3.91s in my 340 now!
Thank you for all the information! Your numbers sound pretty similar to mine... no surprise there, though. A lot of people complain about the high revving rpm at highway speeds for 3.91 gears. It really doesn't bother me (I have 245/60/14 tires, which I believe are 26" tall). I was just a little concerned about the stress on the motor and other integral parts. And of course, the motor running hot.
So I'm guessing the rear-end/differential needs to be exposed to calibrate the speedo. That's definitely NOT me. I am a complete novice when it comes to working on cars... but I am slowly getting acquainted with it.
Beautiful Duster! Love the color! Thank you so much for sharing!
340 with purple cam has operating powerband of 2600-6000 rpm, so why would one want to be at 2000 rpm and still have to rev it to 4000 to pass because not in power band and you will downshift… I think (opinion) that staying a constant but higher rpm is better for an engine than wild rpm variations, especially if that rpm is in the engines powerband.
Fuel mileage I can see 12 mpg highway staying mostly out of secondaries. 6 mpg in the secondaries more as I have the secondaries, I’m using them lol.
My larger concern was never engine, but transmission temperatures, ad non lockup torque converter is not 100% efficient and that equals heat from anything less than where it quote stalls. So I did the math to put my highway cruising RPM above my 3200 stall converter. This in theory is running transmission at max efficiency possible to keep temperatures down. Also being in the powerband is nice. It’s why 3.91 on a built 340 is a great combination.
Engine temps at 3500 rpm is not an issue is you have the correct radiator and are running a 185 thermostat and 18” 7 blade 2-1/4” pitch fan correct for a 340 application. I know, at highway speed the fan spec is irrelevant, but at some point you will be slowing down and stopping again lol. Never had any issues on 90 degree days here, always stays 185-200 degree engine temps.
The speedometer gear is in the side of the transmission tail housing just ahead of crossmember on the driver side. No need to remove anything but the speedo gear housing and speedo gear, probably lose some transmission oil, but very doable. I had a lift so no biggie for me, with no lift definitely would make it trickier and messier but again, still doable I think. Either way speedo will probably read off by 5 mph like mine thru the entire sweep because 50 year old part, needle must be bent 5 mph lol.