3.90 Rear End Ratio

[dcty124]

Just changed the rear end ratio on my '76 124 Spider from the original 4.30 to 3.90:1 (1756cc, 5 Spd.)The speedometer has been accurate up to this point. It sounds like I'll need to change the speedometer drive assembly from the 14 tooth to the 15 tooth of the 1978 and later cars. It also sounds like I'll need to change the worm-gear collar on the transmission output shaft. Does anyone know where these are available? Does anything else need to be changed? The transmission is out of the car, so now is the time to do it. Any other ideas?

[fiat218]

Sounds like a lot of workn

[vandor]

OK, first: are you SURE the speedo was accurate? That would be a first
Usually they are ~5% slow. So changing the rear end ratio by 10%, now it would be 5% fast. The only way to tell is either with a GPS, or with a stopwatch.

If you want to replace the speedo drive in the tranny, then yes, you need both pieces, the drive gear on the shaft inside the trans, and the matching speedo drive. There are a lot of vendors that have used parts. There is a guy whose username is BEEK on here, he has used parts. Also try Aspen Imports in Colorado, Joe's Fiat Service in California, Auto Italia in California, Performance Apex in Washington. Google them to find phone numbers.

[dcty124]

Thanks for the tips. Think I'll just leave it as is and recheck it with GPS once it's all back together. Maybe I'll have the speedometer recalibrated if need be.

[vandor]

You can get custom made little gizmos that change the speed of the cable to make the speedo accurate.

[fiat218]

You can get custom made little gizmos that change the speed of the cable to make the speedo accurate.

And what would that be? And where do you get them

[vandor]

Sorry, I don't know what it's call. A friend had it made for his Spider, but he passed away a few years ago, so I don't know where he got them. If I were looking for them I'd look in Hemmings Motor News, or get creative with Google.

[fiat218]

Sorry, I don't know what it's call. A friend had it made for his Spider, but he passed away a few years ago, so I don't know where he got them. If I were looking for them I'd look in Hemmings Motor News, or get creative with Google.

Ok thanks. I thought maybe you sold them

[baltobernie]

When our cars were new, all automobile manufacturers built a little "fudge factor" in to their speedometers. That's all changed with modern electronic speedos, and virtually every modern car is dead-on.

While our speedometers are (at best) a couple of percent optimistic, the odometers are quite accurate, providing you have tires that are the correct diameter. At a rally last year, we were consistently within one mile showing, over stages 70+ miles in length.

Devices like these are useful if you are using non-standard running gear, such as an auto rear end with a manual tranny, or tires not 23.1" in diameter. Just be aware that if you re-gear your speedo to your personal axle-tire setup, the odometer will be a couple of percent off. (OTOH, you could do what my buddy John does; a small "cheat sheet" that shows RPMs at the three most prevalent speed limits )

[vandor]

>When our cars were new, all automobile manufacturers built a little "fudge factor" in to their speedometers. That's all >changed with modern electronic speedos, and virtually every modern car is dead-on.

The new 500 is VERY far away from being spot-on
What I heard is that in Europe laws are a lot stricter, so European cars have more of a fudge factor, to guarantee the speedo never shows more than actual speed. I remember a car magazine testing a GM product in the late 80s or so with a digital speedometer, and the thing rear ~15% slow!!

[twblanset]

I think the 3.9's only came in the automatics and the speedo drive gear from the auto won't fit in any year manual. I switched my '79 from an auto to a manual and kept the 3.9 rear end. If I remember correctly the speedo drive gear mechanisms were completely different from each other. I remember spending a lot of time trying to figure out a solution and the only one I found was to have the speedo itself recalibrated (which was really expensive). Ultimately I gave up and I just live with a speedo that reads slow. I'm not positive on this, but I'm pretty sure.

[MrWampus]

Interesting. My speedo and odo were both about 4.5% off. Both read faster than actual. I checked the odo on a 10 mile stretch of interstate by watching mile posts and checked the speedo with a GPS. Both were about the same % off. So I bought new wheels from Mark (15" silver Competizione 6.5 wide) and put Yokohama S. Drive (205/55/15) tires. Turns out the tires are 23.8" in diameter which is right about 4.5% bigger than stock and now the speedo is dead on. I haven't checked the odo but I would assume that since it's driven by the same cable, it should be right on target too.

The only issue I have is some slight rubbing on the right front on the inside of the wheel well liner on a lock turn left. I didn't take into account that a wider tire would have a wider diagonal profile (corner to corner) than a narrower tire the same diameter. The left side is OK though, so I may just need to do a little bit of "NASCAR adjustment" on the fender (i.e. bend it out a little with a crow bar. )