Speedo not calibrated correctly

[studmuffin]

After owning a Tomtom SatNav for 3 years I finally used it today in my TD5.

According to the satnav my speedo under-records by 3mph. Not a big problem unless you are going along a 50mph average speed section on the motorway.

So - question 1 - is the satnav always right?

and question 2 - Is it possible to easily recalibrate a TD5 speedo?

Any help gratefully received but-----

Please bear in mind if replying that I am a tad computer illegitimate!!

Cheers

Barry

[western]

Td5 speedo although electronic still runs off the speedo drive gear in transferbox, this gear depends on what size tyres you are using.

a over read on the speedo would be of more concern. i.e. speed limit is 40, your driving at 40 on the speedo but your actually travelling at 43, IIRC it's legally OK for speedo to read less but not over

[Paul Woodward]

It's probably because you have larger than standard tyres? You can change the drive gear in the transfer box to adjust it.

I have 255/85x16 tyres and use the blue gear. Speedo reading is spot on.

There are a few different ones;

http://www.ebay.co.uk/itm/LAND-ROVER-DEFENDER-Speedometer-Drive-22teeth-Blue-FRC3311-/370887472307?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item565a9db4b3

http://www.ebay.co.uk/itm/LAND-ROVER-DEFENDER-Speedometer-Drive-21teeth-Green-FRC3310-/370887472238?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item565a9db46e

http://www.ebay.co.uk/itm/LAND-ROVER-DEFENDER-Speedometer-Drive-23teeth-Green-FRC3313-/370887472364?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item565a9db4ec

http://www.ebay.co.uk/itm/LAND-ROVER-DISCOVERY-1-Speedometer-Drive-20teeth-Blue-FRC3310-/370887472417?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item565a9db521

[ashtrans]

After owning a Tomtom SatNav for 3 years I finally used it today in my TD5.

According to the satnav my speedo under-records by 3mph. Not a big problem unless you are going along a 50mph average speed section on the motorway.

So - question 1 - is the satnav always right?

and question 2 - Is it possible to easily recalibrate a TD5 speedo?

Any help gratefully received but-----

Please bear in mind if replying that I am a tad computer illegitimate!!

Cheers

Barry

This would do it :

http://www.ashcroft-transmissions.co.uk/speedo-correction-kit-c102.html

Dave

[studmuffin]

Td5 speedo although electronic still runs off the speedo drive gear in transferbox, this gear depends on what size tyres you are using.

a over read on the speedo would be of more concern. i.e. speed limit is 40, your driving at 40 on the speedo but your actually travelling at 43, IIRC it's legally OK for speedo to read less but not over

Thanks everybody. Great advice as always.

I meant the satnav reads 43 but the speedo reads 40mph,

I am running standard size Goodrich ATs.

Cheers

Barry

[western]

Probably already got the blue 20tooth gear fitted then, it works with 7..50,235 & 255 tyres, I suspect its ok for 265 tyres as well, just use the sat nav as a secondary instrument, speedo should be the primary speed reading

[Retroanaconda]

Pretty sure it's the other way around Ralph, it's OK for the speedo to rear over but not under.

In most cars you will be doing a little less than the speedo shows. On mine 60mph indicated is actually about 58mph so the speedo over-reads by a very small margin. This is the way you want it, as the speedo under-reading could land you in trouble when you think you're doing 60mph and are actually doing 65mph or the like.

The speedo head can be recalibrated by altering the programming on the EEPROM chip inside it, but it's much easier to change the drive gear in the transfer box to that specified for your tyre size. What vehicle is it? If it's a 90 the factory tyre size could have been 205R16, so fitting 255/85/R16 for example (another standard size) could make the speedo under-read.

[miketomcat]

255/85x16 are 33" ish but 265/75x16 & 235/85x16 are around 31" so with 255 it will under read by about 4mph. I thought all defenders were on 31" tyres by td5 not 205's (29") also the difference between worn muds and new can be 1-2mph.

Mike

[CwazyWabbit]

This would do it :

http://www.ashcroft-transmissions.co.uk/speedo-correction-kit-c102.html

Dave

......

The speedo head can be recalibrated by altering the programming on the EEPROM chip inside it, but it's much easier to change the drive gear in the transfer box to that specified for your tyre size. What vehicle is it? If it's a 90 the factory tyre size could have been 205R16, so fitting 255/85/R16 for example (another standard size) could make the speedo under-read.

There's the 3 answers which are a nice even mix of convenience and price.

The cheapest solution is to change the gear, this should get you close but may not be exact.

Modifying the eeprom would allow you to get it exact but means you would be without your speedo for a few days.

Dave's solution would allow you to get it exact and you don't have to send your speedo away, plus you can fiddle with it if you want in case you got it wrong the first time

EDIT: Just to add Daves solution will allow you to compensate for tyre wear to as you can have a fiddle when you notice the reading has changed