Speedometer calibration 10% out

M

Maplesyrup

Member

Posts

11

Location

Bristol

• Jan 7, 2016

• #1

Hi Guys,

The speedometer on my 2000 TD5 Defender 90 reads 10% too slow according to my GPS (with 235/85/16 tyres)

When driving at an indicted 60mph Im actually traveling at 66mph etc.

Does this suggest that the transfer box has been changed in the past?

It seems like too much variation to just be tyre size?

Thanks for any help

 

Flossie

Well-Known Member

Posts

11,251

Location

Shropshire

• Jan 7, 2016

• #2

That's spot on. Almost every car I've driven reads approximately 10% less than the sat nav.
That's why 30mph limit speeders can't understand when a fine don't drop through the letter box, you can be nearer to 40 and be OK.
Those fools that moan they were caught "a couple of mph over" in a 30 had 40 + showing on ther speedo but they won't tell you that...
OOPS!, read the post wrong, normal is eg- 66 on satnav 60 on speedo- silly me!

 

M

Maplesyrup

Member

Posts

11

Location

Bristol

• Jan 7, 2016

• #3

No worries

I've calculated the tyre size difference between what i assume was standard on my TD5 205/75/16 ? And the 235/85/16 tyres I have fitted and that would seem to suggest it is just the tyre sizes throwing it out at 11.42% difference in diameter.

Does that sound right?

Can I change the speedo cable drive to correct it?

 

Flossie

Well-Known Member

Posts

11,251

Location

Shropshire

• Jan 7, 2016

• #4

I thought td5 speedo is electronic? Dunno, too posh for me!

 

zeaphod

Well-Known Member

Posts

3,110

Location

Belfast, Norn Iron

• Jan 8, 2016

• #5

The speedo will be set for the tyres fitted at the factory. Not sure about the TD5, but there are certainly two types of speedo for older Land Rovers, depending on whether the 205 or 285 tyres were fitted.

 

Brown

Well-Known Member

Full Member

Posts

7,992

Location

East Midlands

• Jan 8, 2016

• #6

Although the speedo drive to the speedometer is electronic on a TD5, there's still a little mechanical bit where a small gear wheel meshes with the output shaft, and then this spins the transducer. you can get different gear wheels to give you different ratios which will enable you to compensate for differing tyre sizes
http://www.land-rover-blog.co.uk/how-to-replace-the-speedo-drive-gear-on-a-land-rover/
You might want to try a gear wheel with fewer teeth to spin the speedo drive a bit faster.