57 Chevy Belair. Bruce Danville

[Bruce Danville]

have a gearing problem, my 57 has a 327 engine . A 350 Turbo trans. Has 3:36 gears . The problem I’m having is when driving down highway at 55 MPH the engine is running at a high RPM. I can see the gas go down ( not really but it’s screaming) seems like it needs another lower gear to go to ! We checked the ring and pinion and the ring had 37 teeth and the pinion had 11 which comes out to a 3:36 rear end. What is my problem BRUCE DANVILLE
THE CAR WAS ORIGINALLY A 3 speed shift standard ! Please help me

[chevynut]

Looks like you should be running almost 3100 RPM at 75 MPH. Your solutions are to change the rear gears to something higher (numerically lower, like 3.08) or get an overdrive transmission. Even with a 3.08 rear gear you'll be running over 2700 RPM at 75 MPH and your street performance will suffer. An overdrive transmission is the best solution that gives you good street performance and lower highway RPM.

[carls 56 (RIP 11/24/2021)]

:?: is it shifting all 3 gears? just a thought.

[Bruce Danville]

Yes

[BamaNomad]

The size of your tires might also be adversely affecting the rpms. My '56 Nomad is running 235/70-15 BFG tires, TCI 350 trans, and 3.08 rear and at 70-75, the rpm is below 2400. Are you running low profile (short) tires? and I'm getting close to 20 mpg with a mild 350 eng...

[chevynut]

The size of tires has no bearing on INDICATED speed vs RPM if using a speedometer driven by the transmission (gear of VSS). In other words you can change the tires and the car will read the same RPM at a given indicated speed. The ACTUAL speed will be different though.

Using a 28" diameter tire and 3.36 gears RPMs at 75 MPH should be about 3024. A 26" tire would be 3257 and a 29" tire would be 2920. So engine speed drops about 100 RPM per inch of tire diameter or around 3.3%. You'd be hard pressed to get the 30% + drop of an OD transmission.


My '56 Nomad is running 235/70-15 BFG tires, TCI 350 trans, and 3.08 rear and at 70-75, the rpm is below 2400.

I don't think so.:eek: According to TireRack.com that tire is nominally 27.4" tall and rolling radius is less than that. Revs per mile are 758 which makes the effective diameter 26.60". Here's the specs on TireRack:

https://www.tirerack.com/tires/tires.jsp?tireMake=BFGoodrich&tireModel=Radial+T%2FA&partnum=27SR5RADTARWL2V3&vehicleSearch=false&fromCompare1=yes

The Richmond calculator says at 70MPH RPM is 2744 and at 75MPH RPM is 2940 with those tires and gears. Here's the calculator:

https://www.richmondgear.com/wp-content/uploads/calculator/street-performance-calculator.html

Either your speedo is off or your tach is off...or both. Numbers don't lie unless the calculator is off. :geek:

[BamaNomad]

Speed was verified independently, but it would NOT surprise me that the Classic Instruments tach was off (or that every Classic instruments gauge is OFF)...

[BamaNomad]

Speed was verified independently, but it would NOT surprise me that the Classic Instruments tach was off (or that every Classic instruments gauge is OFF)... I've mentioned before how sorry the Classic Instrument gauge set is... hard to read, and now.. inaccurate... as well..

[chevynut]

A 350 Turbo trans. Has 3:36 gears . The problem I’m having is when driving down highway at 55 MPH the engine is running at a high RPM.

Going back to the original poster's question, what is your RPM? I just noticed you said it was "high" at 55 MPH. Your RPM should only be around 2200-2400 depending on tire size at that speed. Is your speedometer off? If you have a smartphone you can download a free GPS speedometer app to check it. We need more info to be able to help.

[markm]

If OP car was a 3 spd originally I believe in 57 that got you a 3.55 gear. The 3.36 was std for powerglide. I hear this story all the time people are all used to OD in late models.

[WagonCrazy]

This is a common dilemma by today's performance standards...(high rpms at highway speeds with a 3 speed trans).

Consider swapping to a 700R4 (4 speed overdrive) automatic trans. You'll have a couple thousand dollars into it to get there, but you can probably stay with your 3:36 rear.
Or...if a 3:08 ratio rear pumpkin were to be found locally, swapping that in will yield slightly lower RPMs at speed (as CNut indicated) while staying with your Turbo 350.

It's a domino effect to start swapping...just be prepared for that. We've all been thru it and survived. :p