Reeee-geeeear. I know it's not cheap but will make a world of a difference.
This is from LCE, a major supplier of aftermarket Toyota parts (it's all they do):
"Once you've determined your gear ratio, the next step is to determine which gear ratio is right for your application. This is a
highly subjective subject, but We base our conclusions on the factory ratios. The following chart shows what gear ratio / tire
combination will maintain stock or better performance, in my opinion. The following applies specifically to 4x4's, with either a 5
speed manual or overdrive automatic made through 1995. While We believe it would be applicable to 1996 and later Tacoma's and
4Runners, these models did receive higher gearing from the factory, so using this chart would make the rpms significantly higher
than what you may be used too.
As always, if you are unsure which ratio to install in your truck, be sure to seek a second opinion.
What you see recommended below, may not be to your liking depending on your driving style.
TIRES SELECTION A SELECTION B
225/70R-15 Tires = 4.10 or 4.30 gears
31 x 10.5-15 Tires = 4.56 or 4.88 gears
32 x 11.5-15 Tires = 4.56 or 4.88 gears
33 x 9.5-15 Tires = 4.88 or 5.29 gears
33 x 12.5-15 Tires = 4.88 or 5.29 gears
35 x 12.5-15 Tires = 5.29 gears
For factory stock performance, or near factory stock performance with larger tires, selection A should be used with 5
speed manual transmissions, selection B should be used with automatic transmissions. For better off road
performance and better highway acceleration, selection B should be used, but you will experience higher rpms on the
highway than you may be used too. How these ratios affect fuel economy depends entirely on how you drive and the
weight you carry. In some cases a lower ratio, combined with reasonable driving, will return stock gas mileage, on a
truck with larger tires.
When regearing with an automatic transmission on pre-1995 trucks that have larger tires, We recommend going with the next lower gear ratio to maintain factory performance. I.E. if you have 31" tires, you should use 4.88 gears, if you have 33" tires, you should use 5.29 gears. The lower gearing is recommended to offset the extremely high 1st gear in the automatic and the high overdrive. Toyota installed 4.30s in stock automatics, while similar 5 speed models got 4.10 gears. This is not as critical on Tacomas since they have more powerful engines."
LC ENGINEERING Differential Tech