235/85/16 vs 245/75/16

I find that with my standard 235/70/16's when my speedo is showing 100kph the GPS is reading 96kph :D

Quote:

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Originally Posted by Foxspell

Hi Chris,
I think you are confusing your speedometer with your odometer.
I find that on original tyres Speedometer is approx 7% optimistic however Odometer is accurate.
Your tyres would change this by approx 5%, hence your Speedometer would now be approx 2% optimistic however your your Odometer would under read by approx 5%. It is this 5% you would need to take into account when calculating mileage even though your speedo is pretty close to the mark.
I hope this makes sense.

Lee

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I understand what you are saying, however after testing with several GPS I have found my speedo and odo to be fairly close if not the same and both being around 5% out. Not exactly, but close enough.

As far as I understood it, the speedo and odo should show no real difference in regards to distance etc as they are based off of the same "signal". I could be wrong, but that is the way I understood it at least. I just don't see why LR would produce two different "signals" for essentially the same thing. The only difference between them is one is over a set time (per hour) and one is just a total traveled distance.

If anyone has any corrections I would be happy to hear them. Oh and sorry for the thread hijacking!

Chris

Isn't it just plain physics, the standard fitted tyres to a D1 was either the 215/16 or 235/70/16, at least they are the only two sizes that are mentioned in my owners manual and on the tyre placard on the drivers door.
A 235/70 has a rolling diameter (when new) of 735 millimetres, whereas the 245/75 is 774 millimetres and thus covers roughly 121 millimetres per rev more than the 235/70.
The speedometer by law has to have a reasonable degree of accuracy, so the 5% innacuracy of the speedometer with the bigger tyre is pretty right, about 52 metres further in the registered kilometre.
Maybe these days with the "Rozzers" having all the hi-tech gear in tow to nail you for drifting over the speed limit that could be a problem, but as said by others, just make an allowance for it or treat your GPS like the American Express Card, and "don't go anywhere without it".
Please feel free to correct my shoddy mathematics.

As for speedo and odometer reading the same, my bike speedo is around 8% out whereas the odometer is spot on according to my GPS. Both get their signal from the same sender. Not saying that my GPS is 100% but just that there is variation between the speedo and odometer.

Dave

Thanks for the replies, speedo does not worry me as its out further with 235/85 than 245/75 so I'm already used to it.

Take off is also ok as its a manual V8.

From what I've read I think I'll stick with 235/85s

Steelo,
They have been good offroad, currently a BFG MT, better than anything Ive had on it before offroad but onroad is pretty woeful, especially in the wet so Im considering an aggressive A/T or something similar.

Quick update, should be getting 235/85/16 GoodYear Duratracs in next week or two.

Its about the most aggressive AT I could find so hoping it will serve my purposes better than an all out MT.

Cheers,
Rez