There is a huge difference between the 2 sizes you are comparing. The 215\75s are much smaller. Three inches less overall diameter and 1.5" shorter sidewall for example. They will look tiny compared to the 235\85s. See below.
235\85r16
Section Width: 9.25 in
Rim Diameter: 16 in
Rim Width Range: 6 - 7 in
Overall Diameter: 31.73 in
Sidewall Height: 7.86 in
Radius: 15.86 in
Circumference: 99.68 in
Revs per Mile: 655.57/mi
Actual Speed: 62.14 mph 100.00 kph
215\75r16
Section Width: 8.46 in
Rim Diameter: 16 in
Rim Width Range: 5.5 - 7 in
Overall Diameter: 28.70 in
Sidewall Height: 6.35 in
Radius: 14.35 in
Circumference: 90.15 in
Revs per Mile: 724.82/mi
Actual Speed: 56.20 mph 90.45 kph
Speed Variance: 10.56% too slow.
Diameter Variance: -9.55%
If you want to go to a narrower tire and keep the same overall diameter you need to go to a higher profile ratio. You won't find, nor want, much higher than an 85 that you have. You'd have to go to a bigger rim.
