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Old 01-02-2023, 09:07 PM   #1 (permalink)
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Change rear gearing for better FE?

This has been an idea I have had for some time but was never really able to pull the trigger on. I am again revisiting it for different reasons.

I can't remember where I found it (probably here or from a link someone here posted) but I wrote this down in a notebook at one time.......

"The best FE is found at an RPM range corresponding to a piston speed of between 16.4-19.8 ft/sec.", and "Piston speed is twice the piston length (in inches) times the RPM's divided by 720."

I also have another formula that was given to me here to calculate RPM for multiple variables including transmission gearing, rear gearing, tire size, and speed.

RPM=[(trans. gear)x(rear gear)x(tire rev./mile)x(MPH)]/60

So my thought was to use these two formulas together to try and optimize my RPM range for my usual commute and driving style.

The stroke length for the 2.3l Duratec in my Ranger is 3.70" (94mm), 5th gear for the M5OD is 0.79, the stock rear gear is a 3.73, and my tires rotate 698 per mile. Yes they are oversized by 5%.

So, washing all that information through both formulas I came up with an optimal RPM range of 1600-1900 RPM at 55 mph. I've had the engine down around 1600 RPM in 4th gear (1:1) and it doesn't really like it, so I decided to shoot for the midpoint at around 1750. My options then became a 3.55 rear gear with the oversized tires (1794 RPM) or a 3.27 rear gear with my stock size tires (1742 RPM) I was afraid the 3.27 would be too tall for the times when I was "in-town", so a 3.55 was going to be the answer. A 3.45 was available for the Rangers, but I believe they generate their speed signal differently than the stock ones so the speedometer might not work at all.

Armed with that information I thought I was on the right track until I started looking over the RPM chart I made. With my stock rear gear (3.73 is on the tag) and the 5% oversized tires, I should be running just below 1900 RPM (1885). But it's never been that low at 55 mph. It's always been just a hair over 2000 RPM. Hmmmm. Looking back at the chart 2070 RPM would fit a 4.10 rear gear with the oversized tires. This might explain why I feel like I can only get half way across an intersection before I need to shift into 2nd gear. It also presents me with a bit if a dilemma. If my assumptions about the Piston Speed formula are correct, and I really do have a 4.10 rear gear, I'm getting somewhere between 32-34 mpg depending on the season. EPA for my Ranger is 26/highway and I figure I'm around 80 highway/20 city, so I've always used 25 mpg as a baseline. Do I really want to change the rear gearing? A change from what I thought was a 3.73 to a 3.55 wasn't a big change, so I wasn't too concerned. Now that the math says I probably have a 4.10 back there I'm having second thoughts. Will this work? If the stock rear gear was "supposed to be" 3.73 then the change would still be a small one. Has my ECU "learned" over time where it wants to be? (I drive just over 500 miles a week). If so, will it be able to relearn for a new rear end? Or worse yet, are all my mpg numbers wrong because the odometer is still thinking it's a 3.73 rear gear and I need to figure out how far off my numbers might be? I think part of the solution is going to be to get the truck out on the interstate and check the odometer against the mile markers. Would 50 miles work or should I do a full 100? Also, checking the rear gear to see what it actually is. The math says 4.10, but the tag says 3.73. The diff cover doesn't look stock though. Ill cover how I'm checking the rear gearing later in this thread after I hear what opinions the folks here have first.

Thanks for any insight.

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Old 01-02-2023, 09:48 PM   #2 (permalink)
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Thanks for any insight.
The only issue is the actual ratio of the rear end?
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The math says 4.10, but the tag says 3.73.
If it's on jackstands in neutral you could paint witness marks on the driveshaft and tire, and count one rev (or ten).
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Old 01-02-2023, 09:50 PM   #3 (permalink)
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If it is reading speed off the rear wheel speed sensor like I think it is, the odometer and speedometer will read correctly. The tire size over stock will make a difference but not the gear ratio. Are you running it in 5th gear? It will probably make slightly better mileage in the direct gear with the proper final drive. You will never drive it far enough to recover the cost of a gear change.
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Old 01-02-2023, 10:30 PM   #4 (permalink)
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The cost of having the actual gears swapped out and all set up again IS cost prohibitive, but to just swap a complete rear end isn't too bad. I can pull a used one from the JY for about $300 and it's a direct swap that can be done when I change out the junk shocks that are on there. Sometimes our ecomods won't have a positive ROI but it can be to see what can be done. I've tried driving in 4th (1:1) but at 55 mph I lose 2-3 mpg. At 35 mph in 4th gear the rpms are roughly the same as 5th gear and 55 mph.
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Old 01-02-2023, 10:41 PM   #5 (permalink)
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I once rolled the truck until one bolt on the U-joint was at 12 o'clock. I marked it with tape and marked the tire at 6 o'clock. I rolled the truck forward until the tire made one revolution. The tape on the U-joint was at about 9 o'clock. I figured it made 3 whole revolutions and 3/4 of a revolution. That seems to me to be a 3.73 so that's what I always assumed because it matched the tag. Wouldn't a 4.10 have left the tape around the 1 o'clock mark?

Playing with the formula, the only way I can get the 2k rpms with a 3.73 and the same tires would be with a .87 5th gear but I'm almost certain that that gear wasn't available in any Ranger 5 speed manual transmission.
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Old 01-02-2023, 11:08 PM   #6 (permalink)
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Wouldn't a 4.10 have left the tape around the 1 o'clock mark?
Improve accuracy with multiple rotations followed by a division. double and half should suffice.
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Old 01-03-2023, 11:26 AM   #7 (permalink)
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From the instrumentation tech background: you are relying on a very loosely calibrated tach and speedo for precision measuring even if the data gathered is precise. Based on my ranger experience, your dash gauges are off.

I traditionally put the axle on stands so I can observe when the marks cross the origin point and like freebeard said do multiple runs.

Or you can pull the cover and count the ring gear teeth, but the data I have is simply: there were only 2 main options 3.73 or the 4.10 which will be about a half turn extra on the driveshaft. If like you say it's "9 oclock" then its a 3.73. If its not, but about a half turn further, then it has to be a 4.10.

Finally: being a common 8" diff there are odd gearings available aftermarket, which is why I suggested you remove the cover and count teeth if you are not the original owner.

Further research on older rangers suggests that 6 cyls got the 3.73. 4 bangers got the 4.10. My documents only cover to 2000.
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Old 01-03-2023, 12:23 PM   #8 (permalink)
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gearing

A BSFC map for your engine would be a requirement for the necessary calculus.
The OEM gearing was likely, already optimized for all the transient loads the SAE/ ISO automotive engineers must take into consideration for every conceivable scenario the vehicle will be exposed to.
A 'flat' torque curve, as is more common than ever before, would provide a lot of 'wiggle room' with respect to engine rpm range which would maintain a minimum, 'best-case' BSFC island of operation.
I know of no way to simulate different gearing, without actually changing the gearing.
Since BEVs are 'immune' to BSFC-e variability, and always get 95%-97% efficiency, it's a hard sell, making expensive modifications to an ICE vehicle, which could never compete.
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Old 01-08-2023, 11:28 PM   #9 (permalink)
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My experience with pickups is that they're typically geared assuming they will be towing.

While piston speed does, in theory, have some relevance, in practice increasing load is going to have a much bigger impact on economy. The tallest gearing you can get is likely still going to be short of what will maximize fuel economy.

Take the Mazda MX-5: In the US, the older models had a 4.30 ratio. In Europe and Australasia, a 3.636 was standard.

With the Honda Accord, a 4.4 was standard, whereas the Acura variant got a 4.7. In Europe, the equivalent of the Acura received a 4.05, and real-world fuel economy was much higher.

In both cases, the European variants also had smaller displacement engines as options to pair with the taller gearing, which delivered even better economy.

In old 90's Civics, final drive ratios ranged from 2.95, 3.25, 3.72, 3.89, 4.06 and 4.25. Fuel economy improved almost in a linear fashion with taller gearing.

With the Insight, in 5th gear, the car will not do more than ~75mph with the pedal flat on the floor. On the highway, load is always close to 100%. And, not coincidentally, this is the highest fuel economy vehicle ever sold, approaching 100mpg at highway speeds.

My advice: Don't overthink it. Choose the tallest gearing you can tolerate.

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