Differential gear ratios and cases.

[shastaboat]

Hope someone can help. I have a1979 124 Spider 5 speed that runs real strong. Got rid of the stock intake manifold and installed one from an earlier model with no smog installed at all. I registered it as a classic and it passed the registration smog with flying colors. (Go figure). Single plane off a 1975 if I remember right. The car only has 118K total miles and the current installed engine was the one in when new and actually only has about 60K on it. Long story...anyway I've installed a Weber replacement carb and got rid of the junk stock carb. The engine and car run very strong and pulls 5th gear very well. I want to lower my 4000 rpm at 70 mph to 3500. I understand the differential gear ratio from an automatic transmission model was 3.58 to 1. Did anyone ever make even taller gears say 3.40 to 1? Are either the 3.58 to 1 ring and pinions availalbe or an aftermarket 3.40 to 1 or close ever manufactured and currently available. I'm pretty sure my gas mileage would increase overall about 20 to 25% with this change, and top speed as well. I Will also need a correct speedo gear if available. I have a very good extra complete rearend from my parts car that I will change the ring and pinion and if I'm not happy I can always reinstall the original complete rearend. Thanks for any advice...Brent Johnson, Reno, NV shastaboat@yahoo,com

[Exit98]

Brent,

On my 76 I went from a stock 4:10 to a latter 3:90 rear. Just swapped out the whole assembly. Went from about 4200 at 70 MPH to around 3750. Couldn't be happier with the switch. Unless you have a very hot 1800 motor the auto trans rear may be just too much.

Here's a previous thread, there are others, try the search:

http://www.fiatspider.com/f08/viewtopic ... ilit=+diff

[FiatMac]

See my post in the thread I have attached below for a chart of speed in each gear with 4.30, 3.90 and 3.58 ratio rear ends. These are calculated with 195 60 14 tires. With 185 60 14 tires the rpm will be about 102% of what is in the chart.

Have you verified your speedometer with another car or gps. Your speed vs. rpm looks like a 4.30 rear end. It is possible that a transmission from a car with a 4.30 rear end was installed in your car and the speedometer gear was not changed. If that is the case, then you are actually traveling at closer to 80 mph when you are indicating 70mph. The '79 manual transmission car was originally equipped with a 3.90 ratio rear end.

http://www.fiatspider.com/f08/viewtopic ... &sk=t&sd=a

The 3.58 ratio rear end is the lowest ratio ever supplied. The only way to get lower would be to install a complete axle assembly from another car, and that would be a big deal. A 3.58 ratio will get you to 3200 rpm at 70 mph. I really don't think you would want to go lower than that as you would be well down in the torque range for this engine.

[divace73]

Thought I'd pipe up here just as an example
I have a 2L FI with a 131 brava box (so ratios are a little different to the 124 box) and a late 132 (which I think never made it to the states) 3.4 ratio diff.
When cruising at 110 kph (68.4 mph) I am just under 3000rpm, this is great for freeway driving but not so much when I'm on the track. Next week I will be installing a 3.9 ratio torsion diff, I don't drive on the freeway all that much anymore and was even contemplating 4.3 for quick acceleration...
here is a sample with the current 3.4 set up and speed measured with GPS