3.58 Gear Ratio, pre '79

[vandor]

The '76 is a 4.3.
The '78 is a 3.58 from an auto or a 3.90 from a 5 speed.

No, 1979-85 is a 3.58 from an auto or a 3.90 from a 5 speed. There was no automatic Spider in '78, and the 5-speeds had 4.3:1 gears.

[spiderrey]

That's pretty typical. Lots pf number rods jack the car up right there.

[maytag]

.....

[SpiderJim]

If anyone needs a good 3.9 rear and lives near reno NV let me know. I changed my 81 rear to the Auto rear end last year and just have the 3.9 sitting around and would like to get rid of it.... Free just pickup. And no I'm not willing to ship!. Email if interested. First come gets it. BTW... It is in excellent condition with no noise, slop etc. Also as the disk still on it ready to mount in. I just wanted to drop my Hwy RPM's.

[SLOSpider]

Still got that rear axle Jim?

[spiderrey]

Bump. Still there?

[Dawgme85]

I realize this is an old thread, but thought I'd chime in with a slightly different slant (though Mark has been alluding to it...).

The twin cam motor is designed to REV! It's not a V-8 that can loaf along at 1200 rpm on the freeway, it just doesn't produce that kind of torque. Slowing the engine down, for whatever reason, will also lead to some related problems, such as sluggish performance, plug fouling, etc.

I try to keep my spider ABOVE 3000 rpms for any distance driving, and generally shift above 6500 rpms. I can easily shift out of THIRD at 80 mph and 7200 rpm. It's HAPPY to cruise at 4-5000 rpms, all day long. These are SPORTS cars, not minvans or pickup trucks. If you want any Giddyap at all, the engine needs to be making some horsepower, which doesn't really happen until about 4000 rpm. One caveat, however, is that it helps to have a port-matched head and a 4-2-1 exhaust manifold (at least)! Anything (stock) newer than about 1973 is so choked on smog equipment, that it's far from Fiats original design intent.

Just remember, Fiat designed the Spider to run at 4000 RPM at 70 mph for a reason.

Let it rev, let it breath, be very !

[Tobi]

I have to agree with Dawgme85. My 81 has the 3.9:1 and I have 185/60 13 ( smaller diameter) tires installed and it's a blast. When I cruse at 3500 to 4000 rpms and step on the gas.....it makes me smile every time.
I also don't have the problem that I have to get into second gear on the intersection because I rarely use the first gear unless I start uphill. If anything at all it would be nice to have a longer 5th but since the next Autobahn is so damn far away

[LIQUIDLOUNGE]

The 5th gear ratio on these cars is ridicules. There is no advantage to turning the engine 4k @ 70 MPH while going down most hiways that I'm aware of. Maybe on the track there could be an advantage to to this 4th and a 1/4 gear, if you even consider this a sports car. Cute and somewhat nimble would be a better description of a Spider. 4000 rpms @ 70 only increases engine wear and burns more fuel. You got to remember, by the time the 1.8L motor hit these cars, the 55 mph speed limit was enacted which put the motor into a more acceptable rpm range at cruise.

[santafe308]

i have either a 72 -74 rear end in my 79. i like it ,the original was shot. but after the long hours to get it in but works very superb.the rear end i put in sat for 15 years but still had fluid in it but still replaced it. the front seal to drive line leaks from time to time but i just need to put more miles on it to loosen it up califorina smog is my enemy .......

[QuebecFiatSpider]

Hi everyone, after reading everything you all wrote about that rear differential ratio stuff, I'll post my reply...

I drive a 1980 Spider 2000 FI car. which is pretty much stock appart for the smaller diameter wheels 22.7'' instead of 23.7'' std. (see below)

So if I'm understanding right, I'm accelerating faster with this wheels & tires combination. Is that right?

I'm just getting back from a 1h00 drive with my spider on some backcountry roads (where the speed limit is around 90km/h) and I really had great fun driving that car. It's like that car was designed for that (instead of highway driving)

Since I bought that car only one month ago, I'm still Learning how that car behaves...But one thing I noticed is that this engine really likes to revs between 3,000 RPM and 4,500 RPM and also that it gives the better accelerating feeling also. In fact, I had great fun staying in 4th gear and reving that engine...

So, starting from what some of you wrote, I would have a 3.9 : 1 differential ratio.

What do I need to get my hands on, in terms of replacement parts, to improve acceration? (I really do not care about the engine reving higher RPM in 5th on the highway)

And just to make it clear, I totally agree with what Dawgme85 wrote.

On top of that, have I red right that the older differential assembly weights 20 lbs less ???

[gm404]

So, starting from what some of you wrote, I would have a 3.9 : 1 differential ratio.
What do I need to get my hands on, in terms of replacement parts, to improve acceration? (I really do not care about the engine reving higher RPM in 5th on the highway)

You need to convert back to the pre 77 axle with separate diff pumpkin inserted from the front, and can choose between a number of shorter ratios 4.1, 4.3, 4.44 or even go Lada and use custom 4.7, 5.1 or 5.3.

[baltobernie]

A well-running 2-liter injected Spider with the stock axle is a nice performer (for a 45-year-old car). The few percentage points advantage of slightly shorter tires does not provide noticeable improvement.

Before undertaking costly drivetrain replacement, I'd suggest a visit to the nearest chassis dyno, and ensure you're getting typical horsepower numbers. Driving another "known good" Spider of similar specs is another possibility.