RPM's at 55

WCM · Post by **WCM** » Fri Jun 08, 2018 9:13 am

Good morning,

I found out that someone changed the gear ratio on my 1981 Spider. Also, I have no speedometer but the RPM gauge works. I notice when I shift from 4th into 5th gear, it feels like can shift into another gear at 55. I plan on having the rear end fixed but in the mean time, I'm just curious where I should be at when I'm at 55 MPH. Driving in the city is no problem, I never get out of 4th gear. I want to take it out on the expressway but a little nervous about the high RPMs. Any feed back would be appreciated.

Thanks,

WCM

TX82FIAT · Post by **TX82FIAT** » Fri Jun 08, 2018 10:56 am

I would think you would be just at the 3000 RPM range at 55 mph with standard set up. The difference between 4th and 5th is not nearly as big as we are use to and the car will run at much higher RPM's on the highway. Assume you have standard 14 inch wheels. Larger wheels will help a little and they do have a larger 5th gear or trans swap available. In a Honda, yes, it is time to shift. However, In a Fiat you should get use to running the gears up a little before shifting as well as running at a slightly higher RPM on the highway.

chrisg · Post by **chrisg** » Fri Jun 08, 2018 11:02 am

it's usually only a 500rpm difference between the two. I don't know the actual speed, but in Fiats I'm used to a 4-5k RPM at normal interstate speeds. The nice thing about the Fiat engines is that's fine.

WCM · Post by **WCM** » Fri Jun 08, 2018 11:23 am

Thanks for the help...

WCM

focodave · Post by **focodave** » Fri Jun 08, 2018 2:58 pm

A little late to the party, I know, but:
55 MPH = 3100 RPM
70 MPH = 4000 RPM

5th gear is only about a .88 reduction from 4th gear on the factory gear box in your '81

WCM · Post by **WCM** » Fri Jun 08, 2018 3:43 pm

So, it's ok to run the RPM's that high? I've never owned a foreign sports car especially this old. Driving clutches for so long, I never encountered anything like this. Especially from 4th - 5th. All of this is new to me. Thanks for the heads up.

focodave · Post by **focodave** » Fri Jun 08, 2018 3:47 pm

Yes, absolutely.
These engines are made for that.
They are very rev-happy.
The 2 litre engines are of a longer stroke than the smaller earlier engines, but they are still happy at high RPM's

WCM · Post by **WCM** » Fri Jun 08, 2018 4:08 pm

Nice, Thank you...........

WCM

chrisg · Post by **chrisg** » Fri Jun 08, 2018 4:32 pm

usually there is not a whole lot that's going to happen with a stock engine above the low 5k RPMs, but 3k-5k is pretty much the stock driving range. With some modifications, you'd expect to still be making power beyond that range and all of it is fine for a healthy twin cam. For me, the fact that these sometimes quite unassuming cars call to be driven with vigor (and respond well to it) is part of their charm.

All that said, the quest for lower RPMs on the interstate is a thread that's been alive relative to these cars since the dawn of the Internet. Choices seem to be final drive or gear box changes (4.3:1 to 3.9:1 for '77-back cars is an option, 3.9:1 to 3.6?:1 from an automatic is an option for Spider2Ks, or I believe there are some custom gear box options these days also), getting comfortable with higher RPMs on the Interstate, or (my favorite) driving backroads whenever possible.

WCM · Post by **WCM** » Fri Jun 08, 2018 5:09 pm

Well talking to my brother that previously owned the car, told me the guy he bought it from blew the original rear end out. Instead of waiting for a new one he had a higher ratio rear end installed. My brother owned the car since 2005 but really never did allot of freeway driving with it. This is why I'm a little hesitant about the high RPM's.

TX82FIAT · Post by **TX82FIAT** » Fri Jun 08, 2018 5:46 pm

So, how many RPM's are you running at 55 mph?

chrisg · Post by **chrisg** » Fri Jun 08, 2018 6:10 pm

4.3:1 on a 2L may end up a higher RPM (fill in #) than you'd like & may render 1st a little short feeling of you are used to a Spider 2k, but it won't really "hurt" anything. I put many miles on a 74 that had a 2L and putting earlier rear ends in late cars wasn't uncommon back "in the day" for maybe a sportier feel and also because the lore (true? I don't know) was the early style was stronger.

Now I have a car with a 2L and 3.9:1 and I think I like its manners better.

WCM · Post by **WCM** » Fri Jun 08, 2018 7:35 pm

I do not have a speedometer but last weekend I was guessing I was doing around 45 miles an hour at 3000 RPMs

zachmac · Post by **zachmac** » Sun Jun 10, 2018 9:24 am

I have a 78 with unknown rear gears and transmission as to year (my engine was a 2l swap and I don't know if tranny or rear was also swapped. In any case in 5th gear I am at almost exactly 20 mph per 1000 rpm. 60 mph = 3000 rpm. 70 = 3500, etc. I believe I calculated on day that means I have a ~3.90 rear end.

WCM · Post by **WCM** » Sun Jun 10, 2018 10:57 am

So if I’m running 3000 rpm at 45, that seems pretty high. Yes?

RPM's at 55

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