Topic: Adjustment of speedometer for changed final drive - OT, OTS, OTR

[grt]

I have a query about the way in which Abarth ensured that the speedometer read correctly on the OT FIAT 850 based cars when the final drive was changed.

My FIAT Abarth OTR 1000 has the 9/37, 4.111 final drive and the speedo shows the correct road speed.  Does anyone know how Abarth made the adjustment to compensate for the different final drive?  Was the speedo modified or were the little nylon speedometer cable drive gears in the gearbox changed?

With thanks

Graeme

[evannice]

Hi Graeme,

I'm sorry that I don't have the answer to your question, but I have pondered it as well.  In the past I have posted this question on the 850 yahoo group and on mirafiori.com but have not heard the answer.  A few assorted thoughts on the subject:

First, I do know the 124 speedo drive gears were available in different numbers of teeth and were identified by color.  All the 850 speedo drive gears I have seen were ivory colored plastic, but there was one on ebay last week identified as being for a spider, and it was green.  Makes me think there may have been more than one color and therefore more than one number of teeth.  (But maybe they all started out green and lost the color after years inside the transaxle).

Secondly, there is a Road and Track test from 1969 of a USA spec 1300/124 (3.89:1) where they opined that the speedo drive gear must not have been changed because the speedo read very much too high in their testing.  Seems to me that a numerically lower rear end ratio would make the speedo read too low.

And lastly, a data point: I have a stock 4.875 (8/39) transaxle whose pinion gear has 10 teeth on the speedo end meshing with 26 teeth on the white plastic speedo output gear.  I would be interested to know if others can tell us if they have anything different.

Best regards,
Eric Van Nice

[slim4936]

Hi guys,

I am currently seeking the same information about these speedo drive gears but stumbled upon the problem from the other side.While trying to find a lower CWP for the 850 gearbox (to no avail) I've learnt that Simca used a slightly modified version of the Fiat gearbox in the "rallye & 1000" models. Having larger engines they used a couple of different final drive sets that fit the 850 g/box.

I've managed to find a 8/35 (4.375) CWP from a Simca but it was serperated from the rest of the gearbox. Did Simca have different speedo drive gears or was the speedo different???

Look forward to any info anyone can offer

Cheers, Toby.

[grant gauld]

Fiat and Abarth and Simca changed the plastic drive in the gearbox end.

I pulled apart a Fiat 850 box that had a simca R/P.The simca R/P is slightly longer in physical length than stock 850,but does fit with patience and forethought.The Abarth versions (As my cars have) are the correct length.The 'bitsa' box has a simca speedo drive gear to suit the longer simca R/P and Fiat 850 everything else.

A lower R/P (8/35 over 8/41 eg) will give a higher indicated speed at the same RPMs,(even if the drive is still stock,it just will not be accurate).But when we change tyres it all changes again....and like Pantdinos replica someone may have just stuck on new speedo numbers.(Albeit easier in a 600 than an 850).


Grant Gauld

[viotti600]

I pulled apart a Fiat 850 box that had a simca R/P.The simca R/P is slightly longer in physical length than stock 850,but does fit with patience and forethought.

 Minor correction: the Simca 1000 pinion shaft is slightly shorter than the Fiat 850 version, not longer. That's why the large retainer nut will slightly hang off the end of the pinion shaft when this conversion is done (fitting the Simca ring & pinion into the Fiat gearbox).

http://www.abarth-gmr.be/forum/index.php/topic,2293.0.html


-JS.

[grant gauld]

Yep,it was early in the morning here.
The Simca shaft IS shorter.My bitsa box had the large final nut shortened somewhat which is an option when making this conversion.
Grant

[evannice]

A follow-up data point :  I pulled the speedo pinion gear and plastic gear out of the 1300/124 transaxle today (9/35 or 3.89:1), and they are the same as the 850 sport.  10 teeth on the pinion drive gear, and 26 teeth on the speedo output gear.  This appears to show that Abarth made no provision for correcting the speedo when they changed R&P ratios.

[Pantdino]

A follow-up data point :  I pulled the speedo pinion gear and plastic gear out of the 1300/124 transaxle today (9/35 or 3.89:1), and they are the same as the 850 sport.  10 teeth on the pinion drive gear, and 26 teeth on the speedo output gear.  This appears to show that Abarth made no provision for correcting the speedo when they changed R&P ratios.

Old Italian speedometers were notoriously inaccurate.
I think the best thing is just to use a GPS / satnav unit, drive at the most important speeds, and have someone write down what the speedo indicates at those speeds.  Then mark those points with a Dymo labeller or equivalent.

As was mentioned, if you change tire size you'll have different readings anyway.

Jim Oddie

[Paul vander Heyden]

JIm.

There is another solution.  It is used all the time to make speedo corrections.  Go to the speedo shop in Newport Beach (just down the road from you) and ask for an "in-line" gearbox.

Paul