Disco speed with bigger wheels

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bigrichsri

bigrichsri

Active Member
Posts
254
Location
Derby
Hi, I've bought a set of 35" cooper stt's for my disco, not got round to fitting them yet as I've got to do the body lift and fit the extended arches so that they don't rub.

Obviously with wheels 6" bigger than standard the speedo is going to be out by 21% and instead of doing 60mph as the speedo says, I'll be doing nearly 73mph.... all very well if I'm driving as I'll remember and keep well under the limit. But what about when the missus uses it? I feel several speeding fines heading my way.

My question is, is it possible to get a speed transponder that compensates for the larger tyres? And if not, why not, surely this is a good idea? Or is there any other way of correcting the difference, short of pulling the speedo needle off and moving it round by 10mph?
 
the speedo would need re calibrating or i remember a few years ago seeing an underdrive gearbox that bolted to the speedo cable that would do the job.......

even with a body lift of 2" and a suspension lift of 2" my cousins bobtail still caught the inner arches on full articulation, the outer arches were cut back as far as the inner arch so they looked like lightweight ones!
 
Cheeksgt said:
seeing as the error is percentage based am i right in thinking you cant just move the speedo round?

I would also like a solution to the above problem. :)
Click to expand...

Yes this is true but if the needle was moved 10mph it would cover most speeds, the vehicle would only be doing a few mph over at 60mph and 70mph and under at lower speeds.
 
yes I have posted before there are several speedo drive units for cable operated lt230 speedo drive
 
90/110 speedo drive gears
frc3310 blue --------- 20 teeth
frc3311 green ------- 21
frc3312 yellow ------ 22
frc3313 red ---------- 24


Discovery 1 [200Tdi/V8]
frc3310 blue ------- 20----diesel
frc9339 ------------- 23----[see note]

frc3310 blue ------- 20 ---- V8 efi
frc9339 ------------- 23 ---- V8 twin carb

Note frc9339 diesel for gulf states vehicles,no colour code listed, auto trans Tdi vehicles

200Tdi 110 runs a blue 20t frc3310 on 255/85r16 tyres, speedo is nearly spot on with gps- less than 4 mph difference.
Click to expand...
again

suffex of box may also affect it as input gear on a 300tdi or td5 transfer box is different
 
fanatic said:
yes I have posted before there are several speedo drive units for cable operated lt230 speedo drive
Click to expand...

Is it something inside the box that needs changing as it's just a sensor and a plug on the outer casing of mine. No cable drive to anywhere.
 
why not take it out with gps when your done changing the wheels, it will give you an idea of the speeds you are actually doing and you may not need to do much to compensate.

when i fitted 32" to the disco i found the speedo was actually spot on and with standard 29" tyres it read under. you are more than likely going to find that 35" will give you a reading possibly 5mph over and wouldnt be too much to deal with.

I would be more concerned about replacing the transfer box with one from a defender if you havent already
 
It may be easier just to get a satnav and use the GPS speed? I think Halfords are doing 50% off all satnavs at the mo.
 
bobbler said:
why not take it out with gps when your done changing the wheels, it will give you an idea of the speeds you are actually doing and you may not need to do much to compensate.

when i fitted 32" to the disco i found the speedo was actually spot on and with standard 29" tyres it read under. you are more than likely going to find that 35" will give you a reading possibly 5mph over and wouldnt be too much to deal with.

I would be more concerned about replacing the transfer box with one from a defender if you havent already
Click to expand...

I have 31's on now and it reads almost spot on, so with 35's will read 13% under. 60mph on speedo will be 68mph.
 
bigrichsri said:
I have sat nav but it's just a ballache to keep looking at that instead of the speedo.
Click to expand...

They don't work in tunnels either. Which is why the car's speedo can't be a GPS based one. By law, the speedo can never underread, it can overread by 10% though. But the chances of actually being done for it are virtually nil.

If you can twig which speedo drive cog to swap to, bob's you're uncle. Otherwise the suggestion of re-writing the numbers on the speedo dial is the best one.
 
Did you ever get any joy getting your speedo to work properly? I'm about to put bigger wheels/tyres on my 1995 V8 (285/75/16 is the plan) and I'd rather have the existing speedo work rather than have to come up with a bodge to make do!!
 
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