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3.90 vs 3.73 gears

67belvedere/225

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want to to swap gears , git 3.55 now , but want more launch.
slant six twin Webbers automatic overdrive (gm 700r4 trans)
I fear going 3.90 will make her rev to high on the highways and won't like it.
didn't realize they even made that ratio .
now round 2200 rpm @60 with 3.55
the rpm calculator has it at 3.73 @2270 rpm
and the 390 @ 2380
tempting to go 3:90
the numbers seem off think it would be reving more Around 3000
 
want to to swap gears , git 3.55 now , but want more launch.
slant six twin Webbers automatic overdrive (gm 700r4 trans)
I fear going 3.90 will make her rev to high on the highways and won't like it.
didn't realize they even made that ratio .
now round 2200 rpm @60 with 3.55
the rpm calculator has it at 3.73 @2270 rpm
and the 390 @ 2380
tempting to go 3:90
the numbers seem off think it would be reving more Around 3000
Tire diameter ?
 
The 700R4 has a .7 O/D
What size tire?
3.73 to 3.91 isn't much of a change & won't have much of an effect on RPM's in O/D but when multiplied in 1st & 2nd gear it will matter....
 
I have 3.91 in my 69 Charger and it is needed to get it going…just a heavy car….once I hit second gear the car actually starts to get up and go……
 
Last edited:
I have 3.91 in my 69 Charger and it is need to get it going…just a heavy car….once I hit second gear the car actually starts to get up and go……
I have a sedan and its juiced up slant six , somewhere around 210 hp
 
want to to swap gears , git 3.55 now , but want more launch.
slant six twin Webbers automatic overdrive (gm 700r4 trans)
I fear going 3.90 will make her rev to high on the highways and won't like it.
didn't realize they even made that ratio .
now round 2200 rpm @60 with 3.55
the rpm calculator has it at 3.73 @2270 rpm
and the 390 @ 2380
tempting to go 3:90
the numbers seem off think it would be reving more Around 3000
Every calculator I try has 60mph @ 1898rpm with 26.4" tires, in OD on 700r4 (.70) and a 3.55 rear..but your tach is telling you 2200?

3.91 with a 700r4 OD comes out at 2264rpm.....that's my rpm at 45mph with 3.73s, 28" tires and a 727...
If I had a .7 OD I'd be running 5.13s with my setup!
 
The difference between your 3.55s and 3.90 is 1.1 (actually 1.098) so 1.1times your "around 2200" becomes 2416 rpm.
1.o455 for the change from 3.55 to 3.73s so your 2200 becomes 2300rpm.
Would be an easy choice for me, 3.90s. (With 4.10, your 2200 would go to 2540).
 
just need that push to order the 3.90 and its cheaper than 3.73
one more positive for the 3.90 and im buying em .lol
 
Every calculator I try has 60mph @ 1898rpm with 26.4" tires, in OD on 700r4 (.70) and a 3.55 rear..but your tach is telling you 2200?

3.91 with a 700r4 OD comes out at 2264rpm.....that's my rpm at 45mph with 3.73s, 28" tires and a 727...
If I had a .7 OD I'd be running 5.13s with my setup!
im going out now and going to use my I phone to see how accurate the speedo is
 
want to to swap gears , git 3.55 now , but want more launch.
slant six twin Webbers automatic overdrive (gm 700r4 trans)
I fear going 3.90 will make her rev to high on the highways and won't like it.
didn't realize they even made that ratio .
now round 2200 rpm @60 with 3.55
the rpm calculator has it at 3.73 @2270 rpm
and the 390 @ 2380
tempting to go 3:90
the numbers seem off think it would be reving more Around 3000
These are always the fun things how fast do you want to go how fast to do want to get there
 
That pretty much proves what I said.
I believe a 3 speed auto is 1:1 in 3rd gear, same as a 4 speed.

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