Speedo Calibration???

[BlueRidge1]

Since we're on the subject of angle drives and speedometers I have a question. I replaced my angle drive but are there differnt ratios /ubbthreads/images/graemlins/confused.gif /ubbthreads/images/graemlins/confused.gifto compensate for rearend ratios, mine has a 354 gear no OD, Say if I had a 410 gear would I have to buy a different angle drive or is this calibration made elsewhere??? Bob

 

[Dave Russell]

As far as I know, the drive pinions are not easily selectable. They have a fixed ratio of 2.5 to one. Calibration changes are done in the speedometer head itself. You will see a number marked on the speedo face which gives the revolutions per mile that it is calibrated for. Somethig like 1325 for a 4.11/1 axle ratio.

A typical calibration number:
Axle ratio = 4.11
Speedo drive ratio = 2.50
Rolling circumference of tire = 6.55 ft, on the ground. Roll the tire one complete revolution & measure the distance.

1 mile (5280 ft) divided by 6.55 ft per revolution = axle turns per mile = 806
806 x axle ratio of 4.11 = 3313 drive shaft revolutions per mile
3313 divided by 2.5 speedo drive gear ratio = 1325 turns per mile (original speedo calibration)

For a axle ratio of 3.55 with the same tires:
1 mile (5280 ft) divided by 6.55 ft per revolution = axle turns per mile = 806
806 x axle ratio of 3.55 = 2861 drive shaft revolutions per mile
2861 divided by 2.5 speedo drive gear ratio = 1145 turns per mile (new speedo calibration)

Since the speedo drive is on the output shaft (behind the overdrive) presence of OD has no effect on the calibration.

A speedometer shop can recalibrate to your desired ratio.
D

 

[BlueRidge1]

Thanks Dave, Mine should be ok ,as it's the original speedo and rearend, just was thinking the angle gear might have a different ratio than my original. Bob

 

[Dave Russell]

Hi Bob,
If your speedo face is marked 1145 or close, it should be calibrated correctly.
D

 

[dklawson]

As Dave said a speedo shop can handle calibration but going from 1325 to 1145 turns per mile is beyond what can be compensated for in the instrument itself. The odometer is worm gear driven off the gauge input shaft while the needle is driven by inductive coupling against a spring. To make changes as big as described a speedo shop will first calibrate your instrument for the correct turns/mile and then fabricate for you a ratio box that must be mounted in the drive cable between the gearbox and the instrument. Figure about $100 for the ratio box + a couple of new speedo cables that you supply the shop. Calibration costs for the instrument can range anywhere from $100 to $200.

 

[Dave Russell]

[ QUOTE ]
As Dave said a speedo shop can handle calibration but going from 1325 to 1145 turns per mile is beyond what can be compensated for in the instrument itself. The odometer is worm gear driven off the gauge input shaft while the needle is driven by inductive coupling against a spring. To make changes as big as described a speedo shop will first calibrate your instrument for the correct turns/mile and then fabricate for you a ratio box that must be mounted in the drive cable between the gearbox and the instrument. Figure about $100 for the ratio box + a couple of new speedo cables that you supply the shop. Calibration costs for the instrument can range anywhere from $100 to $200.

[/ QUOTE ]
Hi Doug,
A ratio box "might" be the easiest way. There are two functions to change. The odometer & the speed indicator. For the odometer change, there is a wide variety of worm & ODO drive gears that can be interchanged to get the ODO very close. The combination of 20 & 57 teeth or 1140 turns per mile would be very close.

There are a couple of different magnet wheels available. This plus spiral spring adjustment should get the speed indication very close.

A "good" shop with a stock of old parts should be able to easily make this change since it would be only about 16%. If the shop is not capable, the ratio box is a choice.

In the above case, it doesn't matter since he already has the correct ratio.
D

 

[zblu]

Had to send mine back after being serviced/reconditioned as it was way out!, is close now. You would think that when servicing they could count the teeth on each cog!

 

[dklawson]

If a shop has old Smiths/Jaeger parts they could certainly handle the conversion you suggest. I question whether or not the expense of switching magnet wheels and pinion/worm gears wouldn't go beyond the cost of the ratio box. To keep the center distance correct, both the worm and pinion have to be changed as a pair requiring complete disassembly. I don't know how much it would cost to change a gauge's ODO gearing... it just seems like a lot of work (charged by the hour) to obtain an instrument calibrated for the right turns/mile. The benefit, of course, is one less part to fail (no ratio box).

 

[jtb55]

What is your car with a non od and a 3.54? Do you know the the gear ratios in the box ? I just had my spedo repaired by Neisonger and my spedo has 1325 on the face, Interestingly when I bought it it had 4.11 no od but I changer to 3.54 to race at Watkins Glen in June of this year thanks jtb

 

[BlueRidge1]

[ QUOTE ]
What is your car with a non od and a 3.54?

Not quite sure I understand your question, our car came stock with the 354 no overdrive. I'll have to check the number on the speedo and get back with you on that. Iam not sure what years were equipted with the 354, ours was built march -april 61. bob

 

[Dave Russell]

Hi John,
Since the speedometer drive is on the output of the transmission, it makes no difference whether it has OD or not. The original speedo is calibrated for 1325 turns per mile for a 4.11 axle. The new speedo calibration for a 3.54 axle would need to be about 1140 to 1170 turns per mile, depending on tire diameter. A ratio adapter would need to be 4.11 divided by 3.54 or 116% to correct for the change in axle ratios if the speedo calibration remained at 1325.
D