How do you guys adjust the tire size to match the actual mileage?
I'm running 33s on my 2.5 gen
Using tire spec method!
Suppose you have Yokohama Geolander. Lookup specs
http://www.tirerack.com/tires/tires...5GEOATS&vehicleSearch=false&fromCompare1=yes# click specs.
Stock size 265/70R15 - 700 Revolutions per mile
You've got 285/75R16 - 623 Revolutions per mile.
Take your odometer reading (whatever number of miles)
miles * (700/632) = miles * (~1.1235) = actual number of miles traveled. Calculate you mpg based on that.
Just like Ray said about 12% difference. So say if you calculate based on odometer 15 mpg, your real mpg is +12% = 16.8.
Of course true/real mpg is different. Usually speedometer/odometer is off somewhat. It overstates speed and odometer by 2-5% usually. Hence mpg is as well. To get true value you have to subtract that difference from above percentage.
Use GPS or well know distances to figure that out. From personal experience on Montero my actual speed is only about 2-3 mph above shown on speedometer at 60+ mph on 285/75R16. Which points to only 5% difference in speed/odometer/mpg.
If you have GPS and who doesn't. Get on level interstate. Set you cruise to say 70 mph or 60 doesn't matter that much but higher gives you more accurate reading (77 v 70 is easier to tell than 11 v 10). Stay legal please. Once you get steady going record your speedometer and GPS speed reading. Or better yet travel say 100 miles by odometer and compare that with GPS trip data
GPS speed/distance divided by speedometer/odometer reading will give you true coefficient to adjust calculated mpg. I my case it's only about (1.05) or+5%.