engine and drivetrain 5 speed conversion - speedometer problems

[duckdudess]

I recently installed a 3 piece Getrag 245. The speedometer reads 10% high which wasn't a problem with my old 4 speed. Tire size is 195-60-14 and was within 2-3% on the speedo before the transmission swap. Any ideas on the cause or remedy ?

[Preyupy]

Did you change the differential at the same time?

[duckdudess]

Yes indeed, I did. From a 3.64 open to a 3.91 LSD.

[Guest_anonymous]

Speedometer calibration is based on the diff ratio matched with a corresponding speedo gear ratio which is stamped at the back of the speedo. To achieve a more acurate speedometer reading you will need to change the speedometer to match the diff.

Search is your friend.

[Preyupy]

Yep, that is your problem. You can have your speedo recalibrated, north Hollywood speedometer can do it.

[duckdudess]

You know, I did do the search...but I was looking for things related to the 5 speed . Once I searched on speedo and differential, it was obvious that this is well known problem. Thanks for pointing me in the right direction.

[Guest_anonymous]

something to consider, '76 cars were equipped with 3.9 diffs and corresponding speedos...which were inaccurate due to the face of the speedo sitting with a different orientation, some had recalled overlays put on. While it might be cheaper to find a correct geared speedo than to recalibrate it, you might not be any better off on the accuracy of the readout due to the face orientation.

Something like this. I'm sure I'll be corrected on the specifics

Food for thought...

Daron

[bnam]

I looked for a w=1.393 speedo for a while then bought a new one from W&N. Probably cost approx the same or less to get a new one as to get one recalibrated.

Byas

[duckdudess]

I wonder if they have the "silver dollar" speedos? I checked their website and it wasn't obvious. I'd like to keep the cluster looking original, if possible. Will also check the recalibration guys in California. Thanks everyone for the tips.

[Slavs]

Yep,

That's a common mistake. Some of the guys here making exagerated mileage claims along wildly optimistic acceleration and speed claims have just such set-up.

3.64/3.90 = .93 So your speedo reads 7% too high with perhaps a 2-3%speedo error, this could easily translate to 10%. I once installed a 2002 speedo calibrated for a 3.64 diff, on to a 1600 with a 4.11 diff. There was a huge optimistic speedo rating 3.64/4.11 = .89 or 11% too optimistic.

Pierre - Mathematicus, you can check my math if you're out there and watching.

Slavs

[David Layton]

My 73 has a 76 speedo W=3.9 which works well with the 245 and LSD.  I also have the correct Modell 71 speedo.  Is it possible to pull a gear from the 76 to make the earlier speedometer a W=3.9?

[Son of Marty]

Wouldn't it be easier to switch the faceplates than shift all the guts between them?

[jimk]

1 hour ago, David Layton said:

Is it possible to pull a gear from the 76 to make the earlier speedometer a W=3.9?

Possible, but the speed will be off because the needle is calibrated with a spring setting and the revolutions of the cable per mile. That's what the w number means.

[David Layton]

It appears my options are to have a shop recalibrate the older one or do a faceplate swap.

[Rodolfo Lasparri]

Or you can do this - retain your original Speedo with the below which converts for you (no calibration of your speedo needed) and have the cable cut to the length you need to the box - +1 for Palo Alto - they can do it and are awesome.

 

https://www.speedhut.com/ecommerce/product/2585/SpeedBox™-GPS-VSS-to-Mechanical-Drive-Speed-Converter