Gearing for freeway fuel economy
Record Breaker
Joined: May 2008
Posts: 1,174
Likes: 1
From: North TX
I just ran the numbers for my van...
1986 Dodge ram van:
Tire revs per mile (235/70/15): 720.4
3rd gear ratio: 1:1
Stock Final drive ratio: 3.55
Engine Revs per mile: 2557.42
RPM @ 65mph: 2770.53
Modified Final Drive ratio: 2.76
Engine Revs per mile: 1988.304
RPM @ 65MPH: 2153.996
2770 cruising RPMs seems a little excessive, especially for such a big engine. 2.76 is about the tallest ratio I could find, and seems like it would drop the cruising RPMs to a little more reasonable, but still slightly high 2153.
I would imagine that this would put more load on the transmission and drivetrain at RPM, but it seems like it wouldn't be unreasonable given the loads we're hauling. Anyone have any thoughts?
Also, what about running a good 75w/90 gear oil in the rear diff, rather than the spec 80w140?
1986 Dodge ram van:
Tire revs per mile (235/70/15): 720.4
3rd gear ratio: 1:1
Stock Final drive ratio: 3.55
Engine Revs per mile: 2557.42
RPM @ 65mph: 2770.53
Modified Final Drive ratio: 2.76
Engine Revs per mile: 1988.304
RPM @ 65MPH: 2153.996
2770 cruising RPMs seems a little excessive, especially for such a big engine. 2.76 is about the tallest ratio I could find, and seems like it would drop the cruising RPMs to a little more reasonable, but still slightly high 2153.
I would imagine that this would put more load on the transmission and drivetrain at RPM, but it seems like it wouldn't be unreasonable given the loads we're hauling. Anyone have any thoughts?
Also, what about running a good 75w/90 gear oil in the rear diff, rather than the spec 80w140?
On a Impala forum I am on many have found that the 2.?? gears which were supposed to help highway mileage actually hurts it worse than going with 3.08 - 3.73 gearing. On these cars optimal gearing is 3.43 over 3.08's.
Thread Starter
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Registered User
Joined: Apr 2011
Posts: 6
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Okay, good to know. Is that experience with the 3 speed, 1:1 final drive? Or is it with an OD transmission?
Record Breaker
Joined: May 2008
Posts: 1,174
Likes: 1
From: North TX
Good rule of thumb is to keep the engine near and just below it best torque rpm. Most Magnum series engines make best torque at 2k rpms. Not sure about the older LA engines.
Amateur
Joined: Jun 2008
Posts: 37
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Look at the revs per mile change by going to a 235/85/15 highway ribbed commerical tire. That should be about as effective of a change as a diff ratio. You might be really suprised at the result. But don't go any wider, cause that robs horsepower.