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-   -   Rear end questions (http://www.hotrodders.com/forum/rear-end-questions-226070.html)

Paul E Cain 11-14-2012 05:15 AM

Rear end questions
 
Recently purchased a 1941 Ford with a chevy 350 and automatic 3 speed transmission. @ 65 miles per hour Tack reads 3,100. Should I change to a four speed transmission, or can a different differential be installed. Looking to better fuel millage. Engine is a high performance Edelbrook.

gearheadslife 11-14-2012 06:02 AM

what size tires on the rear.. so we can get the height in inches .. to figure out your reargear...
and what rear is under the truck.. the stocker.. or a later one?

cobalt327 11-14-2012 06:31 AM

Quote:

Originally Posted by Paul E Cain (Post 1611043)
Recently purchased a 1941 Ford with a chevy 350 and automatic 3 speed transmission. @ 65 miles per hour Tack reads 3,100. Should I change to a four speed transmission, or can a different differential be installed. Looking to better fuel millage. Engine is a high performance Edelbrook.

You could do either. In most cases it will be cheaper to change the rear gear ratio, but there's also something to be said for an OD transmission because of the low first gear ratio plus a .7 OD ratio (or thereabouts depending on the tranny you use) gives the best of both worlds- a low first gear along w/a high cruising gear. The cheaper rear ratio change will give you a good cruise rpm- at the expense of good acceleration in low gear.

Some numbers:

The way you sit now, with a 30" diameter tire (middle of the road size) your rear ratio works out to be about 4.25:1.

With a .7 OD the effective rear ratio becomes about 3:1 and at 65 mph the engine drops to just over 2000 rpm. This is a hard to beat combo IMHO.

You can play with these calculators to see what you can come up with: Wallace Racing - Automotive Calculators.

Good luck.

big gear head 11-14-2012 07:04 AM

I'm assuming that you mean a 4 speed automatic overdrive transmission. A 4 speed manual would not help you. There are several overdrive automatics to choose from. The 700R4 has a 3.06 first gear which wold be about useless with a 4.xx rear end gear ratio. The 200 $r would be a better choice because it has a normal first gear and a better overdrive ratio, plus it's the same length as the TH350 and the cross member doesn't have to be moved.

If you do go to an overdrive transmission you will need to get a good drive shaft. I would recommend at least a 3 1/2 inch tube to beat deflection at high drive shaft RPMs. Otherwise you are going to get a vibration somewhere around 60 MPH or higher.

delawarebill 11-14-2012 07:26 AM

cheaper gear change...
 
what rear r u running now ??? and do u know the gear ratio also. if not jack up the rear, mark the d-shaft and tire and then rotate the tire counting the turns on the shaft.. u'll be close without pulling the rear apart.. if your in the 4's then move to a higher gear (lower number) in the low 3's...

SSedan64 11-14-2012 07:26 AM

1 Attachment(s)
With the 2004R Trans the Driveshaft doesn't have to be cut if the TH350 has a 6" Tailhousing but, the Crossmember does need to be move back about 5-11/16".

Automatic Transmission Identification

cobalt327 11-14-2012 03:04 PM

Check rear gear ratio


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