Why an 8 to 10 mpg difference between our 2006 & 2008 Prius?

We own both a 2006 Prius and just purchased a 2008 Prius. The 2008 seems to be getting about 8 to 10 mpg better mileage which I'm puzzled about. We had bought the 2006 used with 27K miles. Initially the 2006 was getting 50+mpg. After about six months we bought new tires and added a trailer hitch and our mileage seemed to drop into the 44 to 48mpg range. This was all in warmer weather, so it is not due to the normal cold weather mileage drop. I know the tires make a difference although the Michellin tires we put on were rated excellent for rolling resistance. We now have over 100K on the car and just replaced the tires again with Michellin Fuel Saver tires. No real improvement. The 2008 we just bought has the original Goodyear Integrity tires with 23K miles on the car. We are getting 54 to 58mpg with the car. I realize the trailer hitch could be a factor but it is a small hitch weighing only about 50lbs and it tucks up underneath the car pretty well so it shouldn't hurt aerodynamics all that much. I'm wondering if the battery pack in the 2006 may not be up to par? Oh, and I did also check the auxilary 12v battery using the MFD in the testing mode. It actually seems the battery in the '08 which is getting the superior fuel economy has the more marginal 12v battery. The '06 auxillary battery tested out just fine, so evidently that battery had been replaced at some point.

 

I really appreciate all the input! This forum is an excellent resource. Maybe the tire diameter is a factor I hadn't considered. When looking at mileage I am going by both the MFD readout and actually checking the mileage over several fill ups for both cars. That normally shows the MFD readout being a couple of MPG's higher than when I check it manually over several tanks of gas. As to drivers, I do tend to get about 2mpg better mileage in whichever car I drive vs. my wife (I'm more on the hypermiler side of the scale) but the mpg difference between the '06 and '08 is pronounced and normally between 8 to 10mpg different with the same weather, similar drives, same gas, etc. I'm not able to get anywhere near the same mileage with the '06 when I drive it vs. when I drive the 08. So it really does seem to be some difference between the two cars. I will try to check the odometer readings over some set measured distance to see if there is a descrepancy in what the odometer is reading due to tire diameter. Even then I can't imagine that alone making a 10mpg difference! I'm still convinced there is something else going on here including potentially the traction batteries in the '06 having diminished somewhat. After all there is nearly 80,000 and 2 years on the '06 vs. the '08. I also notice that the '08 seems to be able to run in pure EV mode a bit longer, nothing scientific, but when I drive the same routes, it seems I am able to keep the ICE shut down a bit more in the '08. As far as the note about potential drive train differences, unless there was some minor computer tweaks I don't know of any difference between the '06 and '08 drive trains. I don't think Toyota claimed any improvements except between Gen I, II and III. While the mid 40's I'm getting in the '06 is respectable, it is still very puzzling why I have been acheiving well above 50mpg in the '08 thus far, don't you think?

 

I like this idea and certainly may try it at some point. It would be a bit time consuming and weird/wasteful to drive around following one another. To minimize variables I would even swap drivers a couple of times to even that out and also even switch back and forth from following to leading to offset any drafting advantages. From what I've experienced thus far though I'm pretty sure we would see the same results, but it would eliminate some considerations such as the tire diameter question, terrain, temps, fuel type etc. Good idea though, thanks!

 

Thanks for your post. So you have answered the question that the two cars should be getting around the same mileage. The mystery continues!

 

I understand that different drivers will get significantly different results and we do have two drivers involved. However even when we switch drivers between the cars their is still a large descrepancy in the resulting MPG with the '06 underperforming the '08 by 8 to 10mpg. When I drive the '06 model I am never able to get the mileage I get in the '08. So I still feel there is something with the '06 that is bringing down the mileage. That is what I'm hoping to discover, not only for my benefit but for the benefit of other Prius owners who may experience this. I probably will try driving the two cars together at some point. I won't be able to do anything as long as 400 miles, but even a shorter run should help to confirm that with everything equal as to gas used, temp, wind, road conditions, speed, etc. how much difference there will be. As mentioned I will even swap drivers at some point to offset any factors in driving style and switch back and forth equally which car is leading the other to minimize drafting effects. That will eliminate enough outside factors to definitely show if there is indeed some issue with the '06 causing this. If it were 1 or 2mpg I wouldn't be concerned but 8 to 10mpg definitely is puzzling. Thanks again for the thoughts on this issue!

 

You may be right that I will never figure it out and eventually I will just accept it. But for my own piece of mind and for the benefit of others it would be great if it can be narrowed down. That way we will have some good information available on the types of things that might be causing lower mileage when it is quite likely something with the car itself.