Not safe or legal....

[DifferentPointofView]

Changing the tire width had an impact I agree but reducing the tire diameter meant I needed less fuel to reach an rpm

the smaller tires themselves spinning while in gear will cause the engine to spin a higher RPM, even when your not using gas.

Also, have you corrected the speedo and ODOMETER? smaller tires will equal more spins and that means more miles per tank. so if your ODO says 500 it will be false if you have smaller tires, as you may have only traveled 390. Only after you have changed your odometer will your mpg be correct.

[theunchosen]

The speedometer had been recalibrated(I have since fixed) the odometer had also been adjusted( I have since fixed). I had to replace the whole cluster shortly before switching the tires and calibrated it for 14 inch tires. The numbers for a week were very low and I did in fact get pulled over for speeding(I was going five over, but speedo said dead on). Fortunately the officer let me off and sighted me for a dead headlight while she was requestig I not drive so fast. I happened to have been going to autozone to grab headlights and had them in the car so she let me off on a citation as well.

Yes it does take more revolutions, but the losses are much much smaller. It's kind of like reducing engine size. The smaller engine has to have a higher number of turns per tire rotations, to get the job done.

Of course to a point. Too small of an engine and you lose efficiency, too small of a tire and you have to complete far too many revolutions. If it were true that shrinking the tire always caused loss of mpg we would all have 18 inch tires. All econo cars have very small diameter tires. Also its not true that smaller is always better, I am pretty sure 12-14 is the optimum range.

Also found some logs of someone who increased tire diameter only. Increasing overall tire diameter 4 more MPG
The thread talks about enlarging tire size to dupe the prius into allowing them to do 45 instead of 41 inEV, His logs since the tire change have been as erratic as they were before but most of his short trips have greater mpg than the long hauls. Could be attributed to his prius EV mode though.

I've got my old tires. I might throw them on for a few days just to get the data.

[CapriRacer]

Originally Posted by JQmile

Not that this would be safe or legal, but has anyone tried running four space saver spares? I'd think a 155-series tire or whatever they are combined with 60+psi ought to show a significant increase in mileage.

Space saver spares are designed to be used for 50 miles at 50 mph. That short of distance means that the tire designer did not have to be concerned about fatigue resistance - and in fact, the tires' casings are designed with 1/3 the durability.

DO NOT - under any circumstances - use space saver spares except for what they were intended for - TEMPORARY use.

[DifferentPointofView]

^
I wouldn't trust a temp tire anyway seeing as they clearly state what you said on the side. If they can only go 50 and only last 50 miles, they definitely aren't safe for daily use.

[Unforgiven]

Originally Posted by JQmile

Not that this would be safe or legal, but has anyone tried running four space saver spares? I'd think a 155-series tire or whatever they are combined with 60+psi ought to show a significant increase in mileage.

Never tried that myself as it is illegal to run like that. Found out the hard way by getting the ticket when I snagged an old cavalier and drove it home from the junkyard with only donut spares on it. Got 2 miles (out of the 40+) away from home and Mr Police Officer kindly pulls me over, tickets me, AND calls a tow truck to take the vehicle away, which added another 130 bucks to the cost of the car.