3.78 vs 3.54 rear end and speedometer/miles
 

I have to replace ring gear in my roadster. Presently it is stock which I think is 3.78. If I put in a 3.54 gear & pinion, how will this change the mph and odometer reading ? Thanks, Bill

Re: 3.78 vs 3.54 rear end and speedometer/miles
 

There is roughly 6% difference between those two ratios, so your speedometer/odometer will be off by about 6%.

With the 3.54 gears, your speedo will read about 6% too low.
When your speedo says '50', you would be doing about '53'.

Re: 3.78 vs 3.54 rear end and speedometer/miles
 

I run 3:54 in two cars and it seems to give about 6-7 MPH more with the same RPM. You will need to change the speedometer drive gear in the cap on the torque tube. All the parts houses have them listed.

Re: 3.78 vs 3.54 rear end and speedometer/miles
 

Buy the correct speedometer gear for the 3.54. I think Bratton's has them. Get the whole assembly, don't buy the new gear for your housing.

Charlie Stephens

Re: 3.78 vs 3.54 rear end and speedometer/miles
 

Charlie - Why? I changed the gear for my 3.54 and it is fairly accurate.

Re: 3.78 vs 3.54 rear end and speedometer/miles
 

I guess I am just a perfectionist. Once the rear end is out for a rebuild it is easy to replace the gear.

Charlie Stephens

Re: 3.78 vs 3.54 rear end and speedometer/miles
 

Thanks, I was concerned I may have overlooked something.

Re: 3.78 vs 3.54 rear end and speedometer/miles
 

thank you for the info. Bll

Re: 3.78 vs 3.54 rear end and speedometer/miles
 

A 3.54! One of the best things I ever did! Rebuilding the rear is a PITA, however it's worth it. If you live in a flat area and have a strong engine, you can go to a 3.27. I am in a very hilly area, so I played it safe and went for the 3.54. The amazing thing is, it climbs the hills around here better than the old 3.78. My engine mods are just a head cut .040 and a "B" carb with a bored out manifold. Yes, there's some improvement, but no one would call it a "Hot Rod". It's more of a dream on the highway. I gave up nothing and gained on all counts. Yes, By all means do it. Be sure to follow Tom Endy's tutorials on Rebuilding ford differentials. It's not a simple process!
Terry

Re: 3.78 vs 3.54 rear end and speedometer/miles
 

That's a heck of an endorsement. I've been wondering about what ratio to go with also. 3.54 sounds good, especially because I have 4.11 now.

Re: 3.78 vs 3.54 rear end and speedometer/miles
 

I look at it from a different perspective 26%od with a 3.78 or a 3.54 at crusing speed of 62 mph? The 3.54 drops engine rpm down 130 from the 3.78..from 2130 to 2000 or so..better effciency,no lugging..

Re: 3.78 vs 3.54 rear end and speedometer/miles
 

My late 30 pickup also had a 4.11 in it and had a shop put in 3.54. Rebuilt engine standard head. Nice to now be able to keep up with other club members at 45 mph without the engine at really high revs. Good power for hills, max top speed is 55 mph. I do not drive highway speeds so this is OK with me.



From what I've read people who drive highway speeds run a Mitchell with 3.78 rear end. Do not remember which ratio Mitchell most run.

Re: 3.78 vs 3.54 rear end and speedometer/miles
 

My engine is modified, probably double stock power. She pulls in high (OD,2.96 or so) without slipping the clutch taking off..in low (3.78) its too low..3.56 should be nice both ways..

Re: 3.78 vs 3.54 rear end and speedometer/miles
 

I would do it! I've done it twice! Please for your sake, read the Tom Endy articles. Cost, $465 for the gear set. Approximately $ 40 @ for the 4 tapered bearings. $16 @ for two "cups" or races. I think the double race (Pinion) is around $40. A Best gasket set to set the inch lbs of the pinion and the carrier bearings. $15. You will also need 2 torque wrenches, one in ft lbs, and one in inch lbs. You will need a vice and a fixture to hold the axle housing. I made mine form a 2x6 and a 2x4. Put two clamping bolts (1/2 inch ready rod) to hold on either side of the axle housing. These things are done vertically. Always remember, the ring gear is on the left side! Otherwise, you'll have 3 speeds in reverse and 1 speed forward. Altogether, the costs add up to about $700. Don't try it without Tom Endy's advice, but that's the gist of it. It's a great time to fix up anything (axles, brakes, spring perch bearings, etc) that it may need. Oh yeah, you'll need a spring spreader too.
Terry

Re: 3.78 vs 3.54 rear end and speedometer/miles
 

You can find Borg Warner overdrives for that kind of money..

Re: 3.78 vs 3.54 rear end and speedometer/miles
 

I'm sorry, I'm a little slow. What are you trying to relate in this post? I am having a hard time understanding...

Are you saying, go with an O/D with 3.78, go with an O/D with 3.54 or just go with the 3.54?

Thanks

Re: 3.78 vs 3.54 rear end and speedometer/miles
 

The gain from rpm reduction at freeway speed.OD/3.78 or OD/3.54..at 62mph OD 3.54 the rpm is 130 less and at the bottom of the torque range..makes it a little more pleasant to drive.700 bucks (Terry's cost for a 3.54 upgrade) more pleasant? that's debatable.