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Incredible but true: Gas mileage vs gear ratio test

2458 Views 14 Replies 7 Participants Last post by  JimGnitecki
Some of you know that I am doing the research that needs to precede a change I am planning in final axle ratio. I did some testing early this morning that generated results that are very encouraging.

By way of background, the SSR comes with a 3.73 rear axle ratio, which sounds "aggressive", until you remember that the weight of an SSR with NO options and NO fuel or driver aboard exceeds 4800 lb. With that kind of weight, 3.73 is not aggressive at all, but actually not enough. I will prove that in a separate posting on gearing.

In this posting, I address the question of "how much gas mileage do I lose if I change the gearing to an even higher axle ratio?"

In the past ("the good old days"), a competent mechanic would tell you that increasing axle ratio appreciably would improve acceleration performance notably (not just under hard acceleration, but also responsiveness at part throttle) but would really hurt your gas mileage.

A theoretical physicist would have objected, saying that work done to oversome wind drag and road friciton.
=rpm x torque actually needed to overcome wind drag & friction
is the same whether you use more revs and less torque per revolution, or more revs and less torque per revolution.

The mechanic would point out that there are 3 flaws in that theoretical approach:

1. A number of accessories on the engine run at engine rpm regardless of actual load, so the power consumed to drive them goes up with engine rpm even if the external drag loads on the vehicle are the same. So, more rpm at the same road speed means more power lost to drive these accessories.

2. Certain driveline frictional losses go up with rpm. So, more rpm at the same road speed means proportionately higher friction losses.

3. Carburetors (this was before fuel injection!) work in an inefficient manner that causes fuel mileage to drop if the bores are opened either too much or too little compared to the narrow range they were tuned for. Remember that carburetors did NOT "calculate" the real amount of fuel needed taking into account all variables, but rather were "mapped" by trial and error ONCE at the factory, AND used primarily air venturi effect to draw fuel into the air. (VERY "approximate")

The physicist today would say that 1. and 2, still apply, but fuel injection works WAY better than carburetors, if the fuel injection is mapped at all decently, because it actually has a computer that calculates the ideal fuel amount needed many times per second.

So, I did an experiment this morning.

I ran the SSR on crusie control at 60 mph, changing between overdrive and 3rd gear every 3.0 miles, first northbound and then soutbound (at a time when wind velocity was officially reported at "1 mph or less" anyway), on a hilly divided highway.

Now, 3rd gear is a 1.00 internal transmission ratio, while overdrive 4th is 0.70, so running in 3rd simulates a 43% change in rear axle ratio (!!), or in other words it simulates running a 5.33 axle ratio! I was at 2600 rpm at 60 mph when in 3rd!! In other words, this simulation is absolutely the WORST scenario anyone could picture in terms of losing gas mileage, and far worse than what a 4.11 (10% ratio change) or 4.56 (22% ratio change) would do to your mileage.

For each 3 mile test segment, I did the following:
- Select either 3rd or 4th gear
- Reset trip meter to 0
- Reset cumulative (not instant) MPG
- At 3.0 miles, note and record the cumulative MPG reading
- Repeat for each 3 mile segment (I did a total of 16 segments, or 48 miles, to average out hills)

The result was VERY interesting:
Average MPG for 4th gear: 22.8 (yes, that's not a misprint, 22.8)
Average MPG for 3rd gear: 19.0
Difference (absolute): -3.7 mpg
Difference (percent): -16%

By way of confirmation for skeptics, MTI Racing, who I discussed gearing changes with recently, reports that on C5 Corvettes with automatic, changing ratio from 2.73 to 3,73 (a 36.6% change!), which they RECOMMEND to C5 owners who are willing to listen, typically drops the MPG from 25 to 21, or 16%.

So, I figure an actual change in ratio to "only" 4.11 or 4.56 will cost me far less than the 16% loss with the whopping 5.33 simulation! I figure maybe 8 to 10% range, which I can live with if it gets me better throttle response.

As you will see in my upcoming "gearing" posting, the improvements created by by changing to either 4.11 or 4.56 are staggeringly good.

Stand by for more. . .

Jim G
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Ok I don't know all that much about all this but if I am reading this right, it sounds like you are on to something!! Keep us posted.
JimGnitecki Now said:
Isn't going from 3rd gear 1.0 to 1.0 ratio going to 4th gear which is a 1.0 to 0.70 a 30% change not a 43% change ?
41chevcoe: No, this is basic math: ratios cannot be compared by subtraction. You have to do division!

Check it out yourself in your own SSR: If you shift from 4th to 3rd at 60 mph, your revs go from 1800 to almost 2600, which is the 43% we calculated above.

It is NOT a 30% increase - that would take the 1800 rpm to only 2340.

Like I say, check it out yourself.

Jim G
Don't forget that the torque convertor lock up does not lock in 3rd gear so that will affect your calculation .
41chevcoe: That's only a factor when you hit the throttle reasonably hard. At cruise, the rpm is where the numerical ratio predicts it will be.

Jim G
I would drive mine in reverse but it would be such a hassel to stop all the time to drain the extra gasoline out.
Jim - What year is your SSR? Are you going to do some base line 0-60 runs before and after the rear end swap?
My SSR is an 04.

Yes, I plan to do some baseline runs before and after.

Jim G
Jim, I am curious to know if you have located a source for a 4.10/4.11 gear set for our axles? This axle does not appear to have widespread applications and there does not seem to be too much available for it in the aftermarket.

JimGnitecki said:
41chevcoe: That's only a factor when you hit the throttle reasonably hard. At cruise, the rpm is where the numerical ratio predicts it will be.

Jim G
The torque convertor lock is designed to stop any slippage at light throttle cruise speeds , if you hit the throttle "reasonably hard" the torque convertor unlocks.If you watch your tach and are sensitive to the actions of the transmission , you can feel your torque convertor lock come on even at light cruising speeds so it will affect your calculations to a degree while driving in third gear (convertor not locked).
41chevcoe: That's what I said! I WAS cruising.

Blastfrompast: Yes, our axles is an "8.625". This is a variant of the 8.5. Both a 4.11 and a 4.56 are available from multiple sources, including Motive Gear and U.S. Gear.

In fact, I have the speedshop that will do the work checking to see if there is a 4.33 or thereabouts ratio available for this rear end as well.

I will be ready soon to publish on this board WHY all 3 of 4.11, 4.33, or 4.56 merit consideration, in a separate "Gearing" thread to keep the MPG and gearing issues untangled.

I did a sidestep to do this MPG research first, because I recognize that we have a very highly engineered drievtrain that is pretty optimized for mileage, not acceleration. The internal ratios in our automatic tarnsmission are definitely aimed at EPA mileage testing and not performance - in fact, they are absolutely awful for performance, as you will learn shortly. I am willing to give up a reasonable amount of mileage for better nimbleness, but, unilke with my motorcycles, I am not prepared to lose a LOT of MPG as I put high miles on my 4-wheel vehicles.

Jim G
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Keep us posted. I to have been interested in a lower gear. I could care less about mileage. If I had been concerned about it I would have bought a gas milage car.

Have you learned yet if you will need to change the carrier in the rear end? I'm pretty sure you won't be able to fit a bigger gear on the stock carrier and will need to get a larger carrier too. Not tha it matters because it can be changed, it just adds more cost. Let us know the details when you can.
hpdjv: The shop had already verified with Motive Gear that they will NOT need to change the carrier.

Apparently, the latest approach by the gear makers is to vary the thickness of the gears as needed to make it unnecessary to change the carrier, at east within a fairly broad range of ratios.

Jim G
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