Yes, I agree 3.55 would be the lowest I would go, so OD can stay engaged on slight grades
The 235/75R15(29") are probably only 1" taller than stock tires(28") but you would need to look at the door sticker to see stock tire size it came with.
That 1" taller tire would put 3.55 back close to 3.45(3.43) so speedo may be very close to acurate.
If tire difference is 2" taller then with 3.55 your actual would be 3.31
If that is the case the 3.73 might be a better choice, that would put actual at 3.47
1990 still used speedometer cable so you can regear the cable at the trans end to fine tune.
The 3.45 actual ratio puts engine RPMs at 2,100 @ 65mph in OD(.75 ratio), max torque on 2.9l is at 2,600 so 2,100 is a good place to be, you are within easy range of max torque for any grades in highway, and for 70-80mph as well, lol.
3.08 would put RPMs down at 1850, better for MPG but it would have to be very flat ground or trans would need to shift to 3rd all the time which negates any MPG gain.
Last edited by RonD; 07-12-2015 at 11:54 AM.