Different Tyre SiZES for a Disco TDi ??

[Robmacca]

Gents.....

For those in the know, would the following be correct:

Std Tyre Size = 235/70/16 (speedo should be reading correct?)

Tyre Size 235/85/16 = Increase speed of 9.59%(31.73" tyre)
Tyre Size 265/75/16 = Increase speed of 9.31%(31.65" tyre)
Tyre Size 265/70/16 = Increase speed of 5.71%(30.61" tyre)
Tyre Size 255/70/16 = Increase speed of 3.81%(30.06" tyre)

The Disco I recently bought came with 235/85/16 and I want to try and work out speedo error along what my TRUE ECONOMY IS....
The car still seems to go OK with these 235/85/16 tyres on them, which surprised me as being almost 10% bigger I would have thought there would a dramatic drop in performance from what I've read on here but that was more related to the "wider tyre" of 265/75/16....

Is there a performance difference between skinny and wide tyres of equal size ???

I'm assuming that there's out there running with the 235/85/16 tyres on their Disco's ??

advise would be greatly appreciated.....

Rob

[Dougal]

Tyre diameters are a good starting point. But the actual rolling diameter is the belts in the carcass.
The best place to get this information (revolutions per km or mile) is in tyre manufacturers data sheets. Most have it on their websites.

Stock speedos generally read high by about 5%.

[loanrangie]

Dougal is correct, stock disco with 235/70's will be out about 5%,

245/70 almost spot
265/70 under by about 1-2 kph
265/75 under about 4 kph

I'm on 265/75's and at 95 on the speedo gps reads 98.

[Rick1970]

My Disco reads 1km/hr slow with 245/75-16's

[hodgo]

I intended to use my navman read out to get this information when I get mine on the road I will now be able to compere my speedo and navman and the chart.

Hodgo

[Robmacca]

Seems like it may vary from vehicle to vehicle :)
I know with mine having the 235/85/16 it's every bit of at least 5% out but I need to go for a drive along the Hwy on a flat section to verify this.....

What about the Speedo's Odometer.... Will this be also out with the SAME percentage amount?
The reason I ask this is I know with our 150's Prado, that when we had std tyres on it that the speed reading was out by at least 2% or so but the actual Odometer was spot on as these two are independent of each other....
I'm assuming with these Discos that run a "Speedo Cable" setup that the actual speed and distance covered and tied together ??? Would that be right?



Dougal is correct, stock disco with 235/70's will be out about 5%,

245/70 almost spot
265/70 under by about 1-2 kph
265/75 under about 4 kph

I'm on 265/75's and at 95 on the speedo gps reads 98.




My Disco reads 1km/hr slow with 245/75-16's

[Dougal]

Seems like it may vary from vehicle to vehicle :)
I know with mine having the 235/85/16 it's every bit of at least 5% out but I need to go for a drive along the Hwy on a flat section to verify this.....

What about the Speedo's Odometer.... Will this be also out with the SAME percentage amount?
The reason I ask this is I know with our 150's Prado, that when we had std tyres on it that the speed reading was out by at least 2% or so but the actual Odometer was spot on as these two are independent of each other....
I'm assuming with these Discos that run a "Speedo Cable" setup that the actual speed and distance covered and tied together ??? Would that be right?

Just like your Prado, the needle position doesn't follow the odometer. You need to check them both indepdently.

[loanrangie]

I know with mine having the 235/85/16 it's every bit of at least 5% out.

5% out from correct but 10% out from factory reading with 235/70's, it does differ from tyre make to tyre make.

[cripesamighty]

Mine reads over by around 6% wearing 235/70/16 Bridgestone Dueler H/T's according to the trusty(?) GPS. Will see if it changes when I replace the Duelers with D697's in a few months.

[Robmacca]

Just like your Prado, the needle position doesn't follow the odometer. You need to check them both indepdently.

Thats very interesting as I would of thought that being mechanically driven via the cable (I'm assuming the speedo is cable driven off the Transmission) that the Speed and Odometer would be tied together for sure.... ?

Rob

[DiscoMick]

Legally, the speedo can't read lower than the actual speed or the vehicle may not be roadworthy, I believce.
On the original 235/70/16s at 100 km/h my speedo read about 105 km/h.
On 245/75/16s its almost spot on at 100 km/h.
Hope that helps.

[Robmacca]

Legally, the speedo can't read lower than the actual speed or the vehicle may not be roadworthy, I believce.
On the original 235/70/16s at 100 km/h my speedo read about 105 km/h.
On 245/75/16s its almost spot on at 100 km/h.
Hope that helps.

Thanks for Mick, but I might have it wrong here but shouldn't that be the other way around?

ie: travelling at 100kms/hr and speedo reads 105kms/hr ??

[Dougal]

Thanks for Mick, but I might have it wrong here but shouldn't that be the other way around?

ie: travelling at 100kms/hr and speedo reads 105kms/hr ??

Isn't that exactly what he wrote?

[Robmacca]

Isn't that exactly what he wrote?

Yeah, your probably right...... After rereading it a few times :-)

Not sure what I was thinking.....;)

[Robmacca]

Well, just worked out today from a little drive up the hwy that my speedo is about 5% out compared to the GPS and the Odometer is 9.8% (or basically 10%) out when compared to the GPS Distance travelled....

That sort of matches to what I worked out, but I wa surprised that the speedo didn't match the odometer.... but I guess that's the "safety" factor that manufacturers built in....