Ideal rpms at highway speeds?

[Guest jdllaugh]

I know this question depends on the engine and how it's set up, but what would be an "ideal" rpm range for cruising the highways at 75 mph with a 350 sbc? Ideal being good gas mileage and the ability to pass without a downshift (if those two aren't mutually exclusive).

 

I've used the calculator on Hybrid's home page to run some different tranny combos. With a .70 final drive gear with 15 or 16 inch tires hooked to 3.34 and 3.54 diffs, rpms were in the range of 2400-2600 rpm. That seems high.

[Mike kZ]

2400 - 2600 isn't bad. I have 3.73 gears with a T56 6 speed and at 75mph my rpm is 1767. I'm getting around 21, 22 mpg with a EFI327.Passing at that rpm in 6th can be done, but you can't put your foot into it, or it will bog.

[grumpyvette]

as a general rule cruiseing rpms should be in the lower end of the range between 1/3 and 1/2 of the engines red line rpms , now figure with most sbc engines thats 4000fpm of piston speed so a 350s piston travels 3.5" x 2=7" in one revolution (RPM) so you divide 48000 inches by 7"=6857 rpm red line so your looking too be in the 2263rpm->3428 rpm range so figure the ideal rpm from a stress/efficiency compromise point of view on the engine at about 3428rpm-2262rpm=1165 x.33=384+2262=2647rpmas about the ideal long term goal to gear for as far as both low stress and reasonable torque useing standard compression ratios, cam durations and volumemetric efficiency and port size, cylinder scavageing ETC.

[Guest Anonymous]

With overdrive tranny and a 350, I will be much disappointed at 75 mph if the tach reads much over 2000 RPM

[Phantom]

The reason the T-56 has overdrive 5th & 6th gears is GM's need to improve the highway fuel economy numbers on the F body cars & the Corvette. The .5:1 6th gear puts the engine at the very bottom end of its comfortable operating range (about 1550-1600 RPM at 60 MPH) and allows a shift to the .75:1 5th for passing. The difference in fuel economy running at a constant 70 mph on a level road at 1700 RPM or 2500 rpm is probably about 1 mpg. The aerodynamic drag which is the primary contributor and the mechanical losses in the driveshaft, transmission and differential which are the next most serious HP losses are all essentially equivalent through the speed range. The only difference is the internal friction losses in the motor.

The biggest impact the lower rpm has is reduced wear on the pistons and cylinder walls but then there is actually more stress on the connecting rods and crank which can translate to shorter bearing life.

Well, I'm confused - how about y'all?

[strotter]

A request to those responding from experience: what actual mileage are you experiencing? I'm hoping to use my hybrid as a touring car, and am highly interested in mileage at cruising speeds... Mike's number seems a bit low, given that F-bodies can consistently get upper 20's lower 30's...

[Guest Slvrhase]

I will be running a 200 4r and hoping that the RPM will be in the range as "grumpy" stated. Anything would have to be better than TH350.

 

Is nobody else impressed that "grumpy" can rattle off a bunch of numbers, make it readable, but still you feel so inferior.

[Mudge]

www.f-body.org/gears

 

I would say it depends on how you build the engine, I had a somewhat largish cam but stuck with 3.42 rear gears, that was with a 255x50 - 16

[Mudge]

Most guys as I recall lose 2-3MPG when going with a different rear gear, not just 1MPG, otherwise GM would probably have been more flexible for performance reasons.

[grumpyvette]

BTW it occured to me that one of the most important parts of the question was not answered , what is the cars speed AT cruiseing rpms ? well you should be cruiseing at those rpm ranges someware in the 55mph-75mph range thats for sure or your likely to get hit in the rear by most freeway traffic where I live and I don,t know about you but with my overdrive trans I sure want to be at or close too 75mph and no more than that 2647 rpm on a long trip cross country.

now keep in mind that your not going to be useing that overdrive highway final drive ratio while drag racing and haveing a slightly taller set of tires for long trips is an option that helps, also keep in mind that the rear gear ratio and trans gearing must be set up to match your engines rpm/tq curve for best performance.

 

lets look at a few examples

my 1985 vette has a 700r4 (SHORTLY TO BE A 4L80E)

the torque curve of my engine says I need to race in the 3500-6500rpm range yet for trips a 383 should spin about 2488rpm as a goal due to its longer stroke while cruiseing useing the same formula

the trans gear ratios are

4L80E 2.48 1.48 1.00 0.78

700-R4 3.06 1.63 1.00 0.70

 

so max performance shows I NEED A 3.73 REAR GEAR AT 126 MPH IN THE LIGHTS TO KEEP THE RPMS IN 3RD GEAR AT about MAX hp (6000rpm)

and still have a 75mph 2490rpm cruise with the 700r4 trans in overdrive,

when I swap to the 4L80E

a 3.55 ratio for cruising would be a better match but Im going to stick with 3.73 which will kick my cruise rpms to about 2775rpm (2633rpm with a 3.55 rear) if I wanted to keep the same launching gear ratio and let the trans shift into overdrive as I went through the lites a 4.56 rear gear would be needed but that would kill the highway rpm range. as it stands if I swap to the 4L80E and let the trans shift into overdrive I be turning about 4700rpm at 125mph (about peak tq so Ill be fine)especially because I prefer the higher top speed the 4L80E can give me as the gearing better matches the hp peak at 5900rpm (165mph) where the 700r4 is geared to high to pull well at those speeds yes I have driven THE 1985 VETTE over 165mph AND YES THE rate AT WHICH IT PICKS UP SPEED OVER 150MPH SLOWS AND YES I MIGHT BE CRAZY BUT HELL I DO LIKE THE FEELING THAT 160 MPH PLUS GIVES YOU!

http://users.erols.com/srweiss/calcrpm.htm

http://www.wallaceracing.com/reargear.htm

AND YES YOU WILL NEED TO FIGURE YOUR OWN BEST OPTIONS,AND WHAT IS MOST IMPORTANT TO YOUR GOALS AND YES EVERYTHING IS A COMPROMISE

[Phantom]

I'll stick with my T-56 with its .5:1 6th gear and the 3.70 rear end - especially since I might maybe someday actually run the car 3 or 4 times down a real 1/4 mile track while the other 99.45% of it's life will be spent as a daily driver. We all know a real man rows the shifter himself!!!