Debating swapping my 4.10 for a 3.42 rear. I'm never going to tow anything with her and would like to get a bit better rpm at highway speed.

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Comment by Russell E Johnson on August 27, 2017 at 10:16pm

I up-sized the tires on the MMP from 8R19.5 to 245/70R19.5 and the RPMs dropped to where I now turn 3000 RPMs at 60, effectively changing the axle ratio from 4.88 to 4.56.

Comment by Philip Wolsonovich on August 27, 2017 at 6:06pm
That's where mine was at thus the 4L80.
Comment by richard nance on August 27, 2017 at 6:05pm

a 4l80e is a turbo 400  with an overdrive.you might look into a bolt on overdrive that bolts to the back of the turbo 400.you need a controller to use a 4l80 e.as far as the speedo you can buy them now that work off of gps or you could just use your phone.waze gives traffic info and current speed or use one of the dedicated speedometer apps. https://www.gearvendors.com/hrgm3s.html

Comment by Daniel Long on August 27, 2017 at 5:34pm

I hear ya. some Allegros have 4.88's which I can't even imagine driving today with that in the rear! at 55 you're revving at 3020! Geez!

Comment by Lakota Wolf on August 27, 2017 at 4:51pm

Your right,,, I miscalculated,,, I had put 373's in My chevy 1 ton to get that extra ummmph out of the stock 3 speed, so I wasnt red lining at 55 mph, It did get me to 62 mph at same rpms

Comment by Daniel Long on August 27, 2017 at 4:47pm

I did and their a bit cheaper but with only a 10% reduction in rpms it ain't doin' so much. I did the math and from 3000 rpm I'll be down to about 2727 rpm. Not bad but still not enough to get the gas mileage up. A set of 3.42's will get her to about 2500 rpm and in the range of what Phil W gets on the road.

Comment by Lakota Wolf on August 27, 2017 at 4:44pm
Have ya thought about 3:73's for gears?
Comment by Daniel Long on August 27, 2017 at 4:24pm

sounds like my buddies that drop Getrag 240's into their E21 BMWs and have to do all the conversion work to get the old mechanics to function. Sounds thought out but cost is about $2800 for all of that and labor I'm guessing. This rear end swap is about $600. I got my Allegro for $2300 in full working order. Glad it worked out for you cause 11mpg is AWESOME for this rig! If I had that I could go across country on about 3 fill ups! WOO! I'm looking for anything better than 6mpg right now and with the rpm range I'm thinking it may be closer to 9 mpg.

Comment by Philip Wolsonovich on August 27, 2017 at 4:17pm
There are some options for the speedometer. I went with a electronic speedo. The controller has to be after market. You would have to mount a T.P.S.. That's not to bad most come with a kit for that. The transmission mount will have to be moved back "4 and the front driveshaft will have to be shortened the same amount. It's alot but if you decide to do it I'll walk you through it. The locking converter is a big plus also.
Comment by Daniel Long on August 27, 2017 at 3:41pm

I checked it out. final 4th gear is 1:.75. My only worry is the electronics on it vs. my old TH400. The ring/pinion is about $250 and then labor and bearings to install. Much less than $1600 for a used 4 speed and then the potential electrical issues or even the driveshaft length.

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