Speedo/Odometer change is simple (and I'll include gear changes):
Old Tire Diameter: OTD
New Tire Diameter: NTD
Old Gear Ratio: OGR
New Gear Ratio: NGR
NTD/OTD * OGR/NGR = percentage difference from stock speedo/odo.
In the case of my previous vehicle, a Tahoe, I went from 31" tires to 35" and changed the gears from 3.73 to 4.11.
This yielded 35/31 * 3.73/4.11 = 1.0246 or a 2.46% change in speedo/odo.
More importantly, the gear change brought the highway traveling speed back into the "sweet spot" of around 2100 rpms and put my MPG back where it was (close anyway) to pre-tire change.
In your case 37/35 is simply a change of 5.7% This is not, in my opinion enough to justify a gear change. I would look into headers and dual exhaust to recoup that small loss of MPG.
Personnally I think I will stick with the 35s on my H2. I do a lot of driving offroad, but not anything that 37s would make a difference in.
Now a set of 40s and a 6 inch lift...hmmm....
