New tires and mileage dropped.
 

After putting 4 new Good Year tires on my wife's Prius her mileage dropped 4 Mpg.The size is the same as the OEM tires. Is it possible that the diameter is larger?

Possible, check the indicated speed against a gps.

Is the tire pressure correct?

Over how many miles is this calculated?




There are several factors that could make the mileage drop a few points.




I would check the tire pressures, and also find out if they are low rolling resistance tires.

Good idea.

After 400 miles i would get 43.5 mpg. Now it is down to 37.7 mpg.

Maybe your older tires were nitrogen filled

Lol, When I first read this I wasn't sure what car we were talking about. I skimmed past it in the original post. I checked your sig and though you were talking about the 420. :D:D

I would think that would improve your gas mileage.

Unless your MPG comparisons are based on strictly controlled driving conditions, it's difficult to say that the tire swap had anything to do with it...although I concur with the possible change in the rolling resistance of the new tires.

Hotter weather requires more A/C usage...not to mention the engine works harder as well.

New tires grip better, giving a more confident road feel...and perhaps more aggressive driving and heavier pedal.

More stop-and-go driving this time around versus prior to the tire change?

The OE tires are special low rolling resistance tires- Are the new ones as well, or are they just standard tires in the appropriate size?

You can't really go by one tank, you need to average at least 3-4.

Check the pressures to, you don't know what the tire shops set them to. I find its usualy pretty low.

If you installed Triple Treds, You definitely lost mileage. They're a heavy tire & cause mileage drop. I know that from my running a Goodyear store & personal experience. Easy to check the weight of the tire is to go to tirerack.com. Look up your previous tire & the new tires and check the spec page on both. Willing to bet money I don't have that they're heavier than your OE tires. Unsprung weight is always overlooked - wheels/tires,etc. Of course, check the basics - tire pressure, similr driving conditions ?, etc.

When I got the General Altimax HP's for the Sentra, I compared them. The triple Treds are 4lbs heavier per tire in the same size.

Unfortunately i don't know the specs. of the OE tires. They were Good Year.The new tires are also Good Year [Comfort Touring]. I drive the same every day and always drive to get good millage. The old and new tires are not filled with nitrogen. I check the tire pres. once a week and keep them at 35lbs. front and rear. The recommended tire pres. on the car door is 35 front and 33 rear. I don't think that would make a difference. I do think that the new tires are probably heaver.

If you go onto tire rack and search for tires by VEHICLE rather than size, it will flag the OE tire and you can click on SPECS and it will display the tire weight, rolling diameter,etc. To easy your search, eliminate all the other brands since Goodyear is both the OE and what you replaced them with. Do you know what model Goodyear tire the new ones are ?

I just checked - using 2006 as the model since you didn't post what year, but I do know that they used the same tire for several years so I'm probably spot-on with the size - 185/65R15.

The OE tire weighs 17lbs, actually 1lb lighter than the new "Assurance FuelMax" Low rolling resistance model, yet the "standard" Non-low rolling resistance Fuel Max is lighter than both at 16lbs.

EDIT: The Comfort Tred Touring weighs in at 18lbs each - one pound more than you're OE tire. Seems insignificant I know, but it does have an effect. The General Altimax HP's I put on my Sentra to replace the horrible Fuzions that came on it are 3lbs lighter per tire. I went from an average of 31.1 to 33.7mpg after the first tank, driving the same route, in the same manner, every day and has remained consistent in the 33.6-34.1mpg average.

Check the tire pressure.

My tires were over filled on the right side and my car a dog to drive. Next time I go to Sams Club for my winter tire change I'll check it myself.