speedo's wrong now!!

[Jake]

Hi all.
I've fitted 235/85R16 tyres to my discovery and now the speedo is reading wrong. Does anyone know how i can remidy this. Or is it impossible.
Ta :oops:

[Thrasher]

....you can - but most of us just work it out ;-)

There is something twirly in the big lump of spinning cogs that works the speedo. You need a different sized one :-)

I'm sure someone will pop along with a part number and the number of hours/days it will take!

[strapping young lad]

your speedo will be slow so you need to get it adjusted.. ive seen an article on doing this in an old lr mag

short of going to a garage and having it reconifgured there isnt much you can do if this article isnt available (and i aint got it)

d

[Jake]

I've susted it to being approx 4mph out at 30 (26 on the clock) but its
the other speeds i have'nt worked out yet. Oh well. I'll just have to carry
on then.
Cheers guys.
Merry Xmas!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

[Paul]

Use a GPS to find out what it should be reading and stick some tape
on it so you can easily work out your speed.

I fitted 235/85's on my Range Rover and it made the speedo read spot
on with my GPS, Must of been reading wrong to start with.

[Sooty]

Most speedometers read about 10% under to allow the manufacturers to fit any speedo to any car without having to calabrate every one off the line to avoid one reading 30MPH at 33MPH and maybe being prosecuted for somebody being done for speeding.
I have 235/75 fitted to my 110 and the speedo is now spot on, all my company cars (unmodified) read 10% under.
As Paul says, check it against a GPS, you may be suprised.

[hobbit]

My 235/75s are the same, my speedo is reading down 2/3 mph, according to the gps, my fathers zafira is worse, 5/6 mph down at 70

The disco at that 2/3 I can live with, least you can sit on the needle and know you are within the limit

[thermidorthelobster]

When I went from 235/70s to 235/85s (the same diameter as 265/75s), the speedo went from reading about 7% under to 3% over.  I can live with the 3%.  At 40mph it's pretty accurate, it's around 70 you have to be aware of the couple of mph difference.

[scorpio]

I put 235/85's on mine and it made the speedo read spot on.

[turtle]

Put 265/75R16 on my defener and its put the speedo right out, gald I have the GPS to keep me in line  :D

[Mark_Sims]

235 70's on mine put it at approx 27 on GPS when speedo reads 30, up to 75 on speedo and GPS reading 70.

Mark

[Mace]

try this website for % changes to speedo readings http://members.shaw.ca/jbarge/tires.html

or this web site for more info on % changes http://www.tyresave.co.uk/tyresize.html

The conversion on a Defender LT77 gearbox involved changing the drive cog at the bottom of the speedo cable where it enters the gearbox. Land Rover sell different toothed cogs for this purpose. It's a pain in the butt to do so most people don't bother for £10 worth of parts. I've not explored the options on an R380 gearbox, but I'm sure it's just the same but probably more difficult to get at.

[Matt_H]

or even better, use my website http://www.m-v.demon.co.uk/tsc.htm  you can enter up to 4 tyre sizes and the 5 speeds andit will give stock and proposed tyre sizes.

My disco came with 205/16s on and I put on a set of grizzly 235/75/16 and it's about right a couple of mph under at the key speeds so no speed camera flashes.  I'm planning on goign to 235/85/16s, which will put it 4.3 mph over stock at 70.

A trick we used to do when chaning tyres on things like minis and imported bikes and cars was to put a sticker at the new 70/50/30 mph zones.  The other thing we used to do was to use a bicycle computer with a magnet on a drive shaft to count the axle revs and work out the speed from that.

As most others have said though you need to know how accurate yuor speedo is to start with.  See if you can borrow a GPS and measure your speed over as straight and flat a road as possible and keep your speeds constant (gps takes a second or two to regester speed changes normally).

Matthew