Speedometer Error

Submitted: Wednesday, Jan 05, 2005 at 10:41
ThreadID: 19081 Views:1545 Replies:4 FollowUps:4
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Hi all

I have a GU Nissan Patrol and originally had the standard wheels and tyres 265/70 R16 and at 100 km/h on the speedo i was actually travelling at 91 km/h.
So i changed tyres to 265/75 R16 and still had about a 5% error, 100 km/h on speedo and actually travelling at 95km/h. (note: speed measured using GPS)
Now the current sensor gear has 15 teeth, i would like to know wether i should go down to the 14 tooth gear or up to the 16 tooth gear.

Thanks a lot.
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Reply By: greghud - Wednesday, Jan 05, 2005 at 10:52

Wednesday, Jan 05, 2005 at 10:52
down a tooth will decreace the amount of teeth for a single turn meaning an increce in speedo reading. never had to do it but i figure thats correct.
i rember a previous thred about this so maby do a search just to make sure. greg
AnswerID: 91406

Follow Up By: Johnno1 - Wednesday, Jan 05, 2005 at 11:05

Wednesday, Jan 05, 2005 at 11:05
I would have thought a 75 series tyres would rotate slower than a 70 series ??

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Reply By: Truckster (Vic) - Wednesday, Jan 05, 2005 at 11:24

Wednesday, Jan 05, 2005 at 11:24
Leave it as it is, since you know what the margin of error is now, you may change it again and it still wont be correct as your tires wear it will change again..
AnswerID: 91409

Follow Up By: Member -Dodger - Wednesday, Jan 05, 2005 at 15:50

Wednesday, Jan 05, 2005 at 15:50
Truckster is right, don't change it as you can vector the error in when doing any calculations.
I used to have a handle on life, but it broke.

Cheers Dodg.

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Follow Up By: greghud - Wednesday, Jan 05, 2005 at 16:47

Wednesday, Jan 05, 2005 at 16:47
YEA, DONT START FIDDILING COS U WILL NEVER STOP, TRUST ME!!!!
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Follow Up By: Member - Russell S - Thursday, Jan 06, 2005 at 11:38

Thursday, Jan 06, 2005 at 11:38
absolutely.... why bother if you know? My cruiser is out by about 6%, so to cruise at 100, I indicate 94. Easy.
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Reply By: alister - Wednesday, Jan 05, 2005 at 13:44

Wednesday, Jan 05, 2005 at 13:44
A change of 1 tooth in 15 gives 1/15*100% = 6.66% change on the speedo, so 100 changes to 93.3 km/h. Sooo... speedo at 93, real speed at 95...
AnswerID: 91427

Reply By: 3.0turbob - Wednesday, Jan 05, 2005 at 14:30

Wednesday, Jan 05, 2005 at 14:30
There was a website posted a while ago (don't know by who or when)
but it calculates speedo readings with varying tyre sizes.
Try www.miata.net/garage/tirecalc.html

although it doesn't answer your question about gear teeth.
AnswerID: 91432

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