Its the radiator. I am not endorsing this as an accurate indicator but it will at least let you know if it's in the right zone temperature wise. Installing a gauge with increments isn't really necessary. If it's gonna bake, you're going to find out when it's doing it and not before. I kind of beat this dog on a regular basis but the factory clutch fan isn't great in any vehicle and it's worth installing electric thermofans (even more so if you have A/C) The benefits will pay for themselves. Better insurance policy than an aftermarket temperature gauge IMO.