I’m not a tire expert but IMHO running a single “80-E” tire with three “85-G” tires, for long term use, would cause a serious alignment issue. The “80” tires are not as tall as the “85” tires and the “E” tire will be softer (and lower) then the “G” tire when loaded. Both of which will cause the axle height of the “80-E” tire to run lower thereby causing the alignment issue. You might get by if you had a pair of “80-E” tires on one axle and a pair of “85-G” tires on the other axle.
To be safe, I recommend you replace all five tires with the same size. However, if you want to keep the “80-E” tire for a spare – remember that the “space saver” spare tire in most cars is smaller than the mounted standard tire and the auto engineers determined that you should be able to get by with it safely for a very short distance in good weather and good driving conditions. But, can you be sure that you will need and use that spare “80-E” tire in good conditions only?
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