lost 20% fuel economy by stepping up one size on my tires. is this normal.

LOL, no, that's exactly what I am saying. After rereading your post I think we are reaching the same conclusion but via a different presentation.

 

The good news for you is gas is $0.80/L in Canmore , and yes your getting "normal" mileage.

 

Gotcha....the link showed me that my -4% adjustment in my ScanGauge was definitely a little high but I'm good with a worst case scenario.

 

The larger the tire the slower the speedo will read. A larger diameter tire will spin slower than a smaller diameter tire.

Try to visualize 2 circles. One large and one small. If you marked a starting point on each circle and rotate them, which one will make a full rotation first? The smaller one will. The larger circle has more area to cover. Hence the larger diameter tire will spin slower giving you a slower speed reading.

 

Here are the results of my RPM Speed and throttle response test. Not to get OT but related so as to index 'cruising' speed at certain RPM's (for my Tacoma model) and for comparison of what might be considered hard driving (throttle response)

Truck - '07 Access cab sport 4.0 manual SR5 4WD 30kmi @ 4,300ft elevation

Mods - Shell, Bilstiens+AAL=2in lift, Tires BFG AT KO 165/70/R17 10ply, removed 2nd charcoal engine filter, otherwise stock

Test conditions - start cruising speed measured off GPS for true speed (it matched perfectly to my stock tires and guage before I went to the BFG's) - start speed was from a dead cruise to eliminate intertia effects, flat smooth road, no wind, 1/3 tank of gas. No shifting, remained in gear shown. At least 3 attempts to get an avg. seconds figure

Gear / RPM / Sec's to accel 10mph at WOT

30-40mph

2nd / 2600 / 2.3secs

3rd / 1925 / 3.3secs

4th / 1500 / 4.75secs

40-50mph

3rd / 2500 / 3.5secs

4th / 2000 / 4.5secs

5th / 1575 / 6.4secs

50-60mph

4th / 2500 / 5secs

5th / 2075 / 6secs

6th / 1750 / 9secs

60-70mph

5th / 2500 / 7.75secs

6th / 2050 / 8.75secs

70-80mph

5th / 2950 / 7.3secs

6th / 2500 / 12.25secs

At the 60-70 test I didn't go below 2050 knowing there be be little to no acceleration response below that (took 12.25 seconds just to get from 70-80 in 6th) so I didn't really test 1600 rpm at 60mph.

I just got my new tires and haven't put enough miles on them to get reliable new mileage readings (but definately lower than 17). I will follow up in this thread after a few tankfuls.