Overdrive

[scampbel98]

Anyone found a way to improve the overdrive 5 speed? better gear? 2speed rrear end? better rear end gear?

[v6spider]

The rear-end ratio used for the manual trans from 74-78 is 4.30 to 1. The ratio for the auto tans models is 3.58 to 1. The gear ratio for the 79 and up is 3.90 to 1. According to my FIAT service manual...I don't know what pumpkin fits into what housing though...You could adapt any rear-end that has the right wheelbase. My V6 driven spider has a rear-end out of a rx-7 GSL installed in it. It is just a matter of fabricating the parts to fit it.

Cheers!
Rob

[124ADDHE]

The 3.9 rear end changed the enjoyment of hwy cruising for me in a very positive way! The 4.3 had me annoyed at times, turning over 5000 rpms on the hwy.

[baltobernie]

http://www.fiatspider.com/f08/viewtopic ... lit=+ratio
If you already have a 3.9 diff, you could swap to an auto trans rear, as Rob suggests. But you'll need a well-running 2-litre engine to pull this ratio without being blown away at stop lights by concrete mixers. Frequent poster Csaba modified his transmission with a Euro-only Fiat-family 5th gear, but it required substantial machining to work properly.

I've solicited a dozen Internet sources of "custom automobile transmission gear manufacturer" with RFQ's. All have declined to manufacture what I consider to be a saleable product. Maybe it's time to give up on finding an American manufacturer, and expand my search.

Another overlooked factor in highway revs is tire size. If your speedometer reads more than 3100 RPMs at 60 MPH, your tires are too short.

[124ADDHE]

I dont know about the stock tire size resulting in 3100rpms, I run 195/65/13's which are much taller than the stock 155/75/13 and I am turning 3700 rpm's @ 100kmph (speedo is verified with my gps). Maybe you have an even larger diameter tire? Or are you speaking of the automatic rear-end should result in this lower rpm?

[baltobernie]

I think the modern tire closest to the 165R13 is somewhere between the 155R80 and the 175R75, with a circumference of about 73". So your 195R60 (70") are 5% too short, accounting for the higher RPMs.

I should amend my earlier post to say that 3100 RPM = 60 MPH with the 3.9 rear. Is it possible you have the 4.33, as fitted to 1977 and earlier Spiders?

[Daniel]

Frequent poster Csaba modified his transmission with a Euro-only Fiat-family 5th gear, but it required substantial machining to work properly.

Hi Bernie

Did Casaba post info on what he did ?

[v6spider]

baltobernie is correct... if your spider is using the 4.33 ratio then a 3.90 from a 2l might be the ticket otherwise if you have the 2l then try the auto trans rear-end. 3.58 may be too tall for your spider if your are running the 1608 or 1800. bigger tires effect the ratio as well but be aware that these mods(including bigger tire diameters will cause your speedometer reading to be inaccurate. Which you may have to make up for with different speedometer drive gears in the transmission.

Rob

[scampbel98]

I'll have to crawl under to see what the current pumpkin/case is. At 75mph I run about 5000 RPM and that 5th gear is just plain useless.

It's a 1980 carb 2L (ish) Federal car. PO had done some other stuff to it already. It's currently a 2L head on a 1.8L block with 8mm HC pistons (love the new rebuild).