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· Senior Privileged Member
4,104 Posts
Discussion Starter · #1 · (Edited)
Before I did my change from 3.73 to 4.56 gearing, I did some formal testing where I ran my SSR under very controlled conditions, at 60 mph, alternating between 3rd gear and 4th gear every 3.0 miles, first northbound and then southbound on the same road.

That test , covered in more detail in an earlier posting on this board, showed that even with using 3rd gear versus 4th, which simulates using a 5.33 axle ratio (!), the loss in fuel mileage proved to be 16%. The mileage on this steady state 60 mph driving went from 22.8 mpg in 4th gear to 19.0 mpg in 3rd gear.

Based on that, and other anecdotal information from others, I predicted a 7% to 10% drop in mileage if I went to the 4.56 gearing.

Well, this morning, i repeated the test runs, on exactly the same stretch of road, using exactly the same number (four) of 3.0 mile segments in EACH direction (northbound and soutbound), and just finished doing the math on the results.

With the 4.56 gearing, my SSR averaged 21.1 mpg, or a drop of 7.4% from the 3.73 gearing! This falls within the range I predicted, and happily enough, near the low end of the loss range! I view that as a very acceptable loss for the giant increase in performance achieved.

So, those of you who have wondered and asked what such a gear swap would do to mileage: now you know.

Why such a small loss even with the 22% stiffer gearing? Because the SSR is HEAVY, and really can USE sitffer gearing. Why do you think that Chevrolet insists on 4.11 gearing for pickups that will haul heavy trailers?

Numerically low gearing IS a help in getting better fuel mileage, but the cost in lost performance greatly exceeds the gain in mileage when the vehicle is heavy, as the engine is working harder BELOW the rpm range in which it is most efficient (an engine is most volumetrically efficient nearer its torque peak, and most fuel efficient somewhat lower than that rpm, but not really low when under LOAD).

By the way, his does NOT mean that my OVERALL fuel mileage will decrease by 7.4%. It will do so at 60 mph cruising. Theoretically at least it won't decrease from its already lower mileage average at city speeds, UNLESS I USE the higher acceleration capability. Realistically, I probably will, as you tend to do what feels better, and the SSR now feels much better accelerating around town than it did!

Jim G

Rear gear concerns

My 2005 I would love more power out of the hole! My only concerns are

1. Will there be a problem with the speedo?
2. What was your total cost?
3. If I,am cruising at 75mph is there ant problems?
Is it cruising at ease or does it feel its about ready for a shift change?

Thanks for all your imput..............Billy

· Senior Privileged Member
4,104 Posts
Discussion Starter · #7 ·

1. Will there be a problem with the speedo? No, you change the value in the onboard computer from 3.73 to 4.56. No problem at all.

2. What was your total cost? $850 to have the work done at a GOOD shop that has done rear end changes before (it requires expertise and experience to do it RIGHT). In addition, I am going to spend another $175 to have the hollow aluminum driveshaft (which acts like a musical instrument amplifying the sound of the sitffer gearing) filled with sound deadening foam. I'll let you all know if this works.

3. If I,am cruising at 75mph is there ant problems? No, I selected the 4.56 knowing that I sometimes cruise at up to 78 to 79 mph on interstate trips for hours on end. It feels GREAT - much better than the stock gearing. The engine turns at 2198 rpm at 60 mph and 2930 at 80 mph with the 4.56 gearing. But, note again what I said about the driveshaft amplfying noise above. Again, i will let you all know if my fix works.

Is it cruising at ease or does it feel its about ready for a shift change? It feels VERY GOOD at high highway speeds, and actually ACCELERATES without a downsift when you apply throttle at all.

Again, wait until I have a report on the noise situation. That drveshaft seems to be a perfect bugle.

Jim G
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