Tires for my Saturn SL

[techanic]

Well I followed through on my threat and traded my Celica for a 2000 Saturn SL. It has a manual trans and early indications are that, around town, it does a wee bit better on gas than the Celica, don't know about the highway yet. One thing in its favor is it is easy to coast with the manual, the Celica's auto made it tough.

I've already installed an Ultra-Gauge and really like it. I've never used a ScanGauge but I'd have a hard time believing it is as good as this.

Now to the point, the car could use some tires. At first, I was keen to install some larger-diameter tires to "gear it up" for highway mileage as I will soon have a 120 mile highway commute on workdays.

But a lot of the stuff I've read makes me wonder if that is a good plan. Many even claim it would lower my mileage, a claim I find dubious at best, although the SL model is geared pretty tall to begin with for sure.

I'd sure like to hear from anyone that has actually done this, particularly on a Saturn (SL or SC series) or maybe a similar vehicle. I was thinking about increasing the diameter in the 4-7% range.

Thanks in advance, especially from anyone who has ACTUALLY DONE IT