Technical Gearbox top speed

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Technical Gearbox top speed

D25

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so if lately come across a little thing that is bugging me A LOT.

so here is my problem when i drive on the highway i need just a little bit more then 4000 rpm to reach 120kmph or 75mph . and thats bugging me because when i am on the highway in the Netherlands the max speed is 130kmph or 80mph. i love my punto but keeping such a high rpm for a couple of hours just feels wrong for her, maybe im wrong?

but here is my question can it be that this is not the original gearbox?
i found some code's on it and they are:
c514 5 1344 46479123
and on the diff housing:
c514.5.13.41 46548384
 
See here
If you check your model you can find what your 5th gear ratio is and what final drive your gearbox has, then use a website (several of them about), put the gearing in and your current wheel/tyre size and it should say what RPM equates to what speed, then you'll know if your gearbox is correct (y)
 
yeah so the gearbox is off i recently went on a dyno (dont now how to call it in english) and there it was measured that in 4 at 4525 rpm the car goes 109,7kmph. but if i calculate it with the calculators on the internet it says that the car should go 122 kmph. so there must be something off

so i have 2 questions.
1: to what car does my gearbox belong?
2: do i need to swap the whole transmission to go faster or can i only swap the diff?
 
What model is your car, 60S, 75SX? etc
1: Don't know the gearbox codes, someone else may be able to help.
2: You can swap the diff to gain one with a longer final drive ratio if you can find one...
Easiest but not a great solution by any stretch of the imagination is to fit bigger wheels, this will increase your gearing in all gears, I intend to do this with my Punto 90 because it has very short gear ratios but trying to find another gearbox for it would be virtually impossible, I intend to go with some 16" Fiat Coupe wheels.
 
Big wheels are not a clever idea, the steering will be much to heavy and the acceleration response will be much worse simply due to the different wheel and tyre geometry and weight, before you even consider the effect of raising the gearing.

You need to explain which model you have and if you think something in the transmission has been changed. If it is standard then you must assume that the gearing was designed to give reasonable performance in varying terrain, and not with only one person in an empty car, which is probably how you are driving it. But remember that small Fiat engines have always been designed to rev and they are happy doing it, just keep the oil clean and fresh and the ignition in good condition. Also check the valve clearances on an older Fire engine and adjust with shims if needed.

If you are overhauling the gearbox then you could raise the final drive gearing (lower number) if you really want.
 
Big wheels are not a clever idea, the steering will be much to heavy and the acceleration response will be much worse simply due to the different wheel and tyre geometry and weight, before you even consider the effect of raising the gearing.

You need to explain which model you have and if you think something in the transmission has been changed. If it is standard then you must assume that the gearing was designed to give reasonable performance in varying terrain, and not with only one person in an empty car, which is probably how you are driving it. But remember that small Fiat engines have always been designed to rev and they are happy doing it, just keep the oil clean and fresh and the ignition in good condition. Also check the valve clearances on an older Fire engine and adjust with shims if needed.

If you are overhauling the gearbox then you could raise the final drive gearing (lower number) if you really want.

I did say putting bigger wheels aren't a great solution, but a solution none the less, assuming it has PAS the steering is a non issue really, Fiat always tend to gear their cars very low to get as much oomph out of the small engines they use, as seen across much of the forum an inch of extra wheel size has never massively thrown off any car I've seen, it might hurt a 55 but a 60 would easily cope with some 15's (the mk2 1.2 8v had 15's and its the same engine), a 75 should have no issues at all. Obviously massive wheels are going to be a little bit too much.

I agree with you that the engines don't mind sitting at high RPM for long periods especially if they're well maintained but the OP has said he wants it to able to cruise at a higher speed with a lower RPM, presumably for noise and comfort reasons rather than worrying about how it will affect acceleration.
 
the car is a 75sx from 1997

bigger tires arent an option because im already running 175 70 14 on stock rims and if i mount bigger tires they will hit the shocks at the front side.

does anybody know a way to trace from what fiat my gearbox comes from?
 
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I agree with you that the engines don't mind sitting at high RPM for long periods especially if they're well maintained but the OP has said he wants it to able to cruise at a higher speed with a lower RPM, presumably for noise and comfort reasons rather than worrying about how it will affect acceleration.

The noise is bareable and comfort is really great in the punto the only complaint i have is that it just feels wrong to go on the highway for a couple of hours with a lot of rpm.

a classmate of mine used to own a 60sx and his didnt need so much rpm for cruising at 130, unfortunaly he doesnt have the car anymore so we cant compare the gearboxes (according to the manual they must be the same)
 
I believe fiat designed the 75 so it can get top speed in 5th at max rpm. So they put in higher ratio final drive to do this. If you put a 60sx box in then the revs should be smaller at same speed but the car will fill less responsive. The 60sx achieves max speed below max rpm power. This is not perfect but makes the car more quieter at speeds. The 5th gear on the 60sx is what is known to be an overdrive gear. This gear speed advantage is on long motorway downhills.
 
If you look at the table didge published in the other thread (link given in post #2 ) you will see that most 45s, 60, 75s and 90s etc all had the same gear ratios, it was the final drive ratio that determined the overall gearing:

Engine Final Drive Ratio
55 3.866
6 Speed 4.923
60 3.563
75 3.733
90 3.563
GT 3.353
TD 3.733

So as Dumbledore says the 75 has a lower final drive ratio (higher number) than the less powerful 60 and more powerful 90. So changing to a 3.563 final drive would cure the complaints of both D25 and Didge3.

The 75 was clearly meant to be the sporty, family model (available in 5-door), just below the 3-door 90hp Sporting. Note that the powerful turbo GT has a much higher (low number) fianl drive ratio.
 
to be totally honest i just would stop worrying about it, the fire engine will be totally fine at 4.5k rpm and higher all day long - it won't damage it at all. Heck I've had 8v FIRE engines up near 6k for 4-5 hour journeys and its totally not a problem. Stop worrying its totally normal and within its limits to rev that high ( #NotEvenHighRevs :p ) for hours and hours and hours.
 
Back in the 80s my mate had a Strada 1.1 CL with a 4-speed box, that used to hammer up and down the autobahn / motorway all day at 80 doing god knows how many revs with no problems at all. Back in the good old pre-cat days with proper leaded petrol... aaah!
 
Also the original 75 would have had much lower tyres as standard than the big balloons the OP has fitted now... 165/60 x 14 I think compared to 175/70 x 14 ! :eek:

Luckily back then Fiat didn't fit a stupid shift indicator in the middle of the dash that helpfully comes on at 2100 RPM whatever the driving conditions or load in the car! :rolleyes:
 
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