Quote:
Originally Posted by RPG
A few thoughts on fuel economy. After fitting 31" mud terrain tires and mega short transfer kit with hall effect pick up my speedo was under reading by 20%. Not a real problem as I drive by the GPS but fuel economy was terrible even allowing for the 20% error. I installed a yellow box speedo correction unit (great Australian made product) and now get 12 to 12.5 ltr/100km which I don't think is too bad for a TJ. One thing a lot of us forget is when we change things this alters the data sent to the ECU and this will play havoc with the mapping. I too like most would like to see my temp gauge a little lower as I find the needle sitting at 100 deg a bit off putting. I ran a cooler thermostat for a while but then began to think if this was causing the ECU to richen the fuel mixture. I have tried to find some data on at what temp the ECU go to full lean running. I would interested in any ones thoughts as fuel economy and TJs are not normally two words that go together.
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my Jeep tends to average 14/100. has been down to 11/100 on long high way trips on flat ground, take it to the mountains and cruise around in low gear low range and it guzzles, and urban driving short trips, stop start, it guzzles.
engine temp too cool, as determined by the engine temperature sensor near the thermostat and the ecu will be in open loop where fueling is defaulted to a richer, air fuel mix. but this is below 65 deg C coolant temp. (not sure of the exact temp)
the other thing that puts it in open loop is wot, otherwise your vehicle should be in closed loop most of the running time, closed loop where the O2
sensors are monitoring exhaust gasses and feeding back to the ecu to control fueling
so, if you do short trips where the engine is cold and runs in open loop for a fair percentage of operating time, then you will see the fuel guzzle
as far as bigger tyres, to correct the speedo you just have to change the speedo gear on the transfer box.you just need to know tyre size, and diff ratio then select the correct gear, there are numerous tables for calculating this and the gears themselves are cheap and simple to change
the gears are plastic and coloured coded as per number of teeth
for eg, if you have 31" tyres, 3.07 ratio then you need a 27 tooth gear
http://www.quadratec.com/c/reference...ar-tooth-chart
Fuel economy
this conversation was one I was having with my wife today as I was working out since going with brand new 31" MTZ's the TJ is drinking like a fish (headers & hi flo K&N air filter) I have noticed going from about 400km around town with ATR's to 320/350km/tank
Moose
