I heated some water up to 170f, gauge was reading close to 212f hard to really tell on these gauges probably closer to 200f. Then I replaced thermostat with a 165f tstat filled with water, ran truck around the block a couple times to get temp up to what it was saying was 200f stuck thermometer in water inside rad. Was reading around 170f took temps around tstat housing and temp probe all pretty close to 175f. Ran truck for about 30min drove it around the block a few times took temp reading inside rad 176f. So I bent the needle a little so it is reading closer to real temp. Ran for another 15min stayed right around 175f. One thing I did notice when driving around 50mph would creep up to around 200 but once stopped at stop signs would drop right back to 180 or so. Would running cheap unleaded gas cause higher temps.
Is there anything else I should be checking I feel that the gauge was just off by around 40f. What kind of water should I put into the radiator.