Gear ratio
Thread Starter
|
Junior User
Joined: Feb 2001
Posts: 69
Likes: 1
Gear ratio
Hi
I am hoping someone out there can help me. I am looking for a formula to figure out gear ratios when running different height tires on front and back. I will be running a 9" in the rear with 3.50 or 3.55 gears and 32" tall tires. On the front I have a dana 30 running 31" tall tires and I need to figure the gear ratio I need. An answer to this would be good but the formula would be better. Thanks in advance.
Mike K
I am hoping someone out there can help me. I am looking for a formula to figure out gear ratios when running different height tires on front and back. I will be running a 9" in the rear with 3.50 or 3.55 gears and 32" tall tires. On the front I have a dana 30 running 31" tall tires and I need to figure the gear ratio I need. An answer to this would be good but the formula would be better. Thanks in advance.
Mike K
New User
Joined: Nov 2003
Posts: 11
Likes: 0
Posting Guru
Joined: Dec 1999
Posts: 1,903
Likes: 2
From: Copley, OH
Tire revs per mile x gear ratio = driveshaft revs per mile
Look up the tire revs per mile at the tire manufacturer's web site or at the Tire Rack.
You need to have driveshaft rev/mile to be the same front and rear or just slightly (1% or less) lower in the front.
The front ratio required will be somewhere around a 3.4, which AFAIK doesn't exist. In the long run it's going to be a LOT simpler and more practical to run the same tire diameter and the same ratio on both axles
Look up the tire revs per mile at the tire manufacturer's web site or at the Tire Rack.
You need to have driveshaft rev/mile to be the same front and rear or just slightly (1% or less) lower in the front.
The front ratio required will be somewhere around a 3.4, which AFAIK doesn't exist. In the long run it's going to be a LOT simpler and more practical to run the same tire diameter and the same ratio on both axles
Thread Starter
|
Junior User
Joined: Feb 2001
Posts: 69
Likes: 1
P.S.
Strangeranger. The tires and wheels I am running are very expensive and it will be cheaper to stager my gears. I know running the same front and back is the best but I really need to go with this option. Thanks for your help.
Mike K
Strangeranger. The tires and wheels I am running are very expensive and it will be cheaper to stager my gears. I know running the same front and back is the best but I really need to go with this option. Thanks for your help.
Mike K
Post Fiend
Joined: Nov 2001
Posts: 14,947
Likes: 6
From: Topsfield, MA
The formula
MPH x GEAR RATIO x 336 / TIRE SIZE * OVERDRIVE RATIO
will give you the RPM in overdrive for a given tire/gear combo for any speed you put in.
Just leave off the last part of multiplying by the OD ratio if you don't have overdrive.
MPH x GEAR RATIO x 336 / TIRE SIZE * OVERDRIVE RATIO
will give you the RPM in overdrive for a given tire/gear combo for any speed you put in.
Just leave off the last part of multiplying by the OD ratio if you don't have overdrive.
Posting Guru
Joined: Dec 1999
Posts: 1,903
Likes: 2
From: Copley, OH
That formula only works if you know the EXACT rolling diameter of the tire which is very different from the calculated or measured diameter. For what he's doing he needs to exactly match front and rear driveshaft speeds. For that he needs the manufacturer's revs/mile data.
