Would you notice the difference!!
Thread Starter
Instructor
Joined: Nov 2012
Posts: 182
Likes: 14
From: Rangiora,New Zealand
Do you know for a fact that you have a 3.08 rear gear.
If you have a 3.08 rear gear, and assuming you are running a 27" tall tire, my gear calculator says with a non OD auto trans (TH350 or TH400) and a 3.08 rear gear you should be turning about 2290 rpm's at 60 mph
With a 3.36 rear gear you should be turning 2500 rpm's at 60 mph.
With a 3.55 rear gear you should be turning 2640 rpm's at 60 mph.
With a 3.73 rear gear you should be turning 2775 rpm's at 60 mph.
My 73 has 245-60-15 tires (26.5" tall), with a TH400 and 3.36 gears.
At 70 mph on California highways, I'm in the middle to slow lanes turning between 2900 and 3000 rpm's.
To check what rear gears you actually have:
Lift both rear tires off the ground.
Mark the inside tire with a chalk line from the rim to the tread.
Mark the drive shaft with a line along the bottom of the shaft.
Turn the rear tire in the forward direction one complete turn and count the rotations of the drive shaft.
Approximately 3.0 turns = 3.08 rear gears, 3 1/3 turns = 3.36 gears, 3 1/2 turns = 3.55 gears and 3 3/4 turns = 3.73 gears.
If most of your cruising and driving is at 60 mph then find the rear gearing that best fits your driving style and needs.
If you have a 3.08 rear gear, and assuming you are running a 27" tall tire, my gear calculator says with a non OD auto trans (TH350 or TH400) and a 3.08 rear gear you should be turning about 2290 rpm's at 60 mph
With a 3.36 rear gear you should be turning 2500 rpm's at 60 mph.
With a 3.55 rear gear you should be turning 2640 rpm's at 60 mph.
With a 3.73 rear gear you should be turning 2775 rpm's at 60 mph.
My 73 has 245-60-15 tires (26.5" tall), with a TH400 and 3.36 gears.
At 70 mph on California highways, I'm in the middle to slow lanes turning between 2900 and 3000 rpm's.
To check what rear gears you actually have:
Lift both rear tires off the ground.
Mark the inside tire with a chalk line from the rim to the tread.
Mark the drive shaft with a line along the bottom of the shaft.
Turn the rear tire in the forward direction one complete turn and count the rotations of the drive shaft.
Approximately 3.0 turns = 3.08 rear gears, 3 1/3 turns = 3.36 gears, 3 1/2 turns = 3.55 gears and 3 3/4 turns = 3.73 gears.
If most of your cruising and driving is at 60 mph then find the rear gearing that best fits your driving style and needs.
Le Mans Master
Joined: May 2006
Posts: 9,993
Likes: 1,136
From: League City Tx
A good converter will get you more acceleration improvement then a mild gear swap. Just make sure to get a GOOD converter, not a cheapy from Jegs. I went from a 3.08 to 3.36 gears, but I already had a 10"/3400 converter and 650hp so the gear change gave me just enough to dial in my combo and still maintain a decent cruise rpm. (70 mph/3000 rpm). On a mild motor, I doubt you'd even notice a difference at all in acceleration. 3.73s would give you a nice bump in acceleration but...also a BIG jump in cruise rpms....70 mph = 3500. Which sucks. A converter will only increase your cruise rpm by 100-200 rpms if its an efficient one. A good converter will knock a half second off of your 1/4 mile time.
Thread Starter
Instructor
Joined: Nov 2012
Posts: 182
Likes: 14
From: Rangiora,New Zealand
A good converter will get you more acceleration improvement then a mild gear swap. Just make sure to get a GOOD converter, not a cheapy from Jegs. I went from a 3.08 to 3.36 gears, but I already had a 10"/3400 converter and 650hp so the gear change gave me just enough to dial in my combo and still maintain a decent cruise rpm. (70 mph/3000 rpm). On a mild motor, I doubt you'd even notice a difference at all in acceleration. 3.73s would give you a nice bump in acceleration but...also a BIG jump in cruise rpms....70 mph = 3500. Which sucks. A converter will only increase your cruise rpm by 100-200 rpms if its an efficient one. A good converter will knock a half second off of your 1/4 mile time.
Cheers Dave
