1990 4Runner is too slow. Replace diff gears?

General Questions
#1

I just bought a 1990 Toyota 4Runner with the 3.0 v6 engine. It has the auto-transmission, on-demand 4-wheel drive, IFS & spring rear.

The previous owner traded out the exhaust manifold & installed an overhead-cam kit. The engine has been pretty much completely rebuilt.

It’s got just buckets of torque. No complaints there.

I got it out on the highway this weekend and it pretty much tops out at 65 mpg. It even downshifts in the face of rises or strong headwinds.

This truck was bought as a daily commuter first, a vacation/camping vehicle next, and lastly as an occasional off-road vehicle (maybe an occasional trip over the mountains on old forestry roads).

As a commuter and vacation car, that 65 mph limit is a bit low. A friend with a similar vintage toyota pickup says that’s about it for his, too.

I’m looking for ways to add a little more top-end to the machine without losing too much off the bottom. This isn’t a rock-crawler, though, and I still have the 4L setting if needed. I know that nothing comes for free and if I add speed, I lose torque.

Is trading out the diff gears for slighly “faster” ones a good idea? If so, do any of you know a kit for this?

#2

GF’s pickup is same vintage with same engine. 5sp though.

It wasn’t always as lame as it is now.

She thinks it’s just getting old.

I think the cat is partially plugged.
If you are thinking of going to taller gearing to increase top speed It will not, unless you are rev limited (which it doesn’t sound like). It sounds like you are power limited.

In which case lower gearing will increase your top speed.

#3

Might be cheaper to buy taller tires.

#4

Looks like you bought the wrong vehical, but if you are really getting 65mpg at least you aren’t using that much money for gas.

Since this is really a commuter vehical why not start at the simple things, the tires, do you have heavy duty off road wide M&S tires, if so switching to a conventional tire can gain you quite a bit.

#5

I used to have an 89 Pathfinder 3.0L 5 speed. Same issues as your Toyota.

I was able to squeeze a little more top end power out of the engine by replacing all of the old vacuum lines - quite a number of them were brittle and / or cracked.

My catalytic converter was plugged - replacing it with a resonator did not result in any appreciable increase in power.

Removal of the air induction system may increase your horsepower.

Before starting to modify your emissions equipment, make sure you’re not violating local law.

I don’t imagine that re-gearing the transmission would be particularly easy or affordable, though I may be wrong about that. I look forward to hearing if that’s a viable option.

#6

Just saw you asked about re-gearing the diff rather than the tranny.

WAG - probably not the improvement you’re looking for.

#7

The thing is breathing through a HNK(?) air filter. Looks like an over-sized black coffee filter. Supposed to be good for power. Colorado is venomous about pollution laws so the catalytic convert probably has to stay.

Larger tires might make a diff, they’re currently BFG All-terrain T/A tires. Kinda large, knobby, & sexy (like me :D) but also brand new. Unless there’s a huge need, I’m not going to dump $600 worth of tires until they’re worn out.

I was hoping that pulling the cover off a pumpkin & replacing a gear might be a quick way out of this. Maybe not.

#8

This isn’t clear. Do you mean he replaced the stock manifolds (there are two) with performance manifolds, or with headers? And I don’t know what an “overhead-cam kit” is - the stock engine had overhead cams. Do you mean he replaced the stock cams with performance cams?

Don’t mean to pick on you, but “pretty much completely” sounds rather vague to me. What exactly was done?

That doesn’t sound at all as it should be. It suggests a significant lack of power.

I have doubts that it will solve the problem. It might alleviate the symptom somewhat, but that’s not a sure bet. It ain’t cheap to do this. I would have the engine performance evaluated by a competent shop before moving in this direction.

I have fears that the previous owner may have done some “hot rod” modifications to make it quicker off the line and/or pull harder, at the expense of normal top end power.

#9

This suggests that you aren’t familiar with what’s involved in a ring and pinion gear replacement. Both the ring gear and the pinion gear must be replaced as a set, and it’s a pretty big job, with a pretty big price tag. Setting those gears up is a specialty, and needs to be done by someone who KNOWS what he’s doing.

A simpler and usually less expensive alternative is to replace the rear axle/differential assembly with a used one. However, Toyota may or may not have offered a choice of ratios, and if they did, your speedometer/odometer calibration will be off.

Nevertheless, academic at this point, as I really think the first step should be to evaluate the engine performance.

#10

You’re suffering through my lack of car knowledge here. The pipes that lead the exhaust from the engine, both sides, have been replaced with ones that are supposed to add power.

I thought it was a cam conversion but maybe I misunderstood. They must be “performance” set, then. He explained that the rattle under the hood was these cams and that it was alright that they were a bit noisy. “Just turn up the radio” he said.

Quick answer: I dunno. I know it was rings & valves. Cylinders over-bored a bit but just enought to smooth them out (few thousandths). New seals.

If anything, it was to increase response in the low end. Guy was first building an off-road/rock crawler but stopped after he bought one already fixed to his liking. He was then continuing to build for his wife’s use but stopped when she wouldn’t give up her sports car. Now I bought it because I needed a vehicle badly & the price was right.

#11

I meant to ask what the engine RPM’s are at 65 mph. That could be helpful info.

I don’t like that there’s noise associated with the cams. I’m guessing the valve clearance setting was increased. While this won’t cause a burnt valve (which would be mucho expensive), it could cause premature wear on the cams. And, I can’t help but wonder if it relates to the lack of top end performance.

I know just enough about high-performance engine modifications to be dangerous. :slight_smile: What I do know, though, is that stock set-ups are carefully chosen compromises to give reasonable power and economy through a range of conditions, and that often performance modifications augment one aspect (e.g. low end torque) at the expense of another (e.g. high end power). It might be necessary to go back to stock valve clearance and/or stock cams to regain your top end. Someone more knowledgeable than I in high-po work could probably shed more light here.

#12

At the vehicle’s “top speed”, what gear is it in?
What is the RPM at that speed?
I’m going to say you’re WELL below the horsepower this thing came with.
That truck is underpowered by today’s standards, but 65 is ridiculous.
I made a crude facsimile of your car in CARTEST 4.5 and came up with a top speed around 106, a zero to sixty time between 12 and 13 seconds and a quarter mile in 19 seconds with a trap speed of of 75 MPH.
Your larger problem is why your output is so low.
Your gear ratios are NOT the bottleneck, at least not yet.

#13

It depends on why he can’t exceed 65 mph, if it were because the truck redlines at 65mph then a taller tire would help, but if it’s due to lack of power then it seems like a smaller tire would help, which seems to be the case as the truck downshifts

#14

Running about 2300 RPM at 65 MPH. It can downshift twice if I push it hard at that speed - Not running at red-line at all - I can more than double the RPM count by allowing the downshift.

I assume it’s running in overdrive at 65 MPH normally.

Seller said his pro-auto-mechanic brother said that wear wasn’t an issue with noisy overhead cam. Not sure why that is.

#15

Okay. It’s in 4th gear at 65 at 2300 RPM. That’s about what my simulation shows, so my numbers aren’t too far off.
I’m showing that at 65 MPH you’ll need about 70 HP to maintain speed. My model of your car is a little imprecise, but it’s a ballpark.
A very generic horsepower curve for your car shows it making around 70 HP at 2300 RPM. I wouldn’t be surprised if either calculation was off by 10 HP, but I can see that your car really wouldn’t have ANY leftover power in 4th gear driving at 65-70 MPH.
If you’re missing some of your vehicle’s top end, I could imagine that the car has lost its ability to cruise at highway speeds in 4th gear.
Is this the slowest vehicle you’ve ever had?
If you let the car run in third gear, it should go faster than 65. Does it STILL refuse to accelerate if it downshifts into third?
If you have the tranny I think you do, you’d be turning 3000 RPM at 65.
At that point, in stock tune, the 4Runner should have 100 HP at its disposal, good for just under 90 MPH.
I don’t suggest going 90 MPH, but it’d be good to know if your vehicle can’t accelerate in third at 65 MPH.
Finally, as a datapoint, the truck makes 150 HP at 4800 RPM. Shifting into second you at 65 you should have access to 99% of that 150 HP, assuming of course that you HAVE the original rated horsepower. Of course, you’re perilously close to redline in 2nd at 65, so there’s not much accelerating left to do there…

#16

Beofre we go too crazy here, go take a look at the tires and write down their size. Then find the tire inflation tag on the truck and write down the size(s) listed. Report back.
I thinking that the previous owner went to tires that were 14 sizes too large to rock crawling, found out what the correct gear sets would cost to install then decided to sell the truck.
If the tires are the correct size then I’m with Gary T you need to have the performance of the vehcile checked out, something is wrong.

#17

Hit submit too soon. If the tires are too large the speedo is already off 65 might be 75 or 80 or whatever.
If in doubt $10 at a speedo shop will get you a calabration slip (65 indicated is really XMPH)

#18

First - if you are thinking of changing final drive ratio - keep in mind that you’ll need to do both front and rear differentials.

Second - the larger tires it sounds like you have will hurt your highway performance in at least a couple of ways. First, they’ll cause your speedo to read low so an indicated 65 will actually be faster, increasing your rolling and wind resistance. Second, they’ll reduce your effective gear ratio (make it numerically lower), meaning that there will be less driving force from the engine making it to the ground at any given speed. Third, they will have higher rolling resistance at any velocity than a comparable smaller truck tire that is not so biased towards off-road use.

Third - my brother-in-law has a Toyota 4WD truck with the 4cyl and a 5spd, and my brother has a Toyota 4WD truck with the V6 and a 5spd, so I know from personal experience that they should pull just fine at any reasonable kind of highway speed (certainly to 90 and beyond), although you might need a down-shift to accelerate briskly or climb hills.

Not to raise any flags, but your description of how the previous owner described the vehicle (and what his brother? did) doesn’t give me a warm fuzzy feeling. If it’s in your budget, before you go considering really expensive modifications that may well be ill-conceived, might I suggest that you take it for a pre-purchase-type inspection at a reputable mechanic or Toyota dealer. Hopefully anything that the owner did can be simply undone. Once these trucks are running right, they really are satisfying to own, and very reliable. Best of luck, and please report back in with the final diagnosis.

#19

I’m not a vehicle maven, but I am astounded you guys are talking about gear changeups on a 16 year old incipient beater truck intended for commuting. Aren’t mian gear replacement mods like that at least a thousand dollars plus?

#20

Astro,

If he uses junkyard parts and does it himself, he might get away for under $1K…
But yeah, the car books out between 1000 and 4000 dollars, dealer retail.
Then again, if he takes care of it and gets whatever is wrong fixed, he may receive years of service…