A lot of import cars used to have a "sport/eco" button associated with the tranny. These are rare now but this used to change the shift points. I suppose your vehicle has combined the OD button to this function.
Chryler and Hyundai I know of as fact employ fuzzy logic, which makes the button redundant. The fuzzy logic adapts to your driving style, if you are agressive the computer adapts the shift points to your driving style after a few minutes.
Sterling is correct that the OD button simply does not allow the vehicle to enter top (4 or 5) gear. It does not change ratios, but may put the tranny into a sport mode. Certainly not a factor on a 91 Ford.
Most imports today, and GM products have full selectability on the shift lever. GM's read in the indicator P R N OD (D WITH A CIRCLE) 3 2 1
Fords of this vintage would have read P R N D 2 1, "D" being either 3 or 4 depending on the OD button selection. Dodge read P R N D 2 L. Now for Hyundai product, the Accent's 4 speed auto reads P R N D 2 1, with an OD button, and the Elantra has P R N D 3 2 1, and no button.
Barrie mentioned the early 4 speed autos at GM had problems. My brother had an S-15 with this transmission, and the problem. This was very acute and noticeable where at normal in-town speed 50-60 km the tranny would very noticeably hunt 3-4-3-4-3-4-3 constantly. I'd say pay attention to your vehicle. If at speeds you normally drive you find your tranny hunting, shut the OD off. If it doesn't leave it on, you'll save fuel. Common sense should prevail eh?
