The Stovebolt Site Forums The Shop Area Driveline Cruising rpm

What are your thoughts on cruising at 65mph at 1800 -1900 rpm with a 235ci? Engine seems to respond fine still and will still accelerate decent. Never had any lugging issues pop up to me. Just wondering if I'm being hard on the girl on road trips.

What rear gear are you running to have rpm's that low at 65 mph? Overdrive? At 65 I'm turning right at 3,000 rpm with a 3:90 rear gear. I run it like that for hours on the highway haven't hurt anything.

What truck, what does it weigh?
My 1 ton weighed 4,400 lb and was comfortable running about 2,400rpm for days on end. 1955 model 235, about 50-55 mph.

If your truck is lighter you can get away with lower RPM, and if going faster you generally need more RPM to offset the added wind resistance, which adds up real fast.

Each situation will be a little different and each engine might seem more content at a different RPM.

I think Wollfcub has a T5 tranny. Not sure what his tire size and OD gear are though.
For reference, at 65-70 MPH I run 2000-2200 on the highway with my 230 inline 6 and I also have a T5. My engine is very happy.
I have heard from many folks that the inline 6 engines run fine without any issues at 3000 RPMs. The nice drop in RPMs from the OD does help improve MPG and hopefully prolongs engine life.

Go ahead and run that cruise speed if you wish- - - - -you'll lose 4-5 MPG because the power valve in the carburetor will be open all the time. Try running a vacuum gauge and get back to us about the 5 or 6 inches of vacuum that the low RPM will cause instead of the 12-15 you should get at a less-loaded 2,000-2,200 RPM cruise speed.
Jerry

Great idea Jerry. I will run that experiment. I can check MPG differences easily by just not using 5th gear. I will hook up a vacuum line to the manifold full time and record numbers at different RPMs and speeds. My engine is a good guinea pig because it freshly rebuilt and so is my Rochester B carb. This will be interesting. I was getting 18.5 MPG highway using the OD gear with my old worn out 250 engine and same carb.

I would love to have a portable 4-gas exhaust analyzer like I did at one of my school shops. With a 110-volt inverter onboard we could monitor exhaust gases while on a test drive, and it was quite enlightening what a relatively minor change in vacuum did for Exhaust gas oxygen concentration and unburned hydrocarbons.
Jerry

What brought this up for me again was I was in need to do something about my current setup. From day one I was using a much smaller tire size than stock. The OD was 24.5" vs the +/-28" stock was. This together with the 3.73 and 3 speed early I was first running the truck made it non friendly to drive. The 3200rpm area in third was harsh as with the stock cam it sounded like it was going to rattle apart. So changed to a T5. The T5 was better as it gave me the OD and things were a bit better. Still with that 3.73 and smaller tires it would just blow thru the gears. I was in second gear within feet all the time.

Then after more stress and blowing time and money I went to a 3.23 rear ratio but not the 3.08 that was recommended. The 3.08 would need a full new rear carrier and I just never wanted to dish out for that at the time. The new 3.23 made things better. I could almost clear an intersection without needing second and the highway cruising was better. I could do 62.5 in 5th at about 2200. I Put a cam in after that and that made the engine sound like it was happy all the time with no more struggling and ran much smoother. Drove it like this for the last summer of 2015 and a bit this year.

This all really bit me in the [censored] last week or so when I just finished doing a back yard rebuild on a 57 inline that was shot in a 57 belair for a guy. I got the engine running and even with almost no compression and other items in such bad shape this thing pulled like a son of a [censored] in second and nicely in third. This got me all going about ratios again so I sat down and messed around with numbers.

I always thought I was ok with my ratios and tire sizes but I was wrong. I was fed up of my rpm gauge looking like a window wiper and me always blowing past the torque curve and needing frequent shifts. I took the plunge on Friday morning and got some new rear rubber for the truck that was almost close to the 28" stock size. Came in at 27.75" actually. I'm happy now and wish I came to this result sooner. Its amazing how it changed everything. Now with the larger size and the additional load on the engine when accelerating it goes like snot. You can feel the inline actually working and not just rev shift, rev, shift, rev...... it's even quieter and smoother also as it has load behind it when accelerating. I have such a [censored] eating grin when I drive it now and I'm no longer ashamed of it anymore as I was before. I was always happy to tell others of the work I put into it but never 100% expressed how it really felt on the road, and never really drove it hard as it never would have impressed anyone.

Now that's what brought up the cruising rpm question post. Prior to the tire purchase 5th would put me in the 2200 rpm area at 62.5. I just noticed that I accidentally used the wrong rpms and speed noted in my first post. I was currently at 2200 at 62.5 and not 1800-1900. The 1800-1900 is where the new tires were going to put me. I was tired and in bed when I made the post.

Now with the new tires on I can confirm the 1800-1900 area is where I am at and it really made a difference on the engine in 5th. I can hold a steady speed in 5th at about 65 but it's very hard to accelerate and I know I'm not doing well having to hold the pedal as low as I am to keep it going. A quick shift to 4th and its back up to 2400rpm, and then I have no issue accelerating and going way faster. I'm now on the possible hunt to find out if I can see if another T5 5th finial ratio might better be better suited then the current .72 one that the T5 has installed. Or I just ignore 5ht and use it as a 4 speed? lol.

Sorry for the mile long post but I will stand here all embarrassed in the hopes that someone else might learn from my mistakes.

I believe the tire change made the ratio a step too tall. If it suits you, fine and good, but that engine will be a lot happier at around 2K RPM at 65 MPH, and 2500 is about where you'll get the best gas mileage. Just because you can hear the engine running doesn't mean it's over-revving. They were made to run wide open throttle all day at 3K or so while pulling a load without doing any engine damage. Lugging one down below 2K kills both gas mileage and performance. Ideally, a shift to the next higher gear in the transmission after achieving the HP peak in a lower gear should put the engine very slightly below the torque peak RPM.

Example: a 1958 235 cubic inch passenger car engine achieves its max BHP of 145 at 4200 RPM. Max torque of 215 Ft./Lbs. happens at 2400 RPM. A properly-geared vehicle with the right tires should max out in one gear around 4K, and after a shift, it should drop back to 2200 or so and be able to have close to 2,000 usable RPM's to pull in that gear. If the noise upsets you, I recommend a set of Caldwell noise-canceling electronic earmuffs from Midway USA. They're the same ones I use when running my chainsaw and shooting my .30-06.
Jerry

Do you still have the 3.73? That would be the best ratio for the .72 overdrive and the 28 inch tires. You make that change and you won't be able to wipe the smile off your face.
You will be able to pull stumps, and drag them down the highway at 65 MPH.
Carl