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I have a 1975 Convertible L48 four speed.
I do not have any documentation. I know there was some significant work done to it before I got it.
According to the on-line calculators, it has 3.70 gears in the diff based on 3100 rpm at 65 mph with 26 inch tires. However, the stamps on the differential indicate 3.08.
The looks like it has been opened before.
If you are skilled at decoding, please look at the picture and decode. Is it possible the gears were changed?
Thanks for the replies.
I am mixed on learning of the 3.70’s. I think 3.08’s would suit me better.
The car is a stock L48 - so it will never be a tire scorcher unless I rip open the engine. Not why I got the car.
I want a nice reliable cruiser I can take to a show about 30 miles from my house a few times a year and in-town cruising otherwise.
It would be nice to be able to go 65 on the interstate so as not to get run over.
How will running at 3100 for 40 minutes or so wear on the engine? Too much?
I could, as an alternate, take the back roads at 45-50mph to the shows and allow an hour travel time.
Thoughts?
PS - don’t have the cash for a 5-speed conversion.
I don't see an issue as long as you are using the appropriate high zinc oil for a flat tappet cam. An example would be Valvoline VR1 10W30 conventional.
I don't see an issue as long as you are using the appropriate high zinc oil for a flat tappet cam. An example would be Valvoline VR1 10W30 conventional.
Good point. I use Amsoil Z-Rod with the correct ZDDP for old SBCs. I researched that issue a lot with my 82.
I am not sure what WOT is on these engines, but 3100 just sounds high to cruise. It did not feel like it was working hard when I ran it yesterday, however.
I will go half the distance on the interstate then drop off when I start to cringe!!!
The old engines did not come with OD trans, 3000-3200 was not uncommon at cruise and will work. The consequences are the engine life will be about 1/2 of one with an OD from day one, your fuel economy will go from bad to worse. 350 with 370 gears and 1:1 final trans is not going to get more than 12-15 mpg cruising, depending on the condition of the engine and how it is built. Around town,11-12 even with a 308.
The old engines did not come with OD trans, 3000-3200 was not uncommon at cruise and will work. The consequences are the engine life will be about 1/2 of one with an OD from day one, your fuel economy will go from bad to worse. 350 with 370 gears and 1:1 final trans is not going to get more than 12-15 mpg cruising, depending on the condition of the engine and how it is built. Around town,11-12 even with a 308.
My second vehicle was worn out 1970 F100 with a 302 and automatic trans. Don’t know the rear end gears. (Back then in the early 80’s I only cared about chasing tail.) There was no tach. I wonder what my rpm’s were when I drove 55-65 on the interstate? Never even thought about it. Probably in 3’s.
Now, my 2023 F250 Powerstroke with a ten speed tranny will be around 1800 rpm at highway speeds. Amazing how much gearing has changed.
I've run my 77 on many very long cross country trips. Yes I spin those RPM's and higher. For hundreds of miles at a clip! All day long stopping for lunch and fuel! 75 -85 miles an hour. Spinning 4 grand all day! And never , not once even gave it a thought! And I still do! Ran up to Cairns and back about 2 years ago in my 77. About 1,000 miles each way. Very rough roads!
I've run my 77 on many very long cross country trips. Yes I spin those RPM's and higher. For hundreds of miles at a clip! All day long stopping for lunch and fuel! 75 -85 miles an hour. Spinning 4 grand all day! And never , not once even gave it a thought! And I still do! Ran up to Cairns and back about 2 years ago in my 77. About 1,000 miles each way. Very rough roads!
Good to hear it does not damage the SBC. I’m afraid I have gotten used to seeing 2K-ish on the tachs of my cars when cruising.
I just got this 75 on the road after working on it for a year. I will get used to it.
Differential Code
