Replace 15 Rims With 16 Rims

Will 6 lug rims from a 99 tahoe fit my 89 sububan? How much of an increase in mpg could I expect?

Some of the newer Chevy trucks have METRIC studs and lug nuts which are fatter than the corresponding earlier 1/2" equipment you probably have. Test on a '99 stud with one of your lug nuts. If the holes in the new wheels are larger, they could "saw" your studs off over time.

Wheel size alone does not affect gas mileage. It's the overall diameter of the wheel and tire combination. Your stock set-up is probably 29" tall. If you go to 32's, you might see six or seven percent improvement on the highway. And you won't be able to measure it accurately without re-calibrating your speedo/odometer.

I think you are in luck. I believe your '89 is the last year with a mechanical speedometer gear, which is easily and inexpensively changed. My '90 is the first year of the ELECTRONIC speedo, where you have to invest hundreds for a new card to change the calibration.

Your 89 suburban must be a 4-wheel drive if you have 6 lugs. No-the center holes in the 99 Tahoe rims will not fit over your front axle hubs-I know-I already tried it.