..my first modification(s). Rubicon Expess 3.5 SuperFlex, RE Front Adj UCA's, Bilsteins, Tom Wood's front DS, Rubicon 17 inch wheels wrapped in BFG MT 255/75/17. I have NOT corrected the speedo. But I do know that when my speedo reads 60 mph, my gps shows i am doing 65 mph. This is a 8% difference.
Sounds about right. 255/75 R17 is pretty close to 32" diameter, with the 32s I had with stock gearing my speedometer showed very close to actual speed I was doing according to a GPS.
When you go higher on tire diameter without changing speedo gear or going to shorter gear ratio on the axles, you get closer to actual speed what your speedo is showing. Go large enough, and speedo is showing less than what you're driving.
Interesting that your speedo shows lower than what you're actually doing. My 98 with 33s shows 80 mph on the dash and my gps shows 70 mph. Thats with long arms and 33s.
Well that's kinda weird really and should not be physically possible, if you're running stock gears and stock speedo gear.
Before these mods, I would average anywhere from 15.xx to 16.xx mpg. Now my travel computer states I am getting 10.2 to 10.4 mpg. Taking into account the 8% error rate i am guessing its actually closer to 11.0 to 11.2 mpg.
11mpg sounds a tad low, but that really depends how much stop-and-go traffic you're doing and is the area you live in hilly or pretty flat. You can get the MPG to drop to as low as around 11-12 doing city driving on stock gears and 32s. I was averaging 20-22 litres/ 100km with stock gears doing 50/50 city/highway driving (or even more on cold winter mornings), That calculates to around 10.5-11.5 mpg.
The factory gears are 3.73's. Because of the larger tires I am assuming the extra fuel consumption comes from the extra effort it takes to get the vehicle up to speed. So my question; If I were to change the gearing to 4.10s or 4.56s would this help to regain some of my lost fuel economy?
I highly suggest doing so, although you most likely will not gain your money back from filling up less often within the near future - if ever.. Regearing is expensive. So it's not so much about the money, but more so because you will get your performance back to what it was, and you also reduce the risk of burning your tranny (niners do NOT have an auxiliary tranny cooler, which they should have!), and if/ when you go wheeling, it won't suck because of too tall gears on larger and heavier-than-stock tires.
I changed to 4.56 gears, and I'm now averaging ~16-17 mpg on the highways doing between 55-65mph, and ~13-15 mpg in purely city driving. I am also back to about stock performance; the heavier tires and other off-road mods have raised the weight of my Jeep by about 200lbs., so all that shows in both performance and gas consumption even though I have the shorter gears. edit: If you are looking more into reduced MPG than performance, you might want to look for 4.11 gears. Then again, if you ever go higher in tire size - which is more than likely, if you start taking your niner offroad
- you will want shorter gears.