Howdy Chaz:
Yes, a slipping clutch could have a negative effect on MPG. Think of it this way. If you're cruising along, with a tight clutch, at 50 mph and getting say, 20 mpg, now your clutch starts to slip and say your rpm goes up 10% because of the slippage. Your mileage just went to 18 mpg.
In reality, that's the last situation you'd notice clutch slippage. Most likely it will be noticeable accelerating from a stop, or a sudden application of power in 2nd or high gear.
How much of a lose can best be figured by you. Compare your fuel mileage data from before to what you're getting now.
Adios, David
