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Discussion Starter · #1 ·
Guys,

Whilst I'm trying to figure out what's wrong with my speedo, can someone tell me what speeds a standard S1 1750 is doing per 1000 rpm in 4th & 5th, so that I can calculate whether or not I'm breaking the law.

Cheers,
Wazza
:D
 

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Road Speeds

Hi Wazza

I just happened to trip over your question whilst looking for something else and have a copy of "Original Alfa Romeo Spider" by Chris Rees on my desk.

Calculation obviously depends on diff ratio and tyre size, but if standard (4.555 and 155SR x 15 respectively) then this shows the following mph per 1000 revs:

5th - 19.9mph
4th - 15.5mph
3rd - 11.5 mph
2nd - 7.8 mph
1st - 5.1mph

Best bet is to use a SatNav that shows speed and distance then you will be able to adjust for errors in tacho and speedo instrumentation as well.

Chris
 

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Apologies ..... because I was looking for something else, I misread this. You've got a Series 1 1750 GT(V). These have a std diff of 4.1 and std tyres of 165HR 14. I think you'll find that the calcs will come out nearly the same. Sorry for the confusion.

Chris
 

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As he says above use your Tom Tom or Garmin GPS to see what spped you are traveling to verify speedo needle.
 

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Discussion Starter · #5 ·
Guys,

My '69 GTV didn't come with SatNav, and being a bloke I don't need a woman on a computer to tell me where to go.

I've got a wife to do that.;)

Cheers,
wazza.
 

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Guys,

My '69 GTV didn't come with SatNav, and being a bloke I don't need a woman on a computer to tell me where to go.

I've got a wife to do that.;)

Cheers,
wazza.
You can select a Brit Bloke if Laura or her other female friends turn you off on your Tom Tom.

Some other guys available, too.

I am kinda used to a women telling me what to do though.
 

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PhilB, The aspect ratio of the tire you used was 165 ""60"" 14 should be 80-83 series. 4.1 diffs should bring about 21.9mph/1000rpm in a .79 5th with stock dia tires 165 80 14. This is from memory not a recent calculation.
 

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Wazza, there's no point doing any calculations based on theoretical RPM as there's little chance your tacho is anywhere near accurate. Mine was about 1500 RPM optimistic near the top end. So using satnav, or just timing yourself between km markers on the highway while driving at a fixed rpm, is best.
Neil
 

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Odd fact. Running a 4.3 rear axle but keeping the 4.56 speedo drive on 175/70-15 tyres gives you a 100% accurate speedo reading :cool:.
 

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There is a road that I use often which has 2 different sections with those radar boards that tell you how fast you are going. I went by them several times at different speeds to find out my mph (speedo indicates kph), and I came up with: 120kph indicated is 68mph; 110 is about 56; and 100 is 45mph. I just committed these numbers to memory, haven't got a ticket yet.

So the moral of this story is to find a section of road that has these monitors (without camera's). Or as others have said, get or borrow a GPS unit and go for a drive.
 

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Discussion Starter · #11 ·
Guys,

Thanks for the input so far.

Using the figures supplied by Chris, I'm assuming around 20mph per 1000rpm in 5th. Sometimes my speedo works fine up until 40mph before it goes mental, which lines up with 2 grand on the tacho.

On the technical front, I've been told that the speedo cable receptacle in the gearbox can cause this erratic behaviour. Anyone heard of this?

Cheers,
Wazza.
 

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Wazza, Chris admitted his figures were for 4.56 not the 4.1 that came in your 1750. 21.9 mph/1000 for your car when stock. Tell us your tire size and I am sure someone will calculate it.
 

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Discussion Starter · #13 ·
Hi Brad,

I'm running 195/55 x 15's.

Thanks.
Wazza
 

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With your smaller than stock dia. tires I calculate 21.5425 mph/1000rpm in 5th and about 17 mph/1000 in 4th.
 

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Nothing more than a theoretical exercise unless you have an confirmed accurate tacho.
 

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Tyre sizes used by manufacturers are nominal and there is a wide variation in actual size between manufacturers and indeed between models in the same manufacturers range.

Wrap a piece of string around the circumference of the tyre & then measure it, though as eedneco says you need an accurate tacho for the calcs to be accurate.
 
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