2 lane blacktop trivia, actually it was Dennis Wilson, "she ain't breathing right, might be the jets"
You have a 454 in a 1958 Cadillac. Changing gearing to save you a few hundred RPM isn't going to be too cost effective when it comes to fuel economy. That whole gigantic engine in an aircraft carrier with the aerodynamics of a house is going to get ya.
It's a fallacy to think that lowering your RPM will result in better MPG. It will only to the point you are 'lugging' the engine.
IN the '60's we all ran 3:90-4:10's some guys 4:33's. I never had a tach or a speedo that was correct. Just ran with traffic. Nobody ran 70 in those days or you would be stopped/ticketed. CHP's were like rectums in those days: one behind everybody.
Yes Sir. I'm getting the vibe that I'm good with the motor and the gearing that I have. My next question is does anyone know what RPM is going to give me the highest amount if torque. My specs are in my first post. And horsepower would be nice to know too. Does anyone know where I'm at with either HP and torque. Once again, Thank You every one.
Logically, you would think so. In reality it doesn't always work that way. In my modern OT car, which has a Direct Injection 5.0 V8 I have found that I get the same MPG at 67-68mph as I do at 55mph if not a little better.
I got 11 mpg at 70 and 13 at 75. Go figure. I'm gonna have to do 95mph to get the mileage I need. Oh well. Can't argue with the laws of physics. Thank you everyone for your input.
You'd almost have to dyno the engine or car to find the RPM where it produced the highest torque. Not a bad idea at that, then gear to nail it as close to that number at your chosen cruising speed. Generally, and I do mean GENERALLY, that is where you will find your best running speed for the highest MPG. That's what I did with my car and it's made for a very happy combination... As I said earlier, almost too happy!
That was good advice back in the days of 2-speed automatics and 3-speed manuals. The OEMs are going to 7, 8, 9 speed trannys to have the perfect choice for every situation. With only a 4-speed automatic, you can still drop it a few numbers and have good all-around performance. Question is, where are you going to find aftermarket gearsets for a '58 Cad rear? Answer, doubtful. jack vines
You guys are lucky if you can run 70mph. Top speed by law in NZ is 100kph (a touch over 60mph) Sucks.
My '32 Highboy has 31" tires, 3.73 rear, 700R4, crate 350 Vortek, Edelbrock 600. AT 70, I am taching 1975 RPM's. My '40 2-DR has 4.11 banjo, stock 3-sp trans, 69 327. At 70, it tachs 3200, way too much and a bad combo. Your Caddy might be a bit heavy to gear it for 1500-1700 RPM's at 70 (just an opinion). The formula for obtaining RPM at whatever speed is: Speed X 336 X rear gear DIVIDED BY Dia. of tire X .70 (for 700R4 overdrive) = RPM's. You might get away with a somewhat larger diameter tire. This formula helped me to decide on 31" Hoosiers for looks and long trips. Good luck.
Hey, Scumdog, if you can get to the LonestarRoundup someday hopefully, you can run 85 legally on the 130 Tollroad around East Austin. Sucks lots of fuel, but it is fun to "blow the soot out". Hope you make it, Mate. Ah, now I feel like an N.Z.
Hiya Gary, I've never been to your part of Tx but I HAVE been around Azle to DFW a fair bit, nice roads but a tad busy for this boy!!
With what you currently run with a 30" tall tire(I am guessing), you would drop that 2900 to 2030 with a 700R4, or equivalent.
Contrary to the cruise at the max torque / RPM idea, when I did my 27 I planned for about 2000 RPM at cruise, based on what I thought a stock V8 might do and still get good mileage. Since the V6 has a blower, it more or less acts like a little V8. Anywho, with a .70 OD, 3.50 (best guess) gear and 28" rear tires I run right around 1900 RPM at 70. Just loping along. And at 80 it only creeps up to about 2200. I've been getting 23 MPG at that speed and I'm not lugging it at all. Better yet, according to the factory specs, my motor makes max torque at 3500, way too fast IMHO to cruise and get good MPG. Since there is about a 1500 RPM change in each gear, the boy-racer in me figured that the rise in RPM between 4th at 80, and downshifting to 3rd would spool the motor up to right around max torque... just perfect for some spirited highway driving. And it works just fine! Zoom zoom. Gary
As I re-read the 1st post, your "RV" cam is probably at peak HP somewhere around 48-5200RPM. The peak TQ is perhaps closer to 38-4500RPM, and that's probably a generous estimate. For the best highway manners and efficiency I always like to be right where the TQ numbers begin to rise. Not at their peak, but to clarify, where they start to "make a move". If we suppose a smogged up stocker makes peak TQ numbers at 3300 then it's easy to see where it could feel as though it's dropping off as speeds increase. Your combination might make a higher peak TQ, but the low RPM TQ numbers could exceed that of most 'generic' engine choices. Your peak number might be 460lbft at 4200RPM, but there should be an abundance of TQ much lower as well. Just over 2000RPM it's probably well over 300 and climbing fast. This where you feel the engine's ability to move all that mass with little effort because there's no shortage of "work" available. Try to borrow some really tall tires as a cheat for a taller rear gear. I don't think you'll gain anything in mileage, and the overall MPG numbers might suffer because you've increased the load at the short end of things. The best mileage you'll see will come from using cruise control (if you have it) or begin to drive for it. Stay out of the secondaries, drive at more steady speeds, etc. A big ass Caddy and gas mileage is like the most traditional oxymoron available, isn't it?
I have 400 SB - 30 over with 700R trans with 350 rear gear and I it runs about 2100 @ 70 miles an hour and get about 18-19 MPG that is in a 37 Nash.
First off, your engine is fine right where it is. It would be a long payback to pick up 1-2 MPG figuring you are around 14 or so now. A little taller tire might be helpful come tire time. I took your specs and did a simulation for power and torque; The red line on the bottom chart is power lost to friction in the engine which is RPM dependent. That is flywheel HP with a 250/250 460 lift RV Marine cam, stock intake, 660 CARB, dual exhaust. Peak power is in the 3,000 to 3500 range around 230 hp AND 495 FT Pounds at 1500 RPM. The early intake closing kills HP over 4000. You said RV cam, if you got more specific I could give you a better picture. Just retarding that cam 4 degrees picks up 20 HP so the actual cam would be very helpful.
I feel for ya! Out here in the desert with long expanses to cover of absolute nothing, our 75 mile an hour limit is pretty much worthless. The limit is 75, but you are pretty much immune from prosecution on up to about 100, unless you are doing something really stupid. When that is your daily speeds, it makes pretty hard to drive the 55 that is still common in our Eastern states. On truly open, no traffic sections of freeway out here, I have had the Plymouth out as far as 145... The aerodynamics run out at about 135!
I'm running 3.73 gears, with 30" cheater slicks behind my 464 BBC and turning 2900 rpm at 70. I'd like to be a bit lower rpm's, but I don't cruise at 70 mph often and don't want to change from the 4 speed, so I'll leave it alone.
2900RPM @ 70mph for me and the 57' F100. I wish it was a little lower but I can live with it. Putting a OD trans behind a BBF is a major investment.
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