Speedo under reading

[Martin OD]

Hi all. I am getting more confident in my newly acquired Triumph Vitesse 2L MK 1. Whilst driving through our village the local speed sign thanked me for doing 37MPH when the speed limit was 40MPH. However, my speedo was reading 30MPH. Assuming the speed sign was more or less correct my speedo is way wrong. Is there any obvious reasons for this? What should I be looking for?

[Josef]

The big things that can affect the speedo reading are the diff ratio, the pinion/worm ratio for the drive from the gearbox, and the TPM of the speedo (and the speedo being broken, but let’s leave that for now). You can figure out your diff ratio manually by jacking the rear and counting the prop shaft turns per wheel rotation. The TPM is given as a number below the fractional miles of the trip counter. I think 1152 is right for a 2L Vitesse but the same style speedo was used on the Spitfires, but with different internals. The gearbox would require the tunnel out and removing the pinion, so I’d check that last

[JohnD]

An even easier way, is to mark the wheel next to the ground, and push it until the wheel has turned one revolution.  Meanwhile, an assistant ( or you, if you have a gullible assistant) count the turns of the propshaft just over four is 4.11, just under is 3.86 etc.

John

[johny]

31 minutes ago, Martin OD said:

Hi all. I am getting more confident in my newly acquired Triumph Vitesse 2L MK 1. Whilst driving through our village the local speed sign thanked me for doing 37MPH when the speed limit was 40MPH. However, my speedo was reading 30MPH. Assuming the speed sign was more or less correct my speedo is way wrong. Is there any obvious reasons for this? What should I be looking for?

Have a look at your tyres as well because the correct size is 155/70x13 but a different one will change your engine rev/speed ratio. As a rough guide the correct diff and tyres on a 2L Vitesse will mean that first gear is pretty much redundant except for steep hill starts. Also this website is pretty useful for checking the speed you should have at different revs and by playing around with the different variables you might see where any error is coming from...

Gear Ratio Speed Calculator for Standard Triumph Drivelines - RichyRichRacing

[cliff.b]

Does the odometer/trip read distance correctly. If so then that suggests it is the Speedo mechanism at fault and everything else is ok.

As is the case with my Spitfire which also under reads speed.

[Colin Lindsay]

Try driving behind someone in a modern and have them hit their hazard lights when they reach 30, or 40 or whatever speed you require, or if you're able, talk to them hands-free on the phone. That will let you know just how far out the speedo is. I passed one of those detectors last week and it said I was doing 38 with the speedo reading about 34, so I must admit I'm happier it's reading over, than under, or whichever way it reads less than the actual speed.

Edited June 29, 2023 by Colin Lindsay

[Chris A]

I checked my recently repaired kilometre version against an old GPS unit I keep in the Triumph, I easily get lost, and found it very accurate, as long as the GPS was of course.

[Chris A]

1 hour ago, Colin Lindsay said:

Try driving behind someone in a modern and have them hit their hazard lights when they reach 30, or 40

As long as there are no police about, isn't having hazard lights on while the vehicle is moving an offence?

Instead of putting the hazard lights on they could just stamp on the brakes, you then also get a confirmation your brakes work as well as the speedo, No on second thoughts . . 

[Chris A]

I forgot to add that when I got my 13/60 it was fitted with 155/70 which were a bit old so I changed them for 155/80 and did notice that the speedo then read more accurately than previously.

[dannyb]

My speedo is accurate up until 40mph according to my tomtom, but then starts to over read. 80mph is about 72 on the sat nav. 

Danny

[DanMi]

5 hours ago, Colin Lindsay said:

Try driving behind someone in a modern and have them hit their hazard lights when they reach 30, or 40 or whatever speed you require, or if you're able, talk to them hands-free on the phone. That will let you know just how far out the speedo is. I passed one of those detectors last week and it said I was doing 38 with the speedo reading about 34, so I must admit I'm happier it's reading over, than under, or whichever way it reads less than the actual speed.

why not just use your phone, they all have some navigation app with GPS and show accurate speed.

[Martin OD]

Hi. Thanks for the suggestion but the phone is not practical. I will find and fix the problem. As a matter of interest I don't think my car has a cigarette light to charge the phone anyway.

[Martin OD]

1 hour ago, dannyb said:

My speedo is accurate up until 40mph according to my tomtom, but then starts to over read. 80mph is about 72 on the sat nav. 

Danny

All cars by design are made to over read. If I get get the 70mph my car shakes itself to pieces 😊😊😊🤔. I will be getting round to that problem later 

[DanMi]

30 minutes ago, Martin OD said:

Hi. Thanks for the suggestion but the phone is not practical. I will find and fix the problem. As a matter of interest I don't think my car has a cigarette light to charge the phone anyway.

I only meant use the phone to check speedo accuracy not a permanent solution

[Chris A]

1 hour ago, DanMi said:

why not just use your phone, they all have some navigation app with GPS

No they don't, I have a 'dumb' phone, it does have a camera function though

[Martin OD]

49 minutes ago, DanMi said:

I only meant use the phone to check speedo accuracy not a permanent solution

Ahhh. That makes more sense. I know it is underreading but I am surprised it is so far out at such a low speed. I will follow the suggestions in good time.

[Pete Lewis]

put some markers on the glass to replicate the truer speed   

Pete

[Mark B]

 

My mk2 gt6 Speedo was 10mph off across the range. Sideways forum put me on the right track and worked for me. Not sure if your Speedo is 7mph under across the range. You may want to have a read. I also have a gps dash top speedo, (ebay) used for more accuracy in the 20mph zones that are now everywhere ane enforced locally by mobile police cameras etc.

https://sideways-technologies.co.uk/forums/index.php?/topic/9438-gt6-over-reading-erratic-speedo/

 

Edited June 30, 2023 by Mark B

[Martin OD]

1 hour ago, Mark B said:

 

My mk2 gt6 Speedo was 10mph off across the range. Sideways forum put me on the right track and worked for me. Not sure if your Speedo is 7mph under across the range. You may want to have a read. I also have a gps dash top speedo, (ebay) used for more accuracy in the 20mph zones that are now everywhere ane enforced locally by mobile police cameras etc.

https://sideways-technologies.co.uk/forums/index.php?/topic/9438-gt6-over-reading-erratic-speedo/