How do I know if I have upgraded gears?

[tahoeman960208]

Last week I was on the highway and I noticed that when I got over 70 mph, the speed on my GPS was lower than the speed on the speedometer. At 70 there was about a 1mph difference, but at 85 there was a 4 mph difference. It is a 96 4 door, 5.7, 4l60e, 4wd. The previous owner of the car used it primarily for towing and there are 2 screwholes under the dash where a brake controller probably sat. Is there a way I can find out if he put in higher ratio gears by looking at the rear axle? It feels like I get off the line pretty quickly, but I don't really have anything to compare it too. Thanks.

 

[drumrpunk]

With only a minimal difference, I doubt you have different gearing. BUT your GPS is not as accurate as your spedo. My GPS is sometimes 5 mph off in my 07 stock uplander. When I switched rear end gears on my mustang you could totally tell it was way off. Sometimes your intuition may be more accurate and you may have different gears. But try not to rely on a GPS for calibration. Have someone drive next to you in a stock car and see what they get. Also you never mentioned what size wheels you have on it (stock or not). That can throw everything off too!