Performance after re-gear

I am curious what kind of performance you'd see if you re-geared for larger tires, like 33's, and then put 31's back on. Has anyone done this that can share their experience?

I am wondering if my 98 Tacoma was re-geared at some point prior to my owning it, as I get pretty bad mpg with just 31's (14 city, maybe 17 highway if i'm lucky).
The gear ratio from the door sill says I had/have 3.91 stock, but I just have a feeling that might not be what's in the diffs now.

 

Just confirm your gearing. Jack up the truck, spin your tires and count how many times your driveshaft spins.

 

^ this

You'll also notice high RPM at highway speeds

 

I regeared my truck to 4.88 from the stock 4:30, but ended up with 31 inch tires.

Lots more low end torque, a bit less power at highway speeds, but I'm getting 24 mpg on longer drives now.

 

This sounds like what I'm experiencing, minus the good mpg at highway speeds. Lots of torque at low speeds, but highways feel like I'm way overweight, and can't maintain speed going up even slight grades. I've had to downshift to 3rd in some cases just to be able to increase speed.

 

Neither of these descriptions makes sense. A regear makes more torque available at the wheels at all speeds, at the expense of higher engine speed. Easiest may be to plug your speed and tire size into a regearing calculator and figure out which diff ratio you have.

 

Have you ever gone down in tire size after a re-gear?

 

I would not describe 14-17 as 'pretty bad'. Many on here would be happy with that.

What trans do you have Auto or manual?

What RPM are you at going down the highway at 70 mph?

 

Jeez I would not want 3.91 with 33s without a power adder. I had 4.10 with 31s and that's my baseline - still bad mileage, still have to downshift going up consistent grades. Now I have 35s with 4.10 but plenty of power for other reasons. The lower the gear ratio, the better highway mileage you should get in theory, but they are diminishing returns if you have to hold the throttle too high to maintain speed. Ideal RPM is around 2300 for min BSFC (and ~80% load). (Lower gearing = greater load. Conversely, higher gear # = greater load)

 

It's a manual transmission. Highway in 5th gear at 70mph I'm at about 2,600rmp.

 

sounds stock. is that GPS speed, or speedometer speed?

 

It's the speedometer speed

 

Are you adding the tire factor to your mpg calcs?

 

With 31s that RPM is about stock. You might find that the actual speed is as much as 5 mph lower if you check it with an app that has GPS.

What is the actual code on the door sill for the diffs?

 

At 70 mph in 5th gear (assuming a R150F with .838 ratio) and 31s...
3.91 ratio would be 2486 rpm
4.1 would be 2607 rpm
4.3 would be 2734 rpm

This of course assumes the speed and tire size are accurate.

 

I'll be jacking up the rear end and checking the gearing using rotations today, but it's already sounding like I've got my stock 3.9.

I'm only wondering because someone hammered in the pinch welds on this truck long ago, and I've always felt very under powered on the highway, but maybe that's just normal.

I have no desire for 33's for any reason, but I was considering 32's with the stock gearing.

I'm on 31 10.5 r15 with 15x8 wheels, and I belive 15x7 wheels were stock on this specific truck. So I'm not too far off from stock

 

It's B06A, which if I read correctly, should be 3.91. But please let me know if that's not right!

 

Interesting. The difference between 3.91 and 4.10 is not very much.

 

Door jamb sticker will tell you. Also wheels mean nothing - it's the outer circumference of the tire that matters