I don't under stand it.I wen't from 265/75/16 to 285/75/16 with a 355 gear.I was running about 1650 at 65 now I am running 1700 at 65.The rpm's should have wen't down not up.

 

speed based on what? gps? radar gun? or did you re do the calibration for the speedo?

 

Go here for a tire size/speedo error calculator.

http://www.miata.net/garage/tirecalc.html

according to your tire sizes, your speedo is now 3.7% slow.
60mph on your speedo is actually 62.2 mph ground speed

 

Seeing as he has an auto. that could be the difference, more trans slippage.

 

now this gets me confused??? the more I read about this the worse it gets, I did the H2 swap and want to fix my speedo, was all settled down to the thought of telling the dealer to set for 601 rev per mile, using the above calculator it tells me different? It says going from 265 70 17 to 315 70 17 to set rev per mile to 587........ who is correct? if I am going to do this I want to do it right so I know my speed....... insurance is bad enough without points, I want accuracy! 8.7% off is what I get told by that calculator. this also translates to almost 200 rpm engine speed which will aid my mileage? this sure seems like a beat down subject but conflicting info is keeping it alive. Any gps folks have backup info for the rev's per mile????

 

I have a chart from DML tire and wheel calculator.It says going from a 265 to a 285 at 65 my true speed will be 62.66 and my gear ratio will drop from a 355 to a 342.This should lower my rpm's by about 50 or 60 but it did not. I also timed out a 53 second mile today.

 

To start with, tires are not necessarily a standard diameter. My Michelen XPS Rib tires are 32 inches. They are 235/85 16R. The Goodyear G159 tires that they replaced were the same size except that they were 30 inch tires. You need to determine the rolling diameter of the tires you took off and the tires you put on.

 

I switched to 285's...truck came with 265's. I have not corrected my speedo and at 60 mph I run 1625 RPM with same rear end as you.
Remember one thing...with new tires you are no longer going 65 when your speedo reads 65...your going about 68 mph; higher not lower like your calculator says. Has to be higher..each rev of the bigger tires are covering more ground!

53 second mile sounds way off..that's an 11 % difference. Mine only shows a 3.5% difference, which the math on # of rev's per mile confirms! See here for Revolutions per mile: http://www.michelinman.com/assets/pdfs/doc_ltxms.pdf

RJ