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Working out gear ratios


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All,

Can anyone help me with the calculations to work out what gear ratios you have in your 'box?

It's well known that you can estimate the final drive ratio in the diff, by rotating the rear wheel once and counting the revolutions of the prop shaft.   Just over 4 prop revs = 4.11:1 and so on.     Can this method be extended to estimate the gearbox ratios?

If the car has the back raised on stands, is in gear with the plugs out, it would not be too hard to turn the whole drive train by a socket wrench on the bolt on the front of the crank.   Then, with assistance, count the engine revs  to turn the back wheels by one rev.     But how to calculate the ratio that the gear box is using?

Fourth gear is easy, it's 1:1, so in the above example just over four engine revs for one wheel rev.     In a lower gear, there would be more engine revs per wheel rev.   If, for argument, you put the 'box into first?     On a Spitfire that's 3.5:1.   So one wheel rev would need 14.385 (!) revs, or 4.11 x 3.5?     

But practically, what we would note is that the engine is turned  a bit over 14 times, say fourteen and a quarter, for one wheel rev.    Can we divide 14.25 by the final drive ratio to get an estimate of the gear ratio?  14.25/4.11 = 3.47.    Then by looking up what gear ratios were available for your gear box, you could match your estimate to what it really could be.

Of course, you could do this conversely, by turning the wheel and  counting the engine revs!

Either way, is the calculation, Engine rev/final drive ratio=gear ratio valid?

John

 

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I think the issue is the differential. Any drag from the rear brakes might make the diff start to turn one wheel more than the other. Better to count the prop shaft rotations going into the diff.
Or use Mintylamb web page (mintylamb.co.uk/gearspeed/) and use the rev counter and speedo (or phone GPS speedo) to check the gearbox ratios.

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Then you need to know rolling radius.  And precisely which speedo you have - like many other parts, our cars are Bitzas now!

I agree that prop revs less likely to be confused by differential (!) wheel rotation - slack off the rear brake adjusters first?   Otherwise engine vs wheel or prop revs going to be more accurate than indirect methods.

But is the arithmetic correct?

John

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Calcs look fine to me. But I think it may be a bit of a struggle to turn the engine over with the diff connected. Probably the sort of thing you should have done before fitting the gearbox (isn't hindsight wonderful?)

I think I would try disconnecting the prop, in this case at the diff end as you have splined sleeve to the box. 

The other point is that unless it is a "special" there are very few variants to the T9 ratios. From memory, the std 4 cylinder, then the 2.8V6 with the longer input shaft and better layshaft bearings, then the diesel (with long input shaft and rubbish first gear, sometimes mistakenly bought thinking it is  V6 version)

There is a "popular" coversion sold to raise first to 2.98 in the standard 4 cylinder box, plus other special ratios. 

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Edited by zetecspit
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Thank you, zetec!    I was thinking of a generalised method, but as you know I have the T9 and the V6 version, so thank you again for the table.   It will now be interesting to apply those known ratios to some speed/revs, to see how a Triumph's 13" wheels affect them when they were originally on 14 or 15", and if I have the right speedo.

John

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2 hours ago, JohnD said:

Thank you, zetec!    I was thinking of a generalised method, but as you know I have the T9 and the V6 version, so thank you again for the table.   It will now be interesting to apply those known ratios to some speed/revs, to see how a Triumph's 13" wheels affect them when they were originally on 14 or 15", and if I have the right speedo.

John

John, many of the cars were 13" wheels. The Capri 2.8i was fitted with 205/60 13 tyres. 

There is a plastic speedo drive which varies between cars, very much like Triumphs with J type OD. At one point I had a very accurate speedo (1% out from GPS measured at 100mph) but that drive slipped on the shaft and chewed teeth, so was replaced with a spare that has an extra tooth, so the speedo now under reads by 5%

The mintylamb site should help you with calcs. Except it seems to have disappeared:blink:

 

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John. Do you not know the ratios in the box you have? Or is this another thought experiment?

If you are planning on actually doing this then trying to count wheel rotations is NOT the way to do it. Do as Mike says as it is a damn sight quicker and simpler to count rotations of the gearbox output shaft/prop than back wheel in each gear if you want to go through the ballache. Although simply checking first is usually good enough.

Regards speedo drive the most accurate, again if that is what you are trying to achieve, is to use the method described on the Speedy cables website, which takes into account actual wheel/tyre rolling diameter since this is of course dependant not only on rim and tyre size but also the weight of the car and the tyre pressure.

Then get them to calibrate your speedo to suit rather than mucking about with speedo pinions and/or having to remove and strip the box to change the worm drive on the mainshaft. And then still not getting it quite right.

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Thought experiment?  What am I, Einstein?

Yes, you're right of course to count revs of the gearbox output shaft.  You need the cover off for that, but you do anyway to check diff ratio.

No, I don't know what ratios they are on the T9, but Zetec's post above only lists two 5-speeds, and mine is the V6 version.  So I'll go with those.

But I need to check indicated speed in the upper ones, or else deal with  a weak mixture at top revs.  Can't get it to more than 95mph, even on a long straight.

Still, a winter project.

John

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9 minutes ago, JohnD said:

Thought experiment?  What am I, Einstein?

Yes, you're right of course to count revs of the gearbox output shaft.  You need the cover off for that, but you do anyway to check diff ratio.

No, I don't know what ratios they are on the T9, but Zetec's post above only lists two 5-speeds, and mine is the V6 version.  So I'll go with those.

But I need to check indicated speed in the upper ones, or else deal with  a weak mixture at top revs.  Can't get it to more than 95mph, even on a long straight.

Still, a winter project.

John

We know that all the T9s have 1.1 in 4th. So that should help. 

Yoi are certain it is a proper 2.8V6 box? the diesel box has teh long input shaft, but different (worse) ratios, and is sometimes mistaken by sellers for the better V6 box. I think the 2.3V6 is different again (long input, but same ratios as the 4 cylinder)

The 2.8 version is identified by the retaining plate under the input shaft, shown in the smaller pic below. 

Image

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10 hours ago, JohnD said:

V6? It sure is!

image.png.f80cbc2d7d0749b88c1cf6f06370e849.png

More importantly, the 2.8 V6. It is clear why so many people end up with disappointing ratios. I expect there are plenty of gullible people still buying diesel boxes thinking  they are the 3.36 first box. 

Edited by zetecspit
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1 hour ago, Nick Jones said:

If it has that layshaft bearing reinforcement it’s the pick of the standard boxes as used on the later Granada’s and 2.8 Capris, and will have the longer first and second gears.

Possibly. Apparently the one I have (started life in an XR4x4) has a few other mods too which improve things slightly. I don't think that happened on the Capri boxes, only the late sierra 2.8i stuff.

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