4.56 vs. 4.11 MPG

where can i change my gears i got 33s in a 2.7 dc and its automatic

 

Ok, so about the ECU/Speedo being off. I tested it and you are right it is off, it is off by +.05 per mile (example in a pyshical 10 mile trip my odometer will read 10.5 miles). So If you do the math, the original qouted milage out of a tank is 250 and the mileage differential is +.05 so 250x.05=12.5miles difference. So with my speedo/ecu off by +.05 of a mile I should be getting 12.5 more out of a 250 mile tank of gas. Meaning if my speedo was correct, a single tank of gas should have been 262.5 not 250 and that gives me 16mpg Vs. the orignally qouted 15mpg.

So you are correct the speedo/ECU is off slightly, but when you factor in the speedo differential that in it is yet another vote for 4.56 gears when running 33x12.50

 

All very clear and well thought out. And I agree with you that 4.56 gears are definitely beneficial with 33s as they put the rpms and tranny ratios back in sync. But I think you are making some basic assumptions that will impact your results when calculating miles traveled and fuel mileage.

The important assumption is that your odometer was accurate in the first place. There is a lot of evidence that they are not, that they can be under by as much as 5%. I would speculate that putting the 33s on brought you very close to accurate and putting the 4.56s on put it back close to stock.

 

I can't argue with that statement. My test was just using the Hwy patrol 1 mile speed markers for 10 miles and on the 10th mile looking at what my speedo was reading. It wasn't an exact science if the mile markers were off then my tests were off. Obviously the only TRUE test is to get everything recalibrated. But since i haven't gone that route and don't think i ever will I figured I would post some helpful numbers wether good or bad to help others with the expensive decision of wether or not it is worth it to change thoose perfectly good stock 4.11's to aftermarket 4.56's.

But you are right this isn't an exact science I'm running with on this thread just my inaccurate speedo vs. possibly accurate Hwy partol Mile postings. So everyone make sure you take my numbers with a grain of salt. The only sure thing is that there was improvement in gas milage and power that both of us agree on even if we don't agree on the exact numbers.