Attention: TDR Forum Junkies Recently, I upgraded to 245/70-19. 5 tires and wheels (thank you very much Rickson) and adjusted the my speedometer using the Smarty Jr. I started with the static diameter of the tire mounted on the rim. At this point, the speedometer was indicating about 2-3 mph higher than the gps. And the overhead was telling me that I was getting incredible fuel mileage (22. 5 mpg). I adjusted the tire size down about 5% and on a ~150 mile trip, I compared the odometer to the distance on my Garmin GPSMAP 60Csx, which seems to be more accurate than the vehicle mounted unit.
The odometer was still a couple miles higher than the Garmin over 150 miles, speedometer seemed right on and the fuel mileage now indicated 20. 2 on the overhead. I dropped the tire size another 2/10", reset everything, and on the return trip, the odometer and the gps matched within a tenth of a mile (odometer lower than the gps), the speedometer still seemed right on and the overhead now read 19. 4. This was no load and driving in the mountains in central Oregon.
I'm amazed how sensitive the overhead mileage indicator is to the tire size and, by the way, if your tire size is off, the "hand calculated" mileage will also be off since the odometer is off with these computer controlled vehicles.
On the return trip to Alaska, my overall hand calculated (total distance traveled/cumulative total fuel) fuel mileage was 18. 52 mpg and the overhead claimed 18. 7 which seems to be close enough to believe. On the trip down with stock tires, the hand calculated mileage was 17. 22. The loads down and up were light though different but the weight was about the same (~1000# in the bed + 3 adults).
I am pleasantly surprised that my fuel mileage increased with the 19. 5" wheels and tires. And for any one interested, Smarty Jr. settings were tow and POD=99.
The odometer was still a couple miles higher than the Garmin over 150 miles, speedometer seemed right on and the fuel mileage now indicated 20. 2 on the overhead. I dropped the tire size another 2/10", reset everything, and on the return trip, the odometer and the gps matched within a tenth of a mile (odometer lower than the gps), the speedometer still seemed right on and the overhead now read 19. 4. This was no load and driving in the mountains in central Oregon.
I'm amazed how sensitive the overhead mileage indicator is to the tire size and, by the way, if your tire size is off, the "hand calculated" mileage will also be off since the odometer is off with these computer controlled vehicles.
On the return trip to Alaska, my overall hand calculated (total distance traveled/cumulative total fuel) fuel mileage was 18. 52 mpg and the overhead claimed 18. 7 which seems to be close enough to believe. On the trip down with stock tires, the hand calculated mileage was 17. 22. The loads down and up were light though different but the weight was about the same (~1000# in the bed + 3 adults).
I am pleasantly surprised that my fuel mileage increased with the 19. 5" wheels and tires. And for any one interested, Smarty Jr. settings were tow and POD=99.
