Gas mileage results with the BRM 4.08 rearend

[Jeff Williams]

O.K. what is the hit, for installing this rearend?

I need to know, with gas prices approaching $2 a gallon, and filling up 3 times a week.

I want this mod, but may have to wait, until I get the next Q, and I can have one to play with.

[Q45tech]

Highway [steady state] mileage is pure and simple a function of engine RPM till the closed loop speed is exceed [around 85 mph- 2800 rpm] when the ecu increases richness to create the extra cooling required in the cylinders.

4.083/3.538= 1.154 or 15.4% WORSE at any rpm/speed between 50 and 85 MPH..............fuel consumption per hour of running at that speed.

Just as driving 85 vs 55 uses 54% more fuel per hour, just as using AC requires the HP to turn the compressor and alternator under more load...........5HP requires........at least 2.5 pounds more fuel per hour [2.5/6.2= 0.4 gallons per hour more].

http://www.uwm.edu/~horowitz/P....html

Most people have to drive in City and accelerate some so the steady state loss would be more than the blended average.

A safe bet is to use 10% [rather than 15.4%] as the probable average decrease for most driving. Say 90 gallons per 15,000 miles x $2= $180 per year.

When ever you change the grams per mile of emissions, an TRUELY ETHICAL person would send the government a check to pay for the extra pollution they create..........Just kidding......in that you pay at the pump.

I'm surprised that no one is looking for a lower ratio than the 3.538 oem...............but the Q would have a hard time pulling small hillls in 4th with AC on without a down shift [wear and tear on transmission and using more fuel maybe or at least no savings.

Generally manufacturers use the lowest diff ratio they can get away with for average terrain. Obviously costal California/Florida could use less than Denver or even NC mountains.

[3.538 x 0.694=2.455372 effective gear multiplication in 4th on a loaded 26" tire [= 25"] x 3.14159= 78.5" rolling diameter................5280/78.5 x 12=807 revs per mile.......so the tire rolling diameter is probably 25.1" vs 25"

[Jeff Williams]

I have thought of a 3.08 rear gear, believe it, or not.

I am kinda thinking along your lines, here, with trying to get the perfect combination, based on my driving style:

"but the Q would have a hard time pulling small hillls in 4th with AC on without a down shift ".

I would love to have one car, for "playing around" town in, and one for the commute.

The car gets great mileage, when I am on back roads, and I keep it around 55 to 70 MPH. I have experienced 24 MPG (10 to 20% increase, with a 20 to 30% decrease in RPM).

Usually, I am getting 20 to 21 (not bad), with relatively steady 76 to 78 MPH driving. Most of my driving is around 2400 RPM.

If the 4.08 increases RPM about 15%, then it might only hit me with a 7.5% decrease in fuel consumption ( rough estimates, not plotting a curve).

I downshift, about 10 to 15 times a day, in traffic. If the 4.08 helps keep the car from downshifting, at a 75 to 85 MPH pass, I could save a lot of gas, using this same logic. I have found, if I try to keep a steady state, of 70 to 74 MPH, I am in the group of cars, that is slowing down to 60, and speeding up to 80, while passing other vehicles. This is very frustrating, and I tend to get irritated, and blast to 100+, to get away from them. If I keep the car steady at about 77, I tend to be in my own crowd, between the 80+ drivers, and the 70+/- drivers. I can get into a steady rhythm, and keep from wasting gas & brakes.

That is why I would like to see some real-world results, from someone who did this change. Possibly a 3 to 4 tank average in city/highway driving before and after.

On paper (10 to 15% increase in fuel consumption), it doesn't make sense for me to do the swap, if I am really concerned about gas mileage. But, I could justify a 5 to 8% change, if it made a big difference in initial acceleration. I like the idea of getting in the power a little earlier. That is why I am considering the rear-end swap.

Thanks for the details, and link. I will do some more reading.

[Q45tech]

It's not on paper, but real world! All one has to do is drive in 3rd gear on the highway for say 120 miles that would be 44.9% more revs [and 44.9% more fuel used] and divided the resulting decrease by 3 to PROVE the exact number for a 4.083

If you don't drop from 24 to 17-16.5 mpg at exactly the same speed, as they say I'll eat my hat and pay for the live web feed.

If you have a Consult you view the injector open time [correct for ramp up opening and view it at what ever rpm [say in 3rd vs 4th and correct for the gear ratio].

If one were to keep the car for 6-7 years the change in gas mileage [assuming it doesn't go to $4/gallon [200x 6+ 1000-1500] and the diff gear swap...[total $2500-3,000]..........would cost less than finding a 5-6 speed AT transmission and trying to match diff gears and tcu and installation/new drive shaft/adapter plate to engine........which I estmate would cost $6,000 and lots of time to sort out.

The 5-6 speed would gain the most acceleration: 11.3 gear multiplication in 1st with 4.083 vs maybe 13-13.5 with a properly match 5-6 speed...............15% vs 35% more 1st gear torque at wheels..............you would have to run a 4.7-4.8 rear diff gear to equal a 5 speed step up.

[AZhitman]

Jeff, you are on to something that Dennis has not expounded on, but my last five tanks seem to confirm:

Average change in mileage over the past 5 tanks has been less than 10%. (First 2 tanks with the 4.08 were dismal, as I was relishing in WOT accelleration every chance I got).

I believe the reason for this is simple. I no longer feel the need to drop the hammer as aggressively, especially from a stop. Roll-in throttle response is much improved, and the 4.08 has another added benefit I'm noticing: You accelerate slowly from a stop, and roll into the throttle, effectively utilizing 1st gear to get you to 35-40 mph. A slight lift results in an immediate 1-2 shift, which is immediately followed by the 2-3 and 3-4 . You are now in 4th at 45 mph, with no lugging and little time spent in 2nd and 3rd.

Dennis can probably give the technical explanation for all this, but like I said, I think the fuel economy hit (on paper) would have to be 15%. The change in your driving behavior tempers this.

A good example is these people who think a 4cyl (or lower-hp engine) will naturally get better fuel economy than a 6 or an 8. Foolish in reality, because if you are heavy-footed, you'll spend a lot less time at WOT with a more powerful engine...

[Q45tech]

The point was/is that a properly spanned 5 or 6 speed AT can improve cruise mileage and increase acceleration for a higher initial cost........probably costs $250 [surely less than $500] over a 4 AT speed to the manufacturer.Unfortunately the bean counters FORCED the engineers to USE a transmission designed for economy1st Gear Ratio 3.54 x 2.76= 9.77 worse than 90-93 [9.85]2nd Gear Ratio 2.26 x 2.76= 6.243rd Gear Ratio 1.47 x 2.76= 4.06 worse than 90-96 [3.538] 4th Gear Ratio 1.00 5th Gear Ratio 0.83 x 2.76= 2.29 vs [90-96]=2.455

Final Drive Ratio 2.76:1 should have provided 7% improvement in MPG vs 90-96

Newer 3.133 x 0.83= 2.60 which should be 6% WORSE by gear ratio alone but a 13.5% TOTAL SPAN improvement for acceleration.

Look familiar in that the 4.083 is 15.4% and the 2004 gears are 13.5% higher.

I'll bet the real world highway cruise MPG at the same speed is worse on the 2004 Q than 2002...........just don't believe there was enough SLACK in MPG to make the 13.5% change in rpm without changing EPA MPG rating................

Whether [probably little doubt] that the newer 4.5 is 6% more efficient [engine, TC, transmission, etc] so the highway cruise of a 2004 Q might be a tiny bit better than the 90-96Q or just equal.

EPA sticker says 17/23 for the M45. Old Q was 16/22...........but 16.500001 is called 17 just as 22.500000000001 is 23.

The M45 has a 1st gear multiple of 11.09 STILL far short of the optimum 13.5.

The M45 need a 3.67-3.80 diff not a 3.133 but US consumers won't like a 19 MPG highway number..........even if it would knock 0.3 seconds off the zero to 60 and quarter.............see why 6 speed AT are the vogue now only way to have cake and eat it too! Or design a 5 speed with better spans.