Member - Sam (NSW) replied:

Its correct. As the speedo and odometer are calibrated for the factory tyre size, increasing or decreasing the rolling diameter from factory standard is going to give an incorrect reading both on the speedo and on the odometer and hence will throw out any fuel consumption calculations. You need to factor in the size increase or decrease. There is a post somewhere on here that gives a URL for an online calculator that tells you the % of change between factory tyre size and what ever you have fitted.

Member - Alan H (QLD) replied:

Kingston

A reliable test is to use GPS on straight stretch of road. Gps is more accurate than your speedo to measure speed but only in a straight line.

Most GPS also have an oddometer display which may help but you would need a long straight stretch of road.

I quess the percentage that your speed is out will be the same percentage as the oddmeter readings.

Others may be able to throw light on more accurate tests.

Alan

Shaker replied:

The diameter difference is 3.29%, at 100kmh your speedo will read 96.7kmh, which is 3.407% slow.

Check it yourself at:

http://www.1010tires.com/TireSizeCalculator.asp?action=submit&reset=yes

Sky Pilot replied:

Don't know which state you're in but the RTA in NSW has speedo check posts on most major highways where you can measure the accuracy of your odometer agains kilometre signs - usually one each km for 5 km. Also, if you're in a city, most have a set of posts somewhere that used to be used to check the accuracy of taxi meters - I don't know if they still use them though.

warthog replied:

I have a gu patrol running the same size tyres as you and find the speedo is pretty accurate but the odo is around 10% out. Maybe the speedo was a bit optimistic with std tyres.

Member - TonyG (Qld) replied:

Hi Kingston,

I have a GU also, and had the same issue.

I found out that Nissan install all speedo gear calibrated for the petrol version of the vehicle whicl comes standard with 275/70/R16 tyres (I think).

As I am running a 3.0 diesel version, they came out with smaller tyres as standard 265/70/R16, and as such, the speedo was always out from the start. This was comfirmed by doing a gps speed check.

I understand they did this to improve torque figures on diesel engine vehicles.

With the speedo reading 100km/hr, the gps says I am doing 92km/hr.

When my tyres wore out, I upgraded to 265/75/R16 tyres. This has brought the speedo reading a lot more accurate.

Now when speedo reading is 100km/hr, gps says 98km/hr.

It is something that all diesel owners have to live with.

Thanks

TonyG

Rigor replied:

Just checked my speedo and odo on the weekend with the work GPS and it is absolutely spot on at 100 clicks and the ODO at 10 Kms was 10050 metres on the GPS . I have new standard tyres and I must say I never expected it to be so accurate. First car I ever owned that was.

Cheers Dave L.

BBails replied:

I concur with the others, I have 31 10.5 r15 tyres on my hilux. According to the GPS the speedo is now bang on but the odo reads well under - sorry not sure exactly the difference. Makes sense that the tow will change in the exact same ratio, so your dealer is right.

Most speedos are calibrated to read a bit under - like about 3%

B

Member - Phil G (SA) replied:

Tyre dealer was a bit over the top. Therse 2 reasons why fuel consumption has increased

#1 Diameter change of 3% (tyre dealer needs to relearn some maths)
#2 The BFG AT tyres have an increased rolling resistance, and also weigh a bit more.

Cheers
Phil