Agreed 100%.

I averaged about 16-17mpg driving conservatively in mixed suburban conditions including highway road trips etc. for the first ~20K in my Jeep with stock Sahara wheels/tires (32" Bridgestone Dueler), no lift, [virtually] no other mods, etc. This is a '15 JKU with a 6-speed and 3.73. I can't see any way on earth you could get 22mpg consistently unless you set the cruise at 45mph in 6th gear and drive about 50 miles in a row that way. Normal conditions? No way.

I just recently put 33" tires on, ATs similar to the stock Sahara tires, and I'm getting about 15mpg.

I just don't see consistent 22mpg even with stock tires to be reasonable under ordinary driving conditions.

 

Haha! I think I get 2 mpg less when I hang my foot out of the tub.

 

This is why I started the thread. I will try the GPS idea this weekend. Coming home from work today, I set the tripometer -- 21 miles, spot on. Correct me if I'm wrong, but if the tire size was incorrect, it would log miles faster than it should, no?

Then, I hooked up the programmer I bought when I had it regeared and the tire size was set to 33.5. For S & G, I set it to 34. I can't explain it, but everything SEEMS to be correct.

 

If you set the Jeep tire size smaller than the actual tire size then the speedo will read slow and your mpg calc will be too low, because each revolution of the wheel goes further than the computer thinks. If you calibrate too big, then your speedo reads fast and mpg calc will be high.

FYI with my original tires I had to calibrate to 30.5" to get the speedo to match the gps. Tires were 32". I just reset with the new tires which are almost exactly 33" and had to calibrate to 31.25" to get it right. That tells me your 33.5 was probably close considering your 34" or so tires.

Have you calculated the fuel economy the old fashioned way using the odometer and fuel pump when filling up? I wonder how those numbers compare. I think the computer calculation is often off my some amount.

 

This is Fun. Usually, "JK " and "Fuel Efficiency " are not mentioned in the same sentence.

 

Absolutely. Always at least check it to verify. Never been off by more than 1MPG (usually only a fraction thereof). Plan to take it tomorrow and do the measured mile.

 

You've got read-out error. Report back the actual mileage as calculated by actual distance and actual fuel consumed. I'll guess 14-15 mpg.

On another note, its strange there are no searches producing "fuel economy" threads? I've participated in dozens over the past 6 years...The best thing that came out of all of it for me is: 1) turn off the traction control and save 1-2 mpg, and 2) if mileage counts, put smaller, thinner, stock tires back on your vehicle and run two sets of rims/tires.

 

I've *been* checking the mileage against the fuel consumed (stated earlier in the thread). Did so again, today. It was off from the computer, but not outrageously so. I used GPS (34.1 miles) to track my trip AND used the Google Maps measurement (33.7 Miles), my odometer reading was 34.2 miles. Started with a full tank (filled from a gas can; to my eye, the gauge looked a bit shy of full), and filled at my destination, exactly 2.0 gallons. After changing the setting to 34" tires, computer was showing 26+ MPG. Actually calculates to 17, but as I said, I don't think the tank was topped off when I started. I changed the tire size to 33 (it started at 33.5) and flashed it, again. On the ride home (I didn't refuel, yet), computer was showing 23.3 (which is about 1MPG higher than I was getting before). At this point, I'm content to ignore it. My bigger concern once this discussion started was, logically, in order for the computer to calculate that gas mileage (and have my manual check verify it with in 1MPG), if the reading is way out of whack, I would necessarily have to be logging more mileage than I'm driving. I've checked it against three fairly long trips (20+ miles) with no variance from the GPS, so that's not happening. So long as that's the case, I'll deal with whatever else.

 

you need to check reading over a full tank to empty tank...not a 20mile run. Hell I can reset mine and run 20 miles on a flat freeway and get 20mpg on my 37's and 5.38's but in reality over a full tank I get 14-15ish

measure the tire as the jeep sit on the ground (probably a 33.5in tire) put that in your programmer and run a full tank to empty

 

Either you're driving a unicorn, or something about your measurement is off. After many of these threads, I almost always see it coming back to 14-18 MPG depending on load and speed driven.