I have 4.10's and at about 75mph I pull around 3000 RPM's. It's a pretty major difference than stock. But I started out with 3.73's, and with an auto was kinda disappointed with the improvement. I changed to 4.10's about a year later. Honestly it didn't make a huge difference in my gas mileage, but when I added my stall I can watch my gauge move.
Completely stock I got around 23mpg on the highway at 70mph cruising. With my 4.10's it dropped to around 20mpg. That's not a bad trade off for the performance increase. But I'm pretty tolerant. My car never goes on trips anymore though. I put about 6k miles a year on my car now. I still drive it every day, just not to economical for road trips.
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Tyson
1994 GT Automatic
Edelbrock Performer RPM intake, Motorsport "E" Cam, GT-40 Iron heads(ported), BBK shorty headers, Hollowed Cats, Flowmaster Mufflers, Built AOD-E, 2500 Stall,
Trans-Go Shift Kit, 4.10 gears, MSD 6AL, Accell Coil, MSD Super Conductor plug wires, Smog pump eliminator (all emissions removed), K&N, 24# injectors, EEC-Tuner, 75mm MAF, 70mm Throttle Body, and Oh Yeah 150hp NOS
Yet another question about gears... 
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