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255/85 16 tyres speedo adjustment


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I have to get some new tyres, and the 255/85 16s are an attractive size but are IRC 33". I was wondering if any folk on here who run them have made any changes to the speedo drive to correct the reading as std tyres are nominally 32" I think. Is there a different gear to go in the transfer box, or is there another trick?

Nigel

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There's 3 or 4 different speedo transducers that can be fitted to the transfer box. I'm sure there has been something on here about this before, but i can't remember where!!!

I'm sure someone else will be along soon that knows where it is or what they are!!

Chris

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if this is for a 110 [vehicle not specified in your post] the correct speedo drive gear should be the Blue 20 tooth FRC3310 -- Standard 110 fit, if it's for a 90 then just acquire a FRC3310.

Can also be used with 7.50x16, 235/85x16 & 255/85x16 tyres -- I've had all those on my 110 over the last 16years with no problems

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You may find, as I did, that the speedo "over-reads" the speed anyway with the standard tyre size on.

Comparing the speed shown on the speedo with the value given by a GPS device, mine showed about 10% over, IE. indicated road speed (on speedo) about 33 mph while on GPS at 30 mph, this was consistent throughout the speed range.

Fitting over-size tyres simply corrected for this error and meant that the speedo was spot on.

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if this is for a 110 [vehicle not specified in your post] the correct speedo drive gear should be the Blue 20 tooth FRC3310 -- Standard 110 fit, if it's for a 90 then just acquire a FRC3310.

Can also be used with 7.50x16, 235/85x16 & 255/85x16 tyres -- I've had all those on my 110 over the last 16years with no problems

It's a 110, already on 235s. I must try it with a satnav as I think it reads low already, hence wondering if there is a different gear. Indicated 30 is 30 according to the speed test sign thing in our village, but at an indicated 70 you are passing many things on the motorway. Ignoring a bit of wibbly wobblyness at low speeds.

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I have fitted : B F Goodrich mud-Terran T/A KM2 LT255/85R16 with 30mm wheel spacers on a 07, Defender with standard springs ,and not only are they the best tyres I have ever fitted in 21years of owning defenders but the speedo is spot on, compared to the GPS on flat ground ,their catch the wheel arches very slightly off road some times I may get some longer spings some time but 99% of the time they are OK.. Hope this helps

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  • 1 month later...

Any body know what the true tyre size is from factory when i bought my landy it came on 205/65/16 but they were road tyres so i put some 235/75/16 on and my dad was saying that it will affect the speedo, so the cheeky reply to that was when the tread wears down it will go back to normal :D, I cant see there being much difference from ratios ,and if need be where is the best place to get a transducer to correct the speedo.

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It's not the transducer you want to change (I think there's only one) but the gear that drives it. The gear sits in the transfer box and, as has been said already, you can get them with different number of teeth on them. The transducer has a square drive that is turned by the gear to generate the signal. It connects just the same as a speedo cable into the gear but sends an electric signal via wires instead of turning a drive cable all the way to to speedo head.

Malcolm

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