94 nissan pathfinder tire question

"Shawn" wrote in news:arH0d.28268$D%.23516@attbi_s51:

How do you know the current speedo readings are correct?

I have a GPS receiver, and have used it to measure(and record in a notebook I keep in the truck) the actual speed compared to the speed indicated on the speedometer. I've also checked the GPS speed against police laser radar.

For example, I have a 92 PF SE V6, and two sets of tires.

With the summer tires(31x10.5R15) the speedo is close to the actual speed - it's low by about 3 percent at all speeds.

With the winter metric tires(smaller size - I think the same as your current tires) the speedo is almost exactly

10 percent low at all speeds.

Once you know the 'offset' of the sppedo to the true speed, just adjust mentally - it's a lot simpler than worrying about changing gear ratios.

Hey Shawn! I'm not sure what size tires you would need. I would go see my local tire expert for some advice, keeping in mind that any change in a tires 'outside diameter' will affect speedometer readings. Changing the outside diameter translates into a difference in tire circumference (distance travelled in one tire revolution). Changing to a smaller diameter tire (shorter circumference) means you must work your engine harder to keep up. Remember the 100m in the olympics? The guys with shorter legs needed to move them faster to keep up with the runners with longer legs. Your vehicles gearing rotates faster now giving a false speedometer reading.

Going to larger diameter rims would mean going with lower profile tires to maintain your original tire 'outside diameter'.

Hope this helped a bit!

Thanks for the help thus far, I've checked some other forums in the "Yahoo Groups" and was given this site.

"just plug in your factory stocksize on the left and the new size you want on the right.It will calculatesize and differences for you along with any change in spedo readings." Ihavnt tried the site yet but thought others may be interested in it as well.thanks again for the help. Shawn

Buy a cheap GPS and use it for a speedo. You would be surprised how far off many auto speedos are. My new 2004 motorcycle speedo is off 7 mph to the fast and adds about

100 miles on the odometer for ever thousand traveled.