Vehicle & Technical > Discovery

speedo's wrong now!!

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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

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