[hat_man]

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.