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Author Topic: final drive and wheel sizes  (Read 5238 times)
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Posts: 26

« on: 07 May, 2019, 06:33:34 PM »

I noticed that the final drive is very low geared on the Prisma.
Even with the 5 speed 'box the engine is whizzing away at 4,000 rpm at 70mph.

The original wheels are 14inch and are fitted with 165/65 R14 tyres.

I guess some straight 165/80 R14 tyres would increase the final drive...but I would be loath to fiddle with the original engineering set up.

I suppose the Prisma won't be doing big motorway miles...and we will drive the car for a while before making decisions but just wondered if it was common among Prisma/Delta owners to change the diff final drive or fiddle with overall radius wheel/tyre sizes. Even my 1970's series 2 Alfa Spider has a much longer final drive (under 3,500 rpm at 70mph).
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« Reply #1 on: 08 May, 2019, 12:03:31 PM »

My 1300 Beta has a similar rev to speed ratio which I just put up with. It's predecessor, a pre-facelift version, was even worse.

1979 1300 Beta Coupe, 2014 Ypsilon 1.2 S Series Momo
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« Reply #2 on: 08 May, 2019, 12:51:09 PM »

Compared to more modern cars, the 2wd Deltas/Prismas/Betas are low geared. Unless you use them on motorways a lot, the revs are acceptable. The shimmed overhead camshaft set up makes the mechanical aspect less strained than the typical push-rod system of contemporaries when the Lampredi engine was designed. 4WD Deltas have a Thema style transaxle so the bits to install such are in existence but its not a 5-minute job so who would ?

Leo (have owned Deltas and Prismas since 1983)


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« Reply #3 on: 08 May, 2019, 01:23:58 PM »

The French verison of the 1.6GTie (which is the Delta equivalent to the Prisma Symbol) has a different gear box to all other markets to be able to overtake at the higher motorway speeds in France. Indeed the French version of the gearbox has a completely different set of ratios. It is very nicely spaced between gears and top is definately not an overdrive as it is the same overall final drive as the standard car.

* Delta gear ratios.JPG (39.02 KB, 434x556 - viewed 650 times.)
« Last Edit: 08 May, 2019, 01:50:22 PM by lancialulu » Logged

Its not the winning but taking part! or is it taking apart?
1955 Aurelia B12
1967 Fulvia 1.3HFR
1972 Fulvia 1600HF
1972 Fulvia Sport 1600
1983 HPE VX
1988 Delta 1.6GTie
1998 Zeta 21.  12v
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« Reply #4 on: 08 May, 2019, 06:10:01 PM »

My Fulvia 2c Berlina's a overall gearing 15 5mph per 1000rpm so I also have 4000 rpm+  on the clock at 70mph, but it's never stopped me crossing France Spain, Portugal and Italy, with a few other countries thrown in at 70 mph, a speed my 2c will hold all day with no problems, with occasionally 75+when required.
At least your have a radio to deaden the sound if you don't like it, I'm happy to hear a Fulvia engine on song😁

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« Last Edit: 17 May, 2019, 08:30:38 AM by fay66 » Logged

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« Reply #5 on: 13 May, 2019, 06:40:51 PM »

The 5th gear was always meant to be a useable gear rather than a cruising gear, like on the Spider.
The HF's etc could get away with a taller gear set as they had the torque to pull them.
You could try using taller tyres but the offset is that performance will be markedly reduced, it will feel much less sprightly in the gears.
For the sake of a bit of higher revving at Motorway speeds I'd stick with the 165/65's. 165R14 aren't that easy to find now either without getting 6ply van tyres, what you can find doesn't give much choice.
If you did want to upgrade the size I'd go for something like a 175/70R14 which would give a 5.4% increase in speed to revs, so if my calculations are right that would drop the revs to around 3,800RPM at 70MPH, although of course it would be an indicated higher speed, 74.8MPH off being an effective drop in performance of 5.4%....

This website will allow you to try calculations, just make sure the tyre size you create is actually one that is made!

Flavia 815 coupe
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Posts: 26

« Reply #6 on: 16 May, 2019, 09:35:59 PM »

Thanks for the replies. Just I'm used to the 24v 166 that sits at 100mph all day on 4,000rpm...different era of car I suppose.

The Prisma 1.6 ie handbook gives the final drive in 5th ratio as just over 19mph per 1000rpm, so that makes at least 76mph at 4,000 rpm, so better than I first thought.

The speedo actually reads slightly slow compared to gps, and I suspect the rev counter reads slightly high.

I will leave the car totally standard.
We drove the Prisma 350 miles from Conwy to Kent on Tuesday...including about 100 motorway miles was great.  Soft springs give a very comfortable ride, responsive and fun through the bends with acceptable performance from the zingy engine...a cracking and characterful little car.

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« Reply #7 on: 16 May, 2019, 10:47:14 PM »

Great news.

David Laver, Lewisham.
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