I have a situation with two different vehicles that has me pretty confused. I have a 1955 Olds 88 with an Olds 350 and turbo 400 that cruises at ~ 65 mph at 2600 rpm. 3.42 gears with a 25" tall tire. I have a 1953 Olds 88 with an Olds 403 and turbo 400 that cruise at ~ 65 mph at 3500 rpm. 3.23 gears with a 27" tire. Speedometers don't work in either car. Rpm calculators show the 55 is too low and the 53 is too high. The 55 sounds comfortable at this speed. The 53 sounds slightly strained. Any ideas why the 53 turns such a high rpm ? I have manually shifted to make sure all 3 speeds are there.

 

Did you use a GPS (such as on a phone) to verify your speed?
What type of tach does each car use?

 

They both use the same exact tach, the retro moon equipped. No gps, just seat of the pants driving along the same stretch of highway that I do every day.

 

Either your tachometers are both off or you don't have the correct tire diameters or gear ratios. Or, your speeds are wildly off from where you think they are.

With 3.42 gears and a 25" tall tire (which is absolutely tiny, like a 235/60-14), you should be seeing abouit 3,000 RPM in third at 65 mph. Allowing for a little converter slip would raise that a hundred RPM or so, and the sidewall flex of a radial would further reduce the effective diameter of the tire and bump it up a little more. It definitely won't be 2,600 RPM. With 25" tall tires, 3.08 gears would put you right at 2,600 RPM. 3.42 gears at 55 MPH would also put you right at 2,600 RPM.

With 3.23 gears and a 27" tall tire (like a 255/60-15), you should be seeing right about 2,600 RPM in third at 65. Again allowing for converter slip and sidewall flex, you're still under 3,000 - nowhere close to 3,500 RPM. With 27" tall tires you'd need 4.33 gears to get 3,500 RPM at 65. With 3.23 you'd be going 85 MPH at 3,500 RPM.

 

How do you know you're going 65 MPH?

An online calculator shows the following:
https://spicerparts.com/calculators/...rpm-calculator

The '55 is doing 56.6 MPH at 2600 RPM.
The '53 is doing 87.1 MPH at 3500 RPM.

 

I'm not exact on the speed but i drive this section of I-35 7 days a week and am pretty used to the flow of traffic in my other vehicles that have working speedometers. I'm sure its close. The tires on the 55 are well worn 15's. They measure at 25. 3.07's were an option in 55. I assumed they were the standard 3.42's. I could be wrong. I can guarantee you I'm not doing 85 in the 53, but the sound of the engine says 3500rpm and the tires measure to 27". I checked the rearend and they are 3.23. Could I be getting slippage at the torque converter ? I don't notice any pulling from a stop.

 

With 3.23 gears and 27" tires, you'd need to be in second gear to get close to 3600 RPM at 65 MPH.

 

Using the rpm calculator, with a second gear of 1.48 that works out to about that rpm. I'm going to have to try it again using a manual shift to make sure I'm getting in third and see what happens.

 

Somewhere along the way the classic car community was convinced that any car without OD was undriveable and all engines will explode if the cruise rpm is over 2500. Bunch of nonsese.

 

Probably not what the OP was getting at, but I sure agree with your point. I can remember my 67 Camaro with a 396 engine, traveling back and forth to college along I 80. 75mph was a solid 3100 rpm. And I can tell that the 72 Cutlass that I recently acquired is cruising somewhere above 2500rpm as well. No tach on it, but it is hard to mis the higher engine speeds than what my more modern cars sound like.