14 replies to this topic

[germini30]

I'm swapping my 998 engine with a 3.1 diff to a 1275 with 3.44 diff.
What do I need to change so that the speedo reads correctly, thanks.

[Spider]

I think the easiest way here is to see what speedo drive gears are in your 998 now (assuming your speedo reads correct with these) and then calculate where you need go from there.

[MrBrown]

How do you calculate that Spider? I’m in a similar yet different situ. Thanks!

Edited by MrBrown, 13 July 2024 - 01:49 AM.

[GraemeC]

There’s a nifty calculator on the Guesswork’s site
http://www.guess-wor.../Tech/ratio.htm

[Spider]

How do you calculate that Spider? I’m in a similar yet different situ. Thanks!

 

Let's say you have a 3.1 FD and swapping to a 3.44 as the OP is.

You check your speedo drive gears and find they are 5 and 16 toothed, the ratio here being 3.20:1

 

Work out then what the ratio difference between the final drives is;-  3.44 / 3.1 = 1.11

 

So we need to change the speed of the speedo drive by 1.11, in this case, increasing it's speed;-   3.2 / 1.11 = 2.79:1

That's the ratio that's needed. The nearest available ratio is 2.883:1, using 6 tooth Pinion and an 17 tooth drive gear. Using these, there will be a small error, but one could likely live with it.

[germini30]

I've just been online, you can buy a GPS speedometer, which are meant to be very accurate, and plug into a usb for about £10. I may get one of those and leave the speedo drive alone

[Ethel]

GPS won't work in in certain circumstances, you'd still need a road driven speedo really.

 

Minis existed with your final drive, so there will be a solution.

 

Lifted from Minimania's site - but also read http://www.guess-wor...Tech/speedo.htm

 

Final Drive Ratio    Pinion Colour    Part no.          Spindle Colour    Part no.

All pre- 1980          17 white             22A1881        6 none                2A3720

3.44                        16 green            DAM2905      6 none                2A3720

3.2                          18 red                TXD1006       5 red                   TXF1004

3.1                          16 green            DAM2905      7 blue                 DAM6028

2.76                        15 black             TXD10004     7 yellow              TXF10001

[DeadSquare]

I'm swapping my 998 engine with a 3.1 diff to a 1275 with 3.44 diff.
What do I need to change so that the speedo reads correctly, thanks.

 

 

I'm swapping my 998 engine with a 3.1 diff to a 1275 with 3.44 diff.
What do I need to change so that the speedo reads correctly, thanks.

You need to change the speedo to a Cooper S.

[Ethel]

 

I'm swapping my 998 engine with a 3.1 diff to a 1275 with 3.44 diff.
What do I need to change so that the speedo reads correctly, thanks.

 

 

I'm swapping my 998 engine with a 3.1 diff to a 1275 with 3.44 diff.
What do I need to change so that the speedo reads correctly, thanks.

You need to change the speedo to a Cooper S.

 

 

If they have "S" era worm & pinion. Unlikely if it was 3.1 FD

[Spider]

 

I'm swapping my 998 engine with a 3.1 diff to a 1275 with 3.44 diff.
What do I need to change so that the speedo reads correctly, thanks.

 

 

I'm swapping my 998 engine with a 3.1 diff to a 1275 with 3.44 diff.
What do I need to change so that the speedo reads correctly, thanks.

You need to change the speedo to a Cooper S.

 

 

Not necessarily. There were many base line models that had 3.44 final drives.

[MrBrown]

Wow. Thats a lot of pesky detailing to get it right. I am in another situation as stated.
My car was an automatic:
3.27 FD(22 pinion-72 crown) and 17 tooth pinion drive and 7 tooth spindle.
Remote gearbox I swapped in had a 3.64 FD (17 pinion-62 crown) with a 17 tooth pinion drive and 6 tooth spindle when I got it.
It has a 3.21 FD (19 pinion-61 crown)in it now.

I don’t even know where to begin with this one. May be worth getting that GPS edition haha!

Edited by MrBrown, 28 December 2024 - 11:42 PM.

[Spider]

Wow. Thats a lot of pesky detailing to get it right. I am in another situation as stated.
My car was an automatic:
3.72 FD(22 pinion-72 crown) and 18 tooth Speedo drive (lost the spindle so not certain the tooth count).
Remote gearbox I swapped in had a 3.64 FD (17 pinion-62 crown) with a 6-17 tooth speed drive when I got it.
It has a 3.21 FD (19 pinion-61 crown)in it now.

I don’t even know where to begin with this one. May be worth getting that GPS edition haha!

 

It can be worked out in a few different ways what you need to sort it, but all of it is only from working with Pesky Detail.

 

There's 3 parts that are swappable in the Speedo drive train to calibrate it. The Drive Pinon and Speedo Drive Gear as already discussed here, the other that's been touched on is the Speedo Head itself. These are calibrated in TPM (turns per mile) or TPK for the metric types. On the Smiths Speedos, this is usually marked on the face of them, it'll be a number like 1208 for the imperial ones and 804 for metric.  These numbers can be worked out to suit the final drive ratio and wheel size.

Another way is if you have gears in there to accurately determine what error there is than adjust from there, however you really need to determine the error from the Odometer and not the Speedometer. If the Speedo is working correctly, it will be linear, however if it's out of adjustment, it won't be linear and this is why any error needs to be measured off the Odomenter. You can check it off the Speedo and adjust from that, but it may show underspeed below your reference speed and overspeed above it, or viki-verka depending on how it may be out. You could also do say 3 or 4 reference checks at different speeds, ie, 20, 40, 50 and 60 mph. Once the error is known, it's simply a matter of calculatng the percentage of the error and then applying that to the speedo gears to get you close or you can consider the gears and the speedo head TPM to get it spot on.

There's other options to this as well, one is to see an Instrument repair shop and they can re-calibrate your speedo head to match or swap out to an Electronic Speedo that can be re-calibrated by yourself, on the run and as many times as you like. While this latter option may appear costly, it can be similar costs to having your speedo re-calibrated by a shop.

Personally, I always like to have all Instruments, including the Speedo and Odo working, accurate and reliable, otherwise, you may as well take them out of the vehicle. For what ever it's worth, I went to an Electronic Speedo in my Moke some 15 years ago and never looked back. If I change final drive ratios or even tyres or for that matter, run at different tyre pressures (as I often do when off road), I can re-calibrate the speedo and odometer (important off road !) to always be right.

[Ethel]

If you're not hell bent on originality, there are alternatives https://acewell.co.u...ion=full&id=283

[MrBrown]

Great information! I appreciate it.

[Spider]

If you're not hell bent on originality, there are alternatives https://acewell.co.u...ion=full&id=283

 

That's a great option and nicely priced.