to figure out if you have an MPH or Km speedometer. the we can go from there.

 

[Rocky_LC]

I will give both of those tests a go in the next week, and report back.

Appreciate the responses!

Rocky

 

[Kaybass66]

I have a Smith’s Speedometer in my car.

It is reading very closely to kilometers per hour, if I am doing 64 mph (measured via GPS) it reads 100.

It is clearly marked “MPH”, and the bottom of the dial says:

Smiths S/N 5324/65 1000

Any easy way to figure out if the issue is in the speedo, or the drive gear in the transmission?

Any suggestions as to how to re-calibrate or repair, other than putting a piece of masking tape marked KPH over the glass where the MPH legend is located?

Does the gear come out of the transmission easily? Is the gear marked for easy identification?

Moss used to sell a $6 speedometer gear (now out of stock) and does sell a $56 speedometer gear assembly. What’s the difference (other than $50).

Thanks -

Rocky

This is easy to solve. Contact Powl Speedometer in Lancaster Pa

I have a Smith’s Speedometer in my car.

It is reading very closely to kilometers per hour, if I am doing 64 mph (measured via GPS) it reads 100.

It is clearly marked “MPH”, and the bottom of the dial says:

Smiths S/N 5324/65 1000

Any easy way to figure out if the issue is in the speedo, or the drive gear in the transmission?

Any suggestions as to how to re-calibrate or repair, other than putting a piece of masking tape marked KPH over the glass where the MPH legend is located?

Does the gear come out of the transmission easily? Is the gear marked for easy identification?

Moss used to sell a $6 speedometer gear (now out of stock) and does sell a $56 speedometer gear assembly. What’s the difference (other than $50).

Thanks -

Rocky

This is easy. Download a speedometer app on your phone. Take the car up to 60 mph on the app and record the speedometer reading. Then, contact Powl’s Speedometer center in Lancaster Pa at 717-898-2552. Send them your speedometer cable and the readings you recorded and they will do the calculations and custom make a dandy little gearbox that will splice into your cable just under the brake booster in the engine compartment. You will need to know the distance from the back of the speedometer to the engine compartment,(about 12 inches if my memory serves me) so they know where to cut it. They will install new ends to fit the gearbox to the cable. I’ve had this in my TR6 for ten years and it’s wonderful Bill Stumm

 

[CJD]

Well, that's an option. But!...that will only apply if he has a speedo for a 3.73 gear ratio and is running 4.11 gears. If the speedo gear is correct and the speedo is out of calibration, then adding a compensation gear will throw off the odometer.

We need to see how his odometer reads before we can continue the diagnosis...

 

[Rocky_LC]

OK, Got some data....

I have about a 15% error on my odometer reading.... If I travel 1.0 miles, my odometer reads 1.15 miles (based on the "tenths" digit of my trip odomoter. This held up over longer distances.

Here's a plot of my indicated vs. actual speed. The slope of the indicated speed line seems different from the "actual" speed, so it's not a precisely calibrated straight MPH / KPH error....

Anyway - heres' the data I collected...

GPS Speed	Indicated Speed
40​	64​
50​	78​
60​	92​
64​	100​

Comments & Suggestions welcome....

Rocky

 

[CJD]

The TR's were offered with 2 gear ratios...4.11 and the most common 3.73. Taking your last data point:

64mph X 4.11/3.73 = 70mph. This indicates your speedo is out of calibration, even greater than just changing the differential gear.

Now for the odometer... 1.00m X 4.11/3.73 = 1.10miles. This is still off from your readings, but not far off if you have a 4.11 tranny take off gear with 3.73 differential gears. The .05 difference could be from lower profile tires than original.

So...

My guess is your transmission was once driving a 4.11 differential, and still has the 4.11 speedo gear. This accounts for about 10% of your error and will bring your odometer much closer. Even with the correct 3.73 speedo gear, you will still be off significantly on your speed. This requires a calibration of the speedo, which should not be that hard, unless the clock spring has broken.

You have the option of sending the speedo off for calibration, or I can explain how to do it yourself if you are interested.

 

[Rocky_LC]

Thanks, John -

Looks like my car (a TVR Vixen 2500, which uses the TR6 engine, transmission, rear end and drive/halfshafts) was originally spec'ed with 165 SR-15 wheels (diameter 25.4") and currently has 195-65 R15 on it. So tire diameter is within about .3" in diameter of the original design - my current tire is a little taller (looks like it's within about 1%). Current tire is about an inch wider, but that doesn't matter.

The rear end ratio in the Owner's manual is 3.45 : 1.

I guess I probably need to confirm my rear end ratio, first.

So - are you saying I should consider a different gear in the transmission speedometer drive, as well as calibration of the speedo?

Thanks -

I would be interested in learning about Speedo calibration.

Regards -

Chuck

 

[Ricks74]

My guess is that you have the exact opposite issue I have in my MGB. Changed to OD gearbox and the speedo reads low. Smith made both 1000 and 1280 tpm (turns per mile) speedos. The 1000 is for OD and the 1280 is for non-OD. Switching out the speedo to a 1280 tpm then would correct the issue. If the face of your speedo is compatible with what I have perhaps we could trade?

 

[Rocky_LC]

Rick…. I’ll consider it as an option as I work through this engineering problem…. Thanks!

Continuing (to gain knowledge): I jumped under the car this morning, and determined I have an “open” diff (not posi- or limited slip). So following my instructions, I put one wheel on the ground, and rotated the other.

Luckily, a strategically aligned zerk fitting on the driveshaft allowed me to clearly measure ~1.75 driveshaft turns for each wheel revolution, confirming a stock 3.54 : 1 rear end in the car.

Now maybe I have to do the 52’ 9 1/2” speedo cable trick?

Rocky