Changing rearend to a taller gear

Wayne Thomas · February 07, 2014, 01:47 PM

Has anyone ever changed the rearend for better mileage ?

Lefty · February 07, 2014, 06:51 PM

The Chevy chassis may or may not have aftermarket ring & pinion sets available.. I know the Dodge chassis are virtually non-existent. The biggest problem is that you actually need the low gearing for the torque. If you installed a taller gear set, it would help on flat highways where you would be running a steady speed. It would probably hurt the ability to climb hills and grades.. Especially if starting from a standing start. It may also hurt milage when in hills, because the engine would have to work harder. This could also result in transmission heat issues, as the transmission would have to apply more torque to turn the driveshaft. How much would be affected and how badly would depend on how much you changed the ratio. Going from a 4:56:1 to a 4:30:1 might not make a lot of difference in the power or heat aspects.. going from a 4:56:1 to a 3:73:1 or 4:11:1 might.

A better option is to get a GearVendors overdrive unit. Then you can have the best of both worlds. When in towns doing stop-start traffic, or in hilly areas, you can turn it off and run the original low gearing... But when on the Interstate running steady at 60+mph. you could turn it on and take advantage of the over drive. Installation is fairly straightforward, you would have to have the driveshaft shortened and rebalanced (check local machine shops.. some can do it, some can't... but if they can't they probably can refer you), but other than that it's pretty much a straight bolt on tin place of the original tail housing of the transmission. I've installed a few, and I've used them. They are great for vehicles that never had Overdrive originally.

The costs of the GearVendors setup incl. the driveshaft modification, is about $2,000-$2,500. This is likely inline with the costs of having a custom made ring & pinion made, as well as installation. If you are fortunate enough that an aftermarket ratio exists for your rear end, in the ratio you are wanting, it would prob. be about $500-$750 total to change it out.

I don't know what year exactly that the Chevy chassis and 454 combo was available with the factory 4 speed automatic ( 700R4). Our '88 model had it. If yours is a 3 speed auto (turbo 400), it would make an excellent upgrade. Early 700R4 and 4L60's (same thing, just different name after '91), were not electronically controlled, and can bolt right in. There IS a difference whether it is for a small block or big block!!! Small block trans have 30 splines on the input shaft, and big block trans have 27.... they use different torque converters that are not interchangable. The electronically controlled transmissions are identified by having an "E" at the end... (4L60-E), and would require an aftermarket control box and wiring harness... but they are available. They also made a very HD version called the 4L80-E and the 4L85-E... These are rated to over 18,000lbs GVWR with 4:10:1 gearing. that were available in Suburbans,vans, and med duty trucks... the Hummer H1 had it too.

I used to have a little "math for Mechanics" card that had all sorts of handy formulas on it, like how to figure compression ratios, displacements, and gearing... I think it was a freebie from my Snap On dealer. I have no idea where it is now thougth. But my point is, there is an actual math formula that exists to figure out the ideal rear end ratio needed for any setup. You'd input the engine peak torque rpm, the weight of the vehicle, cruising speed desired, cruising rpm desired, transmission ratio, and tire diameter.. and the result would be the ideal rear end ratio to get the rpm at the speed you want, and still be in the engines torque curve. I have no idea where youcould find it at, maybe somebody here who is way better at math than me could figure it for you... maybe a call to Jegs or Summit Racing could get a result..
Anyways, I just wanted to say that yes, anything is possible... heck, one member stuck a Cummins Turbo Diesel in his!! It all depends on your budget, creativity, talents, and size of your workshop & tool set. Also, bear in mind that for every change there are tradeoffs. A taller gear lowers the cruising rpm, but at the tradeoff of less available torque, and possibly more strain on the engine/transmission. These may or may not be tradeoffs you can accept, but you do need to be aware of them.

Good luck with your project, and be sure to keep us informed of your results no matter which option you decide to go with.
Lefty

circleD · February 07, 2014, 11:20 PM

The only thing I can recall from the 80s selection is that 3.73 gear is great for mileage on trucks at higher speeds.
I've used the 4.10 to make up the difference when putting large tires on a truck and it kind of balanced out the take off torque and higher speeds.
The 4.56 is used for torque/ power/ and drinking the gas. On the highway this gear was not your friend.
Those 3 gears are your only choice for a 14 bolt unless an off road shop custom makes one and good luck finding parts then.
So you have to decide on economy or power and the gear vendor is the only real middle option.

pvoth1111 · February 08, 2014, 08:01 AM

Changing final drive gearing most likely would adversely affect getting that thing moving....adding more gears at the tranny is really the only solution......that's Why todays cars have 5,6,7,8, gears.

Wayne Thomas · February 08, 2014, 11:29 AM

Thank You All . I have a 22' Winnebago Class A , with a 454 , and a 3 speed trany , 4:11 rearend . Ithought a talled gear would work . Wayne

Lefty · February 08, 2014, 07:14 PM

If it has a 4:11:1, it already has the taller gear

Our Georgie Boy was a 29' Class A on a 1988 Chevy P-30 chassis, with the 454... but it had a 4 speed automatic (700R4). It had 4:56:1 rear end in it.

You might could go with a 3:73:1 since yours is only a 22' model... and likely a lot lighter than mine was.

Wayne Thomas · February 09, 2014, 12:04 PM

Thanks ! Lefty . I thought you could buy the chunk , and put that in if you didn't like it you could put the other one back . Wayne

Lefty · February 09, 2014, 08:37 PM

be sure to get a shim kit with the new ring & pinion, as well as some blue marking dye. That way you can set the lash on the gears. Otherwise it'll hum like crazy.