I have an old 1987 corolla fx-16 hatchbutt and a new yaris hatchbutt, and have always tried to drive gently. The corolla averages 40MPG on rural mountain roads. (maybe - see below) The corolla briefly had to settle for a ricer "turbo" muffler in a pinch once, and got about 2-3MPG better until the POS rusted out. I'm somewhat conflicted about the loss of MPG with my new wonderfully quiet muffler. The yaris is still on its first tank of gas, so we'll see how it does later on....
WRT to my MPG: I know my FX-16's speedometer reads 10% fast compared to the radar speed feedback signs on various streets. I don't trust either of them. I really need to take both cars out on the longest straight bit of road I can find and check the odometers against my GPS, and find out how far off my mileage is. I hope I don't fall below the magical 40MPG!
I started poking around here because I need some new tires for the FX-16, and I'm interested in finding some efficient and long lasting tires that are also good in extreme wet - yeah, a compromise.
Recently the fx-16 has had difficulty starting smoothly from a dead stop in first gear when the engine's cold - it jerk-jerk-jerks if I don't work the clutch and throttle just right. I'm not sure if that's an air/fuel mixture issue, or just old-age lack of torque. Forgot to ask about it last time it was in the shop. After that started happening I acquired the yaris to be ready for the corolla's eventual retirement. Very sad...the fx-16 has been wonderful to me over the years.
Speedometer error may not translate into odometer error. It's nothing to worry about unless the GPS indicates you also have an odometer error.
Finding low rolling resistance tires is really a crapshoot. I've seen testing that essentially shows no relationship between LRR specs and testing results. The best seem to be the OEM tires that come on the Prius, if they'll fit your car.
Apparently your engine scavenging is restricted by your muffler. 2-3 mpg is a lot to gain or lose - more than you'll gain from LRR tires.
Whenever the weather is damp my clutch grabs like crazy and jerks something wicked (like hammering the motor mounts bad) until it dries out from some 2nd gear slippage so you may want to see if it is the clutch causing the jerking.
From the behavior it seems like it's just weakness of the old engine or maybe poor air/fuel mix? If i do the clutch and throttle JUST right it starts out fine, or if I just use Brute Force and just give it lots of RPM and slowly let the clutch out. (bad for clutch in the long run) Once it starts oscillating I have to push in the clutch for a second rev the engine and start over again with the clutch. If I'm already in motion and catch it starting early enough I can occasionally power the start of the oscillation away by applying more throttle. It's a common old car behavior. Someone called in the exact same problem to Car Talk once, and they speculated lack of torque but surprisingly it didn't sound like they were familiar with this problem.