The Stovebolt Site Forums The Shop Area Engine Shop freeway speeds in 61 1/2

Hey all:

I just purchased a 61 Apache 1/2 with an in-line 6 235 and a later 60's saginaw 4 speed. (I gotta finish a few things before I get onto the road.) I don't want to blow the engine running it on the freeway. With this transmission and a Rebuilt rearend with Patrick's 355:1 ring & pinion, can I cruise the freeway without worry?

That should be a great freeway cruiser with that setup. What size tires are you running? I ran a 235 with a 3-speed and 3.70 gears, and 14" tires for a lot of years. No problem running 60's, 70's, and sometimes 80's if I was really pushing it! If you're running 235/15 tires, the RPM should be right in the good cruise range at highway speed.
Jerry

It depends on where you live. Here in metro Atlanta (and really on most of the interstates in Georgia) you wouldn't feel safe if you couldn't hold a comfortable cruise speed of 70.

3.55 gears even with the tallest reasonable tires on your 61 still will put you in the upper cruising RPMs for a stock 235 at 70 mph. For example 235-70R15 tires will put you at 2988 rpms at 70 mph. That's not where I would want to keep a 235 for distance driving.

I recommend overdrive or just keep your Saginaw 4-speed and go with an even numerically lower gear to make things easier. If you plan frequent interstate travel then try to get your cruise rpms around 2300 to prolong your engine life. I have overdrive on my 66 half ton and with the aforementioned tires and 3.73 gears I turn about 2300 rpms at 70. When the traffic runs faster the overdrive gives me enough extension to travel faster, yet still cruise. At 80 I'm only at a shave over 2600 rpms. That's still pretty easy on my six since I have upped the intake, cam and exhaust.

-ftyler

hey ftyler
where did you get your formulas for computing the rpm v hwy speed based upon diff gears, tranny type, and tire size? Or better yet, what are the formulas?
Thanks

No fuss, no muss. rpm calculator

Here's a crude Excel spreadsheet I use for calculating the speed for ratio and tire. Gives you a nice table instead of one answer at a time. The gears are for a sm420. Just change the ratios in the cells as per your transmission.

Gears.xls

Thanks for your help.

Remember that forty year old suspensions and drum brakes weren't always meant for today's freeway speeds.

Thanks for the response. I don't plan to travel the freeway extensively, maybe 70 miles per week. I wouldn' have any problem driving at 60 mph during these trips.

This truck also has a new intake manifold, eidlebrock carb, headers, maybe something done to the cam too. Would these modifications allow the truck to travel at lower rpms driving at freeway speeds?
Thanks.

Unfortunately no. The mods you've made would allow your engine to rev higher and get there faster than it would stock. And you would have a higher top end speed and faster acceleration as a result. But it would not lower your rpms. Changing your final drive ratio is the only way to lower your rpms. You can do that either through a ring and pinion change, a transmission change (if the transmission has overdrive) or both.

Great point. But I haven't had any problems whatsoever with my 38 year old truck with it's stock setup. The truck handles great a 75-80 on the interstate. You really have to drive smart and not try to stay with the fast lane traffic on heavy traffic days. I'm going to switch my single master cylinder over to dual sometime before the end of the year so that will help me have some braking ability, should a brake line give way.

-ftyler

Excuse me for my lack of knowledge, but by would these modifications allow the engine to handle running at higher rpms? If so, then maybe the transmission or gear ratio change wouldn;t be necessary. Again, I don't plan to drive fast or far < 100 miles per week.

Better breathing, from the camshaft change, and a good balance job, tighter bearing clearances, etc., will allow the engine to make higher RPM's, and stay together there. It's still putting a little more stress on the engine, which might shorten its practical lifespan. The 350 round track engines I build produce 600 HP in race trim, and wind up to 7,000 RPM or more, but they're worn out in less than 1,000 miles of running those speeds.
A taller gear, and lower RPM is a better choice for a street machine.
Jerry

for those of who want to calculate just about anything go to randy's ring and pinion.com web site there is calculater for a number of different things on is site, it is also fun to play around with gear ratios, tire size and such. ron

My truck has a worn out 250 with a Holley/Weber 5200 carb, 3 speed saginaw with no overdrive, and 4:11 butt gears. The biggest problem I have running at freeway speeds is gas mileage. I'm turning so many rpm that the engine is just SUCKING gas. When I drove around local and back and forth to work I hardly ever go over 55 or 60. I get about 14-16 mpg. Running at freeway speeds I get 10-11 mpg. That's one more consideration.

Boo

Boo,

Not to mention the extra wear on the bearings, rings and rods by turning those high rpms for extended periods of time. So there's a bigger cost of running an engine high rpms on the interstate than just gas mileage - shortened life.

-ftyler

What do you mean by "freeway speeds"? The minimum speed on interstates in Texas is 45 mph, 40 within city limits. My stock '46 1/2-ton will cruise all day at about 48-50 mph, and the brakes weren't designed for speeds greater than that anyway. With no AC, no radio, no PS, no PB, and no GPS (I can read a map and have a compass), there is nothing to do but enjoy cruising along listening to the engine purr while waving at all the friendly folks who wave and toot at me as they race by. Life is too short to drive fast...

my 56 235 3 speed with a 390 rear does just fine at 50-65 when I do take it out. I would not recommend much more even with 355 gears. They were just not designed to take that much punishment. It was a work truck. have fun and enjoy your new ride.

I'm planning on putting a 3.07 '68 truck rearend in my 261 powered truck. I calculated out the gears, and the first three gears (with a 3.07) of the 4 speed Saginaw 3- groove I'm planning on putting in matches the 3 upper gears of an SM420/4.11 almost exactly when plotted on the same graph. So I'm getting an extra gear. It should give me about 2600 rpm at 65mph with my current tires. I'm upgrading the brakes, though.