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Speed vrs rpm vrs gear


Anglefire
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I've taken the Spitfire (1500) out this afternoon to some friends and went past a speed radar sign and I'm pretty sure it said I was doing 35, when I was showing 30 on the speedo.

So wondered what the rough speed should be for say 3000rpm in 4th (non OD)?

When I'm at 3000rpm, the speedo is showing about 50, but based on the attached spreadsheet, 3000rpm should be about 55/56mph?

gearspeeds.v2 lr.xls

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Most of cars I have owned have had wildly inaccurate speedos. 10-20% out, and always under reading. 

Bizarrely the most accurate speedo has been with my type 9. Less than 1% out at 110mph (checked against GPS, and yes, it was on a genuine autobahn) 

Part/most of the issue seems to be with overdrives. Attention isn't paid to the speedo drive gear when fitting. Yes. I am guilty.

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Way back in the 70-90s when I waas involved with  in factory  tachograph  calibration we had a spec for speedo was +10%+4mph   so it could read 70 at true 60 and be within tolerance   they  must not read slow

Manual calibration measure 52.8 ft down the road, attach card pointer to speedo cable  inner  .  push car exactly 52.8 ft count the turns.

X the turns by 100  and thats the turns per mile needs to match the numbers under the oddometer rolls

Pete

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Anglefire,

In addition to the above points on the legal side of speedo readings, in Triumphs they measure the speed of the final drive, as it leaves the gearbox.     This is then reduced by the differential, which of course has a range of ratios, from1:4.5 (?Courier van) to 1:3.27 (GT6, no overdrive).   A 1500 Spifire left the factory with a 1:3.63 ratio diff and a speedo to match.     If, as is quite likely, it has had a higher ratio diff  fitted, or a speedo from a lower ratio car, then the speedo will under read, the car will be going faster than indicated.

John

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12 hours ago, Pete Lewis said:

Way back in the 70-90s when I waas involved with  in factory  tachograph  calibration we had a spec for speedo was +10%+4mph   so it could read 70 at true 60 and be within tolerance   they  must not read slow

Manual calibration measure 52.8 ft down the road, attach card pointer to speedo cable  inner  .  push car exactly 52.8 ft count the turns.

X the turns by 100  and thats the turns per mile needs to match the numbers under the oddometer rolls

Pete

Pete is right, the calibration requirements on the speedo were fairly wide; the speedo was only supposed to be an “indication” of your speed and must not read slow.

However, that's not to say the operating principle of a NEMAG speedo is wildly inaccurate because they can be made to read a lot more accurately than they generally do; it was the calibration requirements which were loose and after decades of use, things will obviously have drifted further out.

I decided to do something about my tacho and speedo neither of which was reading correctly, so I re-calibrated them based on the principles gleaned from TM9-1829A Ordnance Maintenance: Speedometers, Tachometers, and Recorders - 1944 a War Department document. It was my winter project!

I built a lash-up consisting of a large electromagnet a power supply and a geared electric motor and managed to get very good results; you’ve just got to have a lot of patience.

David

 

 

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The speedo on my 13/60 has been recalbrated by a previous owner and using GPS as a comparison it is as accurate as my modern so they can be adjusted to read correctly. How it was done I have no idea. Mind you sometimes it does fluctuate a bit at 60 ish. 

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On the UK 1500 Spit, 3.63 diff, the top gear speed at 1000rpm is 18 mph.

This subject is a can of worms, this is because in 1974/5 the Speedo's on most Triumph models was standardised at 1000 turns per mile. This meant that other Triumph models, Dolomite, using the same gearbox and overdrive would have Speedo drive cog to match the diff ratio and not a different turns per mile Speedo. 

This meant 1500 Dolomite which up to 1980 has a 3.89 diff though it has the same gearbox/overdrive as the 1500 Spit has a Speedo drive cog gave less turns per mile. The 1300 Dolomite had a 4.11 diff so less turns per mile. So if somebody fits a Dolomite 1300/1500 gearbox into your 1500 Spit then you may have an under reading Speedo. The same goes for the 1500 Dolomite Overdrive unit.

Late, 1980, 1500 Dolomite had a 3.63 diff so no problems.

The 1850 and late, 1980, 1500 Dolomite Overdrive where OK as both cars had a 3.63 diff the same as the UK 1500.

The problem may be the Speedo or the above. Check with a GPS.

Dave

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Thanks Dave - little bit more to add to my story!

The engine in the car when I bought it was from a Dolly 1300 - but I'm 99% sure was rebuilt as a 1500. The flywheel was also a dolly flywheel so pre-engaged starter.

It is therefore possible that the gearbox is from a dolly as well. In fact if you replace the diff ratio in the spreadsheet with 4.11 then the calculated speed at 3000rpm is about 50.

I always knew the car was a bit of a triggers broom, but also looks to be a heinz 57 too :lol:

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For warranty testing I had a Smiths drive unit , had two humongous electro magnets that shut down the grid if used in anger,

Giant rpm dial and stabilised drive motor.... when they closed us down it went on the skip along with a starter and alternator tester

As a superb air valve test rig  there Is limit on what you can acrue from the skip  in hindsight  could have set up a nice little earner

Happy days .....gone 

Pete

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38 minutes ago, Pete Lewis said:

For warranty testing I had a Smiths drive unit , had two humongous electro magnets that shut down the grid if used in anger,

Giant rpm dial and stabilised drive motor.... when they closed us down it went on the skip along with a starter and alternator tester

As a superb air valve test rig  there Is limit on what you can acrue from the skip  in hindsight  could have set up a nice little earner

:o The Smiths drive units are as rare as you know what! Double Do'oh

oh Well

David

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23 teeth for 4.11 sounds near, maybe a bit on the fast side.  A 4.11 with 155/80/13 tyres is 16 MPH per 1000 RPM.

As a guide - 20 teeth (Grey) is for 3.89. and 19 teeth (Green) for a 3.63. Though some times the drive cog is also changed on the main shaft as a result I have seen 18 teeth (Brown) used on a 3.63.

Dave

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 Mark, teeth numbers don't sound right, this is probability me . So back to part numbers. Standard UK 1500 Spit has a 3.63 diff. If you are using this ratio then yes you need 219004 as given by Canley. I have checked this number against the 1850 Dolomite which uses the same diff ratio and it is the same, so it looks correct.

The Swiss 1500 Spit uses a 3.27 diff, 219001. The US 1500 Spit uses a 3.89 diff, 219005.

Dave

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