GPS speedometer

[Loadie]

Just wondering if anyone uses one of these five quid gps speedos that you see on Temu?? My speedo reads 20mph at 30 and gets worse the faster you get… I seen all the blurb on needing the right speedo for engine gearbox etc and the numbers say my 1955 convertible has a traveller speedo fitted..
Thinking of a cheap gps thing to just hang on a handy hook. A 12v one even better if such a thing exists

[Bill_qaz]

AP on your mobile for free?

[oliver90owner]

Agree with Bill.

My comments:

Don’t buy anything from that supplier unless you can afford to throw it away when you find it faulty. Cheap trash is my opinion of that suppler.

Legally the speedo must record a higher speed than actual. It can read accurately, but is illegal if showing less than the real road speed.

Turns per mile may be different for the speedo head, drive from the gearbox may wrong ratio, differential ratio may have been changed. They should all be proportional to the speed. IE, double the speed, double the error.

A faulty speedo head, or slipping drive cable inner are other possibilities, I suppose.

All can be additive or subtractive, so can approximately cancel out - but likely not.

Tyre/wheel size can also make a difference, but unlikely in this case?

[Davemate]

Phone app, then little coloured stickers stuck on at the various true (ish) speeds or permanent marker pen
Should cost a lot less than a fiver

[geoberni]

the numbers say my 1955 convertible has a traveller speedo fitted..

The Traveller aspect is irrelevant as such; it depends on what the required TPM is.
If the car has been altered with different gearbox and/or Diff, then the TPM requirement is different.

As standard, all SII cars were 1000 TPM, latter cars were in the range of 1300 - 1500 TPM.

That may explain why you're 50% under-reading....