ive gone from stock tires to 31's 
and have already noticed a change in fuel usage 
:'( but i love the not so stock look
I went to larger tires and compensated for the difference in odometer and speedometer readings using GPS etc. Having done that, I can say it felt like I was accelerating more slowly even though I wasn't, but since I didn't change gearing, I also had to work the engine harder to get the same "feel" of acceleration. Like I said above, I actually was accelerating pretty much the same, but the lack of engine revving made it seem to me that I was accelerating more slowly. Watching the actual speed etc. showed that it was about the same.
Since putting on the new tires, I got a ScanGauge II and used it to adjust my driving (which annoys the cr*p out of impatient drivers) and I'm now getting about 25% better mileage than I did with the factory tires. For example, I now get about 500KM out of a tank instead of 400KM. I accelerate more slowly, but seem to catch up to the group once I'm up to the speed limit, never mind at the next red light! It's a lot less wear and tear on the entire drivetrain, including brakes, clutch, tires etc. due to the more smooth driving. I drive more by throttle opening than speed, so my speed does drop off on hills, though I don't let it get crazy and drop a gear to stay "close" to the speed I was doing.
I also concur that with speeds over about 70KPH, the drag increases considerably based upon the required throttle opening to maintain a given speed. 80KPH is pretty much the best compromise, though I usually do 90KPH with decent results. Once I go to over 90KPH, the mileage drops off significantly. Numbers? Well, on the highway at 80KPH on fairly flat terrain, I can get under 11L/100KM and sometimes can get that in the city too. My "typical" is high 12's and low 13's on anything but a short hop.
The caveat is that I didn't have the ScanGauge with the factory tire size and so didn't have the benefit of the driving changes for a fair comparison.
The subject of throttle opening versus gearing versus RPM etc. is a pretty complex subject, hence why engine computer programming is an arcane "art"
Sure, it's easy to say that "everything else equal", the less RPM, the less fuel used, however it's that "everything else equal" that gets you. If there is a corresponding increase in load due to the lower RPM (higher selected gear or bigger tires or lower numerical gearing), then chances are the computer is enriching the air/fuel ratio, so for a given RPM, more fuel is used. It's a trade-off and without the ability to reasonably accurately measure things in real-time, it's very tough to determine what works best. Oh yes, I did discover that when my mileage was dismal one day, that my tire had also dropped to 8Lbs or less for some reason. The basics apply, maintaining engine tune (air filter, clean engine, spark plugs, wires etc.), slick lubricants, regular chassis lubrication and maintenance, optimal tire pressures, minimize stuff causing drag, smooth driving etc.
Egads... enough rambling.. While I don't buy a Jeep for good mileage, that doesn't mean I don't want to get the best mileage out of it that I can while still having fun!
Topic: Gas mileage (Read 3339 times)