Inliners International Forums General Discussion Engines Best RPM's for crusing @60 - 65 w/ a 235

Hi first post here - thought I would ask a question re/ my stock '54 235. I am looking to change out my current rear end -

I have a chev 10 bolt. I am told it has a 3.73 ratio. I am changing because of width and some other issues .I also have installed a T -5 trans although I am not sure of the ratio right now. So what I am trying to figure out is what the best rpm range for hiway driving would be - somewhere bet. 60 - 70 mph
Thanks,
Surf dude

If the trans is out on the bnech you can put it in 5th and turn the output shaft 10 revolutions and count the turns on the input shaft (so a .82 OD would then be 8 turns +.2 or roughly a quarter, if you have the 1o spline input that's real easy as it's 8 turns + 2 splines, if not, guestimate, it's close enough).

You probably already know that, but I thoguht it might help if you hadn't (old dirt racer's trick I was taught).

i like to aim for a 2100-2200 rpm cruising speed. it also depends on how much torq your engine can produce. tom

It mainly depends on what the specs are on your camshaft to get best mileage.
Is it stock?


MBHD

Put the transmission in first and see how you like it, going ahead and swappping out the rear gears now may be a waist. Everyone has there opinion of what gears they want to run. The camshaft/engine mods will have an impact for drivability and what you are driving, older cars are not aerodynamic so there is a lot of head wind. running something like an OD trans with 3.08 gears you might think that your gonna get better fuel econmy but 3.73 may actually work better.
Example: I ran across a guy last year who had a 40 coupe with a corvette totally stock LS1, 4l60 OD trans and 3.08 gears and could only acheive 21mpg and he wanted to go with 2.7X gears like the Vette's have. I don't know what his outcome was but I am guessing worse. I run a slightly modded LS1 in my 48 with the 4L60 with 3.73 and get 25mpg.

To find best mileage at freeway speeds, you can install a vacuum gauge on your dash and find the rpm that gives highest vacuum.
In theory at least, this should be close to the maximum torque of your cam. In your case probably around 2000rpm or a little higher.
A 5 speed with 3.73 rear and 28" tire dia. would give you about 60mph in OD at that rpm.

I would recommend you use low gears in the rear end. If you have a 0.8 OD X a 4.11 rear that = 3.3 total in OD. A 235 will cruise OK on level ground but struggle on hills with a 3.3 ratio.

I had a 261 in a 52 Chevy with a 3.55 rear and Packard OD and it had trouble on hills.

I now have a 40 Chevy with a 302 GMC with a 4.11 and a T5 with a 0.7 OD and it takes hills well, cruises at 65 at 2200 rpm and gets 18 mpg. It could take a higher rear, but not much higher.

Road Runner, good call on the vacuum gauge. In our world of computer diagnostics, we forget the simple things. There are websites that tell how to use and interpret a vacuum gauge.

One similar topic is engines that don't want to idle. One trick is to put a vacuum tank or can under the hood, to provide a reservoir for the vacuum.

ps: beep beep

Got a vacuum gauge on both my rides and I check it more than rpm or speed.
It is a very revealing tool indeed.

BTW, I have original 3 speeds and 3.55 gears and my cams give highest torque around 2500-3000 rpm, going between 60-70 mph all day on the freeways with 18-20 mpg.
My 261 climbs mountains like a train and I only notice it cos the cars get smaller in my mirrors much quicker than with my old 235 that keeps up with traffic on our mountain freeways just fine.


PS: We got a lot of them birdies around here. They sure can fly across the ground without wings.
My dogs can never catch them.

My 54 has a 3spd with no O/D 10 bolt and 2.73 gears. Cruises nice 65@2100 gets 21mpg! My 53 truck had an older T-5 with only a .90 O/D with a 3.90 first gear worked awesome with 3.07 rear gears. Both were just OK on a steep hiway grade. I would keep your 3.73s and look for a T-5 with a .78 O/D ratio.

Amen on the vacuum gauge. I bought a nifty "Mileage Minder" from ePay, it's hilariously calibrated in MPG as well as InHg, multi-color backlit and chrome cup. Fun, silly 50's gewgaw and useful to boot.

There's a complicated relationship of factors to get max. MPG at cruise; axle, cam and carb. My take on it is, you want cruise to be lowest RPM that has "the right amount" of torque to easily maintain cruise speed with a bit extra for mild hills, before you gotta mash it to climb real hills.

The cam-determined torque peak RPM is highest fuel efficiency (cylinder filling, firing, energy extraction, etc). Here's where I think the trickiness is: the engine is happy with the throttle open lugging at cruise, but the carburetor (...) will go rich at low vacuum, un-doing the low-speed benefit. So with carbed cars cruise is adjusted to be the lowest-rpm-that-maintains-vacuum-so-the-carb-stays-lean, I'm guessing but working at figuring out, that's a few hundred RPM over what might be optimum if the carb would stay lean.

I went EFI on one of my cars (amc 232ci six), but I'm not happy with mileage, though the BLM numbers are spot-on perfect. I'm gonna go EFI on my current toy ('63 Rambler American with the old Nash 195.6ci OHV motor) and will experiment more on that car. The first one was practice!

For the record, my little American gets 24MPG at 55-60 mph (1900-2000 rpm) and 22MPG at 60-65 mph (2000-2200 rpm). It's got a cam like a lawnmower, and a drag coefficient close to 1 (a brick!). I think the latter is the reason for the severe drop in mileage with speed.