highway RPM's?

[cobravii]

I have a 1964 Buick Electra convertible. It has a posi and I think it has 3.07 gears. I have 235/74R15 tires.

What can I expect for rpms at highway speeds (75 mph)?

Thanks in advance

[smithbrother]

GOOGLE, "RPM AND GEAR RATIO CALCULATOR CHART" this will give you all the info you need.

I wish you well, I noticed on the Buick vert, someone changed it back to ONE carb?

Dale in Indy

[NTX5467]

With a 3.07 axle ratio and P235/75R-15 tires, it's probablly running about 26.75mph / 1000rpm. If it's possibly got the 2.93 axle ratio, then it would be right at 28.04mph / 1000 rpm. With a torque converter (non-lockup), there will always be a little slip, even at steady-state cruise, but you can use these figures as "pretty accurate". Remember, too, that all electrical meters have a + / - tolerance percentage, too, of the full scale reading. This tolerance can affect any tachometer readings you might get . . . even with tolerances as close as 5%.

With the 3.07, that would put the rpm (theoretical) at 2804 and with the 2.93, at 2675. Either way, right in the meat of the torque curve.

For these calculations, I was using the tire effective rolling radius of 13.8". That might be a tad small, but the larger effective rolling radius would mean slightly lower rpms at cruise in "high" gear.

Enjoy!

NTX5467

[cobravii]

GOOGLE, "RPM AND GEAR RATIO CALCULATOR CHART" this will give you all the info you need.

I wish you well, I noticed on the Buick vert, someone changed it back to ONE carb?

Dale in Indy

The car came from the factory with the 4 bbl on it and the dual quad option in the trunk. My dad would switch them out depending on his mood :-)

[cobravii]

Thanks for the awesome info!

It reaffirms that with the kind of driving I plan on doing I will need a 700R4 overdrive transmission.

My TH400 is rebuilt and I will keep it with another drive shaft in case I decide to switch it back to original.

[bhambulldog]

Thanks for the awesome info!

It reaffirms that with the kind of driving I plan on doing I will need a 700R4 overdrive transmission.

My TH400 is rebuilt and I will keep it with another drive shaft in case I decide to switch it back to original.

VERY COOL!! So, with two transmissions and two carburetor set ups, you have 4 drive train choices!!:cool:

[cobravii]

LOL...... so if I had a quick change rear end, the possibilities would be mind blowing :-)

[smithbrother]

Are QUICK-CHANGE set ups NOISY?

I have always wondered.

Dale in Indy

[bhambulldog]

LOL...... so if I had a quick change rear end, the possibilities would be mind blowing :-)

I'll have to take my shoes off to count that high!

[NTX5467]

ONE thing which many people might not be aware of is that you can have TOO LOW of a cruise rpm, especially with carburetors! Plus, as long as the engine is in the meat of the torque curve, that rpm is usually the most efficient cruise speed range.

With the 3.07 and a .7 OD ratio, that would put 75mph rpm right at 2000rpm. At that rpm, the engine's throttle response would be soggy as it tried to pull the weight and wind resistance with LESS available torque. End result, OD would not be as useable as you might suspect. This also would mean that at lower speeds, the trans would not shift into OD, too! Therefore, as many who put THM700s in the place of THM350s and did NOT upgrade the rear axle to about 3.42 or lower discovered that OD would not engage during normal driving on the highway. They DID like the much deeper low gear ratio, but didn't realize that OD wouldn't engage until much higher (than locally legal) road speeds (on flat ground).

Carburetors rely upon air speed through the venturis to meter fuel and higher manifold vacuums to keep the mixture in the "economy" range rather than otherwise. With fuel injection, the computer determines what the mixture is. This is why throttle response with fuel injection is much more responsive than what a carburetor on a similar engine might have. BUT . . . every time the throttle moves with fuel injection, the mixture is corrected to accomodate that throttle movement . . . whereas with a carburetor, until air speed increases, no additional fuel (unless manifold vacuum drops into the "power" range) is metered.

In the world of "fuel curves", a lighter-load situation at higher rpm can actually use very similar amounts of fuel as a higher-load situation at lower rpms. One reason why keeping the engine in the meat of the torque curve at cruise (and why many truck lines had to re-gear their tractors for the 55mph speed limits) is the best strategy.

Enjoy!

NTX5467

Edited July 4, 2012 by NTX5467 (see edit history)

[bhambulldog]

That's very interesting!!

[RivNut]

If you're considering an o/d transmission for your Electra, you might want to do some research on how the 700-R4 will fit in an X frame. I've seen both 700-R4's and 200-4R's in Riviera chassis, and the 700 is very close. The U-joint will be right in the X and the speedometer drive will have to be modified. The 200-4R fits like a glove, and there's very little fabrication needed for the transmission crossmember. The gear raios, except for the o/d, are very close to the same ratios as are in the ST400, and the o/d is .67. The 3.06 first gear ratio in a 700 is really steep for a nailheads limited rpms.

There are kits available that will limit o/d until you reach certain highway speeds as well as controlling the lockup on the torque converter. All this plays into drivibility on the highway. As the other guy says, it's best to have a 3.42 or higher final drive to get everything compatible. I've got a 3.91 gear to go along with the 200-4R in my '64 Riviera. The other thing that needs consideration for both the 700 and 200 is the TV (throttle valve) cable. There's no vacuum modulator or kickdown solenoid on the 700 or 200. All up shifts and down shifts are handled with the TV cable. It's attached to the carburetor and is requires very close adjustment. If adjusted correctly, it works well. As far as adapters go, most are made with the nailhead bellhousing to chevy transmission bolt patterns. No worries with any 200-4R, they're cast with dual bolt patterns, chevy and BOP.

One of my other wild a$$ed thoughts for my '64 Riviera is to mount an 8.8" rear end from a Ford Explorer under it. There cheap and abundant. They all have 3.73 final drives, posi, and disk brakes. I'd need to weld the control arm brackets to the Ford rear end and redrill the axles for the 5" bolt circles.

Ed