speedo accuracy

I have checked the accuracy of my my speedo for KPH using my GPS - 7 satellites must be accurate ?? The trip distance I have checked using the 5km roadside distance measure signs that are now on a lot of main highways. My theroy is - if the tyre is bigger you will get a smaller reading over a measured distance. Someone else may be able to confirm this.

Mathematically, here goes.....

If you went from 265-70-16 (LC size?) to 285-75-16, then...

you have gone from a tyre diameter of approx. 777mm to 834mm. Divide 834 by 777 and you get approx 1.07.

Therefore you speedo will be approximately 7% out. ie If your speedo says you are travelling at 100kph, you will actually be doing 107kph.

These are round figures, but close enough.

Yes you will be travelling more kms than your odometer says.

Should you contact your insurance company? Up to you. There are different laws in every state. I'll leave this one alone as I'm sure plenty of others will give you their opinion.

PS If you live in Tassie, you are legal!

Hi Andrew,
You could try this site out for your tire calcs. "www.miata.net/garage/tirecalc.html" Don't know about the insurance query.
Ciao,
Wayne.

Do you know someone with a GPS???

Take them for a drive, and mark with Liquid paper the correct markings on the speedo for the correct speeds.. Otherwise get it recalibrated

GQ/GU's you can buy different gears for the speedo for under $20 to correct your speedo. Change them on the side of the gearbox.

Andrew,

Nissan Dealers sell the gears, Ive posted here the conversion table before, so have a search of the archives.

they are supposed to be 20 min job to change. I havent done it yet, too lazy!

Andrew
How do you find the 285/75/16 (33") on the poor old 2.8. With my 32" tyres she runs away a bit on hills. Not real happy about it. Was pretty good with 31". Checked my speed with a GPS just 2 days ago. With my 32s at 100k on the clock the GPS said 102k. BUT at 60k on the clock the GPS said almost 65k. The indicative and true speeds do not seem to be linear. Be aware of this if you go for one of the $20 gears Truckster was talking about. May make one speed accurate but the others still may not be. If I went by the 60k check and bought a gear I would be underspeed at 100k. Do your GPS checks and just remember how far out you are for 60/80/100 etc.

Andrew,

the Rockhopper low range gears and things have come down in price a lot now, www.bigballsoffroad.com have them cheaper. They were $3200! now $1500 I think.

And I know what you mean, I run 33's and 35's, downhills are eye opening...