Just bought a new (to me) truck and passed my old one down to my son. It's a '93 4X4, 4.9 with the Mazda 5 speed tranny. It came with 33" tires on it, and that's what it will wear as long as I have it. The gearing in the axles is 3.08, and with the 33's the motor is really working hard to keep highway speeds. Also starting requires a little massaging of the clutch.

This truck will be my go everywhere vehicle. I will be driving it on the highway to work, then creeping around the mountains on fire-breaks and pig trails. I need someone with a little real world experience to offer opinions on which gear ratio would work best and help keep the motor in it's sweet spot.

My old truck had 3.55s in it, everything else was the same except for a 5.0 instead of a 4.9, and was a perfect balance for highway and off road. But I realize these two motors make their optimum power at different ranges. I've searched the net for most of the day, and have seen people recommending everything from 2.73s to 4.56s, it really is an information overload! I know that the 3.08s are too high and they have to go. I'm leaning towards 3.55s or 3.73s, I think that 4.10s will be too low for it.

Someone please help me out!

 

welcome to FTE.
you are on the rite path with your choices. personally i would go with 4.10 gears for the pulling power, but if it is a daily driver i would go with the 3.73 gears.

 

with those tires 3.73s will feel like 3.55s and the 4.10s will feel more like 3.73s. Either of those will be a vast improvement of the 3.08s.

 

I'm not sure what the gear ratios are in the M5OD, but assuming OD is about 0.8:1, driving in 4th with your 3.08 gears will be like driving in OD with 3.85 gears (3.08/0.8=3.85). So try leaving it in 4th. If that isn't low enough for you, then you'll want 4.10s. If it seems about right you want 3.73. And if it's too low you want 3.55s.

This won't give you a feel for accelerating a load, but it will tell you what you need to know about cruising speed.

By the way, this can also be a way to save money, both on gear swaps and on gas. If your truck generally drives well but just can't pull overdrive with a load, then use 4th when you have a load. I did that with an F-150 with a 351, E4OD and 3.55s. When I went to 33" tires I thought about changing the gears, but I just tried it for a while first. I ended up deciding that I had plenty of low end for what I needed, and it pulled OD fine empty above 55 mph. So if I was cruising at 55 on winding roads, or pulling a load I'd just take it out of OD. 3.08s may well be too low for you in normal use, only you can determine that. But don't be afraid to drive in 4th when you need it either.

 

Thanks guys! I have no idea what the ratios are for each gear in the transmission. On the highway in OD it moves along decent, until there is any hill or rise. Running it in 4th it does just fine, doesn't sound over-wound. My main concern is in the lower gears like 1st and 2nd. I have to really work the clutch to get it started, and then it doesn't like to move at slow speeds, it wants to start lurching.

Towing is not much of an issue, it has all the pulling power I want but I just need it geared so that the engine isn't under a strain constantly.

 

Nothing Special, tell me if I'm following this right. The rear end is geared 3.08, fourth gear is 1:1, and OD is .80:1. Driving in fourth, which is direct drive, to determine what gear ratio would put me in that range I just divide the current gear ratio by the ratio of the final drive? Then adjust a little up or down to suit my preference?

 

Yep. You would divide your current 3.08:1 ratio by whatever the fifth gear ratio is to get the equivalent rpm/feel of the lower axle gears in fifth gear. Assuming your fifth gear ratio is 0.80:1, driving around in fourth gear with 3.08:1 gears would give you the same engine rpm as driving in fifth gear with 3.85:1 gears, just as Nothing Special described above. If that rpm seems a little high, 3.73's would be a good choice. If you would like that rpm a little higher, 4.10's would be the route you would take.

 

Yes, that's correct.

To work through it so maybe it makes more sense:

To find the overall gear ratio you multiply all of the gear ratios together:
trans ratio x t.case ratio x axle ratio = overall ratio

Since the t.case in high range is 1:1 you can drop that out and it's just
trans ratio x axle ratio = overall ratio
(but don't forget to include the t.case if you're trying to figure out crawl ratios in low range)

So if your trans is in 4th gear (1:1) and you have 3.08:1 axles your overall gear ratio is
1 x 3.08 = 3.08

Now if you had a (hypothetical) 3.85:1 axle running in OD, your trans ratio is 0.80:1, so your overall drive ratio is
0.80 x 3.85 = 3.08

That's the same as driving in 4th with your current axle.

But it's the hypothetical rear axle ratio that you're trying to figure out. To figure that out you use algebra to rearrange the equation to find the axle ratio:
overall ratio / trans ratio = axle ratio

You know the overall ratio (it's the overall ratio in 4th gear with your current axle = 3.08). The trans (in OD now) is 0.80:1. So the axle ratio that gives the same overall ratio in 5th geear as your current axle does ion 4th is:
3.08 / 0.8 = 3.85

 

Got it now, just took me a minute to wrap my head around it!

 

PowerBlock Resource List - Engine RPM Calculator | Engine Blueprint | Transmission Ratios

play around with the numbers and what you want your speed-rpm ratios to be. You should be familiar with where your truck has enough power and what you want interstate rpm to be.

 

Very interesting calculator. How many cylinders is it calibrated for?

 

The number of cylinders doesn't play into the equation here.

 

Its amazing what a little tinkering will do. I've been driving the truck in 4th gear all week because it was a dog in 5th. I had a day off yesterday and gave it a little tune up, just the basics like plugs, wires, distributor cap and rotor. Lastly changed the air filter, it was filthy, and rechecked the timing. The first time I checked timing, it was about 12 degrees off! The new filter and ignition parts really helped out. It drove out fairly good on the highway in 5th this morning. Still need a little bit lower gears, but its not as bad as it first appeared.

 

I had the same combo in a 95 and found it actually wasn't as bad as it should be with those gears. The 4.9 has a huge amount of low end torque so it can walk through the gears easily enough when it's running right.

BUT something to consider for ease and probably cheaper. If your doing OK now but just want a little more gear, it was pretty common to have 3.55 gears in some of the small block V8 trucks, find one of those and just swap rear axle and front drop out. No cost of setup just buy the axles and swap them in. If you get same or close to same year and model of truck it's a direct bolt in.

That 4.9 will pull a set of 33s and 3.55 gears all day long.

 

Well, looks like I've got a busy weekend coming up.

New ball joints were put in Tuesday. Have new radius arm bushings and Moog 844 coil springs for the front end. Found a limited slip diff with 3.73 gears for the rear end. On Tuesday new Maxxis Bighorns will be here, and a complete alignment will be done. The following week I plan to drop the front chunk and re-ring it.

Amongst all of this I'm teaching my son how to wrench on my '88 that I passed to to him when I got this one. Pictures will be in order soon.