fireplug
1/2 ton status
Is there a consensus on what the 'ideal' highway (60mph) rpms should be?
CK5
Register an account today to become a member! Once signed in, you'll be able to participate on this site by adding your own topics and posts, as well as connect with other members.
How many RPM's at 70 mph?
- Thread starter cbbr
- Start date
I doubt you'll find much consensus. GM ran 350's with 4.10's and no overdrives with tires smaller than 31" diameter, which at best is going to be 2600RPM's as an example.
My truck is running roughly 2000RPM at 60, and IMO that is more than I need. My engine makes power down low, it doesn't need to spin that fast to keep the truck moving at 60.
I believe the Vortec 5.7's were running around at roughly 15-1700RPM's. My 3.42's came from a 90's truck and would have run at that RPM based on the .70 OD of the 700.
It really depends on where an engine makes it's power as to where it will be happiest/get the most mileage. Back when OD wasn't an option they were geared such that engine RPM's at cruise had to be higher. With OD, you can have a relatively mild motor with numerically higher gears and still cruise at reasonable RPM's. IMO all the engines from 4.3-7.4 will have no problems running as high as 3000RPM's all day long, but that's higher than necessary.
My truck is running roughly 2000RPM at 60, and IMO that is more than I need. My engine makes power down low, it doesn't need to spin that fast to keep the truck moving at 60.
I believe the Vortec 5.7's were running around at roughly 15-1700RPM's. My 3.42's came from a 90's truck and would have run at that RPM based on the .70 OD of the 700.
It really depends on where an engine makes it's power as to where it will be happiest/get the most mileage. Back when OD wasn't an option they were geared such that engine RPM's at cruise had to be higher. With OD, you can have a relatively mild motor with numerically higher gears and still cruise at reasonable RPM's. IMO all the engines from 4.3-7.4 will have no problems running as high as 3000RPM's all day long, but that's higher than necessary.
thedrip
1/2 ton status
with a 6.2, th350 and 4.56's I decided on a 42" tire... not enough gear offroad, a little sluggish off the line, but at least I can cruise at 60-65 without thinking the world is going to end. I would have liked a slightly taller tire yet so I could cruise 70 at ~2000, but really, who drives 70 with 47" tires?
Mine came stock with 4.10's, 29" or so street tires and the TH400 (No OD).
As already mentioned, every different engine has a different optimum cruising rpm in regards to mileage on the highway.
3,000 rpm shouldn't be a problem for any SBC or BBC engine in decent shape in regards to reliability. Sure, it's not optimum for fuel mileage but it shouldn't hurt it either. I also wouldn't say that it's "working hard"...in general the higher rpm means it is making more power and thus has less overall stress on the engine. In certain cases you can actually put more stress on the engine by lugging it around at too low of an rpm.
Installing taller tires in an effort to get a lower cruising rpm, and thus better mileage, can be a double-edged sword. Yes, you can lower the rpm's down to a better cruising range but larger tires will increase the rolling resistance, aerodynamic drag, and rolling mass and can very well wash away any mileage gains you might gain by the lower rpm. Also because of this you usually need a higher engine rpm at certain speeds to make up for how much harder the larger tires are to spin down the road.
3,000 rpm shouldn't be a problem for any SBC or BBC engine in decent shape in regards to reliability. Sure, it's not optimum for fuel mileage but it shouldn't hurt it either. I also wouldn't say that it's "working hard"...in general the higher rpm means it is making more power and thus has less overall stress on the engine. In certain cases you can actually put more stress on the engine by lugging it around at too low of an rpm.
Installing taller tires in an effort to get a lower cruising rpm, and thus better mileage, can be a double-edged sword. Yes, you can lower the rpm's down to a better cruising range but larger tires will increase the rolling resistance, aerodynamic drag, and rolling mass and can very well wash away any mileage gains you might gain by the lower rpm. Also because of this you usually need a higher engine rpm at certain speeds to make up for how much harder the larger tires are to spin down the road.
Little bump for the best calculator I ever saw. Great Job!
h0wl0ngcanitbe
1/2 ton status
Sorry too newb up this very intense and very educated thread but im tryin to figure out the calculator on grimmjeeper, and im getting lost. i pluged in all that I know but i think im missing something this is my set up and im lookin to see what kind of rpm/ mpg ill be/ should be getting at i guess 60?
350, 700r4, np208, 4:56:1, and 35" tires
Is my estimated rpms at 65 1992? and if so thats good right but we are talkin about a 700, and nobody seems to run or like those?
350, 700r4, np208, 4:56:1, and 35" tires
Is my estimated rpms at 65 1992? and if so thats good right but we are talkin about a 700, and nobody seems to run or like those?
buffblazer
1/2 ton status
i ran it on 4lo.com and thats about right, reading some of your other post about burning through transmissions and how your re-gearing, you should be ok. its about perfect for fuel milage, and power your 700r4 wont be working to hard because the gearing will help take the load off the tranny. when your doing 65 down the highway on flat ground your gonna be ok, the only problem i see is if you hit any slight incline or if your slowly adjusting between 60-70 mph (traffic) the tranny is right at its border line of downshifting into third. so basically you might have the problem of bouncing between gears going down the highway.
h0wl0ngcanitbe
1/2 ton status
Thanks for that info i really appreciate that! I reall wanna keep my rig and all the advice on here is helping me too do just that!
