My opening point was, many new to the automotive world, or those that have not been taught better, believe that a rear end gear in the 4:00 something (4:10, 4:88) is a high rear end gear (big numbers) and that a rear end gear in the 2:00 something (2:45 or 2:76) is a low rear end gear (low numbers) when in fact both of these statements are incorrect. A 4:00 something rear end gear is considered a low gear ratio and a 2:00 something rear end gear is considered a high rear end gear. We need to be sure people learn these things correctly, and if they don't hear it from us, who would they hear it from?
If you are cruising along in your ride at 1800 rpm at 70 mph, and you come to a 5% grade increase on the road, to maintain the 70 mph speed, you have to open the throttle. How much, and for how long you open that throttle for will depend on the torque the motor has. As soon as you open that throttle more then it was opened before the 5% grade, you are reducing the fuel mileage the motor is getting. In the over all picture a one time event probably will not make much difference, but should it happen often, the throttle effect will have more impact on the mpg.
The point someone brought up about the mid 80s 318 2bbl wit the OD and the 2:71 rear gear was interesting. If you actually drove any of those vehicles you fully understand what I am talking about. Even the slightest increase in the grade on the highway would drop the trans out of OD. If the vehicle was running on cruise control, after it dropped out of OD, it would also unlock the converter, and if the hill was big or long enough, it would open the throttle wide open, and might even shift the trans into "passing" gear. I worked at Chrysler as a tech from 86-88. Guess what the biggest complaints about those new OD transmissions were? Constantly engaging and disengaging the OD on hilly country roads. If they were driven and used on pretty level roads, they pulled some pretty good mpg, but throw in a few hills, and that mpg dropped. Chrysler actually limited the availability of getting the high rear gear and the OD in some locations across the country
Nobody ever drove an old 318 2bbl with OD and 2.71 rear end that was stock. They never used the automatic OD with a carb, and the old carb engines were insanely crippled by the emissions equipment of that era so not really relative and they also never used an OD trans with a 2.71 rear gear, 3.21 is the highest gear they used behind the automatic OD and that was in the dakota with a pretty short tire otherwise 3.55 was as high as it went... mine has the 3.55 and it is turning way too many RPM for max MPG on the open road even in OD and LU and in fact gets worse MPG than my 3/4 ton 4x4 4 door long bed with a 6.4 did.
I previously had a 1999 318 NV 3500 4x4 dakota with much taller tires and I could leave it in 5th gear almost exclusively except initial acceleration and it would motor right along even 35 MPH and at highway speeds it would not lose speed up even decent sized hills with lugging it and it would almost always give me near 20 MPG city or highway, that excesive down shifting is just that, excessive downshifting and is the reason I hate automatics, they downshift way more than needed for normal cruising no reason an engine like a 318 magnum can't pull a 2.76 gear with short tires in a light vehicle and stay in OD most of the time.
