help w/ selecting gear ratio

[84BURBMIKE]

hello guys & gals

i'm new to the forum and am very impressed w/ the content and member knowledge so thanks in advanced.

i recently put 35" tires on my 84 burb. i currently have 3.73 gears but need to put a lower ratio in so i can drive it on the interstate. it will not stay in overdrive(700r4) around 65mph. it will down shift w/ just half throttle. i've messed around w/ some of the calculaters on this site and on randy's ring & pinion but i would like to see if someone out there is running 35's w/ a 700r4 and what ratio and/or fuel mileage they are getting. any advice? i'm rebuilding both axles while i'm at it around $800 in parts so i want to get it right the first time.

thanks

michael

[JohnC]

How does it perform in direct? What RPM are you turning at 65 in the 2 gears? What do you have for power? (turbo?)

Do you tow? If so, what?

[DmaxMaverick]

Welcome aboard!

If, with the 35's, it has enough power and easily (within reason) holds direct (3rd), changing to 4.10 gears will move you into OD (4th) at about the same RPM. If you need lower gears, you'll have to replace the entire axle housings (which wouldn't be a bad idea anyway, with 35" tires), and use heavier (3/4 or one ton, older 12-bolt, or heavy Dana axles). If you plan on any significant towing, loading, or off-road, the original 10-bolt axle won't hold up very long (I went through a few before upgrading). The late 70's 12-bolt axle is a bolt-on swap, including the brakes. The only adaptation I had to make was the U-joint (different pinion yoke), but that depends on the axle's previous application (it was way cheaper to just use an adapter joint, than replace the yoke). Lowering the gear ratio will give back (most of) the acceleration from a stop. Keep in mind, larger tires require more power to turn, scrub the surface, and break the wind, so you will never get back all of it. Mileage and power will still be down at highway speed, no matter what you do. That is, of course, unless you are using ultra-skinny pizza cutter tires with stock suspension height. Mud tires are the worst at sapping power, for several reasons, and there's no way around it.

For reference, my '85 is non-turbo, 700R4, and turns 33x12.5's well with a 4.10 geared 1978 12-bolt axle on the rear, and the original 10-bolt on the front. I can still squeeze mid-20's for mileage if I keep the black smoke to a minimum (and even better if I can run downhill, both ways).

[84BURBMIKE]

i have a non turbo 6.2. i already swapped in a 14 bolt in the rear and hd dana 44 in the front (8 lug). the tires are all terrains not aggressive at all. i do not know my rpms----have not installed a tach yet. mostly a daily driver but i tow a pop up trailer 7-8 times a year (2500 lbs) and my jeep occasionally 4k lbs but not since the lift and bigger tires. in 3rd it feels like i deployed a parachute but it holds. 1st & 2nd actually feels peppie.

[Subzilla]

Got 35's on the Sub with 4.10 gears. Preturbo days, it did OK on the highway except for those big long hills where it huffed and puffed, especially towing a popup. I understand what you're going through. I was always downshifting to 3rd to hopefully maintain the speed limit. At 65, it's showing 2100/2200 rpm's I think with the 4.10's. With the turbo and rebuilt heads, it's a different beast. Hills are not an issue now.

[84BURBMIKE]

decided to go w/ 4.56's. in od it should be at 1992rpms at 65mph and i will install a lockup kit to keep it in od. the only elevations i deal w/ in central florida are overpasses.:) anyone installed a tach? which one is best & easiest? thinking of getting rid of the clock to the left of the steering wheel and replace w/ a tach to look factory. suggestions?

[Subzilla]

Autometer makes the easiest to install diesel tach. The signal pick-up attaches to the outside of the alternator with a large hose clamp. The tach is adjustable for the different alt pulleys. The only complaint is at low idle, the needle may fluctuate depending on what electrical accessories are running - especially the heater blower. But once past 1200 rpm's it smooths out and acts perfectly.

[84BURBMIKE]

just ordered the autometer. more $ than some but the ease of installation was the selling point.

thanks