On the day I drove my 1964 Ninety Eight home, I cruised at 55 mph to try to save fuel. I also thought the engine felt content at that speed. With the Holley EFI setup it has, I could see the digital tach showed the engine doing around 2,300 or so RPM at that speed. Recently I was chatting with someone who said they used to have a '64 Olds. I told him it was a shame the car didn't have a 4 speed transmission with overdrive like modern cars, like my Dodge Caravan that does 1,800 RPM at 75 mph. He said that his car "would cruise at 75 mph all day long" and wasn't super terrible with fuel consumption. He thought I should ask the previous owner if he had swapped out the rear axle or changed the gears in the diff. So my question to other owners with this transmission (and a tach) is, what RPM is your car cruising at on the highway?

 

What are your rear gears?

 

I don't know.

 

What kind of Oldsmobile did your friend supposedly own?

Frankly, I'm surprised you got your car to shift properly with an aftermarket EFI. I tried to install an Edelbrock carburetor on the 64 98 I owned years ago and never got it to shift right. Why don't you give us a pic or two of the EFI linkage, specifically on the driver's side. The Slim Jim Rotohydramatic uses a manual linkage between the throttle valve and 4GC carburetor. Setting this manual linkage is CRITICAL for transmission health.
​​​​​

 

Not sure how to post a link (my own personal tech handicap), but look up a thread entitled "Stock gear ratio in 1964 98". That'll show you how to identify your rear end gear, which will go a long way toward telling you what your highway RPM should be. Failing that, there's always the old-school method of jacking it up and measuring driveshaft revolutions vs. wheel revolutions. Less accurate, but should give you a ballpark idea.

 

The EFI was installed by the previous owner and seems to shift well into 2nd and 3rd. I'm not saying there's a problem with the RPM - just asking if 2300 is normal at 55mph? I thought maybe in those days 60mph was the accepted speed on the new Interstate system. Here are pics of the linkage to the EFI:

 

2300 @ 55 sounds high, but could be perfectly normal, depending on what your rear gears are.

When the interstates were built, the prevailing speed limit was 70 MPH in most places.

 

Hi,

Here's a reference from Fabcraft for rear end ratios:

Regards,
Jim

 

Tire diameter also impacts the RPM at a particular MPH. Taller tires reduce RPM and shorter tires increase RPM.

 

I moved the thread to the Vintage forum.

Thanks for posting those pics, and thanks to Jim for posting the info on rear end gears. The manual linkage to the transmission seems to be in order from the pics. The adjustment of those jam nuts is important though. If it isn't done right it can lead to transmission damage.

I don't recall if we've ever had a member post about successfully using an aftermarket carburetor or EFI with a Slim Jim.