RPM's at 55

baltobernie · Post by **baltobernie** » Sun Jun 10, 2018 2:15 pm

Yes, it would. At a steady speed on a relatively straight road, your smartphone or GPS should display a pretty accurate MPH. Even a 4.3 rear with standard-diameter tires would not rev that high. Thirty-five in an old car can easily feel like 45.

DieselSpider · Post by **DieselSpider** » Sun Jun 10, 2018 11:13 pm

A 165/80-13 tire has a sidewall height of 5.196850394 inches, total diameter of 23.39370079 inches and a circumference of 73.49347853 inches so it would take 862.1173098 revolutions to travel a mile and with a 4.3 rear axle and 0.88 fifth gear that would make for about 3265.959005 engine revolutions per mile. 60 mph would be at around 3265.959005 rpm more or less depending on tire inflation and wear.

For a simple calculation you can go with 54.43265008 rpm per mph in fifth gear.

WCM · Post by **WCM** » Tue Jun 12, 2018 7:47 am

Good morning. I was able to download a speedometer on my phone, that actually works pretty good. I will be taking the car out and do a test run to see where I'm at. I will keep you posted.

Thanks,

WCM

WCM · Post by **WCM** » Sat Jun 16, 2018 10:21 am

Update:

Ok, In 5th gear............

55mph = 3100rpm
60mph = 3500rpm
70mph = 4000rpm

It's just a weird feeling running that high. I still feel I could use another gear.

WCM

focodave · Post by **focodave** » Sat Jun 16, 2018 9:00 pm

Yep.
Same as mine.
Sounds like you have a stock gear box and a 3.90 rear axle ratio (also stock).

Don't be worried about the "high" RPM.
We Americans are used to cruising at 70 MPH at 2000 RPM in our modern vehicles -- and the older American V-8's just lumbered along, seemingly under no stress at all.
It is, indeed, hard to get used to running an engine at 4000 RPM at 70 MPH when you have not experienced that before, but I am telling you that it is normal for these engines and they are designed for it.
Just make sure you have good oil in the crankcase and that the engine is running at the proper temperature and everything will be fine.

Enjoy the cacophony of sounds the engine makes and consider it a symphony to your ears.
If you want to hear my 1980, here is a link: https://www.youtube.com/watch?v=zxc9TEcS1Eg

Happy motoring....

DieselSpider · Post by **DieselSpider** » Sun Jun 17, 2018 8:41 am

WCM wrote:Update:

Ok, In 5th gear............

55mph = 3100rpm
60mph = 3500rpm
70mph = 4000rpm

It's just a weird feeling running that high. I still feel I could use another gear.

WCM

What size tires do you have?

3100 at 55 would be with 175/70-13 tires and the 4.30 rear axle

phaetn · Post by **phaetn** » Mon Jun 18, 2018 8:19 am

Indeed. I have the older (pre 78.5) 4.3:1 diff and those are somewhere around the RPM numbers for me at those speeds. I have 205/50-15 tires mounted which are pretty darn close to the diameter of the 185/70-13s that I had on there before.

To be honest, as others have mentioned, the engine really only comes into song after 3,000rpm anyway. It's a delight to cruise at 60mph, put the foot down, and hear the roar as it picks up to 80mph. With my engine/exhaust the drone actually gets to me after a while and I wear headphones or earplugs on trips more than two hours because it's so loud.

In 2015 I drove it 550 miles to Pittsburgh for FFO and back. A year later 1,000 miles round trip to FFO 2016 in Detroit. It's not a problem -- these cars will happily go on the highway. Just make sure you've got good oil in the engine, gearbox, and differential and that the cooling system is functioning well for when you hit stop-and-go traffic after highway cruising.

Cheers,
phaetn

WCM · Post by **WCM** » Mon Jun 18, 2018 8:53 am

185 / 70R13

MIGHTY 8 · Post by **MIGHTY 8** » Tue Jun 19, 2018 7:26 pm

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

Try downloading an app on your phone to verify your speed. Plenty of free ones available

WCM · Post by **WCM** » Wed Jun 20, 2018 10:27 am

Try downloading an app on your phone to verify your speed. Plenty of free ones available;

I did that and posted my results a few days ago.

Thanks,

WCM

DieselSpider · Post by **DieselSpider** » Sat Jun 23, 2018 8:53 am

WCM wrote:185 / 70R13

Here is how I figure it in my Excel Spreadsheet using a 185/70-13 tire, 4.3:1 Diff and a 0.881 Fifth Gear as an example:
185cm X 0.70 X 03937007874 = 5.098425197" Sidewall Height
13" + (5.098425197" X 2) = 23.19685039" Total Tire Diameter
23.19685039 X PI() = 72.87505478" Circumference
(5280' X 12") / 72.87505478" = 869.4333087 Tire Revolutions Per Mile
(869.4333087 X 4.3) X 0.881 = 3293.674203 Engine Revolutions Per Mile in Fifth Gear
3293.674203 / 60 = 54.89457005 Engine RPM Per MPH in Fifth Gear
MPH X 54.89457005 = Approximate Engine RPM at that MPH which can vary due to tire wear and/or inflation.

EX: 60 MPH X 54.89457005 = 3293.674203 Approximate Engine RPM at 60 MPH
55 MPH X 54.89457005 = 3019.201353 Approximate Engine RPM at 55 MPH

spider78 · Post by **spider78** » Mon Jul 09, 2018 6:58 am

zachmac wrote: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.

I have a 79 same mph = rpm

SpaceLion · Post by **SpaceLion** » Tue Apr 21, 2020 6:22 am

So this is interesting. I started looking into this because I just picked up a 1980 carbureted project, and I have family that I visit a lot about 2.5 hours south of me. The speed limit on most of I75 is 75mph. So flow of traffic is about 75-80. that being said, how healthy is it to maintain those high RPMs for hours at a time? And does anyone know what kind of MPG to expect at those speeds? I plan to eventually go dual weber idfs so I expect it will be a bit worse than stock.

aj81spider · Post by **aj81spider** » Tue Apr 21, 2020 8:46 am

I wouldn't worry too much about the engine. It's designed to run at those RPM, and frankly does poorly when running at low RPM. I've seen a lot of variability in the gas mileage that people report. I get in the low 20's mpg - but I don't do much highway driving because:

Your bigger problem will be that you are a tiny low car that no one can see. I keep off the interstate as much as possible, as I've had several incidents where pickup trucks or SUV's almost hit me because they just don't see the car. I also upgraded my horn because they couldn't hear the original horn in the car. Others may be more comfortable, but I find driving the Fiat on the highway to be a not at all relaxing experience. I'm also worried about what happens in a small open convertible if some 46 year old component decides to let go at 70 mph.

18Fiatsandcounting · Post by **18Fiatsandcounting** » Tue Apr 21, 2020 10:48 am

I agree with A.J. Fiats love high rpms, and 4000 or so rpm for hours at a time shouldn't be an issue for a well maintained engine (oil, cooling, good belts and hoses, things tightened down properly, etc.). The main issue is driver comfort and safety, and in addition to what A.J. mentioned, it can be pretty tiring to listen to road, engine, and wind noise for hours on end. Plus the suspensions aren't like modern cars and the ride can be pretty jarring if the road isn't in good shape. Add a cross wind to this mix and you can see where it becomes more of an ordeal.

-Bryan

RPM's at 55

Re: RPM's at 55

Re: RPM's at 55

Re: RPM's at 55

Re: RPM's at 55

Re: RPM's at 55

Re: RPM's at 55

Re: RPM's at 55

Re: RPM's at 55

Re: RPM's at 55

Re: RPM's at 55

Re: RPM's at 55

Re: RPM's at 55

Re: RPM's at 55

Re: RPM's at 55

Re: RPM's at 55