Better Tyre Configuration for rear and front tyres

I'm a regular 43 mpg guy on my 2010 Prius, the car comes with the stock 195/15/55 tyres which i considered ok, but i have this issue where i sometimes scrape the underneath on some bumps around my neighborhood. About three weeks ago, i replaced the two rear tires with 205/55/15, leaving the stock tires in front to document the effect. My mpg went from 42mpg to 47mpg - quite pleased. Recently, i went ahead and introduced two new additional 205/5/15 tires to the front replacing the 195's and took an mpg reduction to 38mpg. In additional, the car feels heavier to throttle response which i know can be as a result of the new bigger tyres which will require some time to break in but i don't think i want to leave these tyres as is.....seriously thinking of going back to the 195 fronts and 205 rear configurations .....I want to know the impact of this configuration, will it have any damage to my car except probably confusing the skid control which will be reading off different rpms from my front and rear tyres?

 

I can certainly relate to this and if this is the only trade off without any long term damage then i may have to leave it that way as against risking a damage to my exhaust or anything like that underneath....the other option will be to be extra cautious while while driving past those bumps and any other such bumps for that matter.

 

Will have to confirm these numbers again, i know for a fact that it is a 15 inch rim. The essence of the change was to have more ground clearance and so if i end up with a "significant reduction" in Outer Diameter, then it defeats the purpose. Though visually, the new 205 tyres are obviously higher.

 

Ok...did confirm, i went from 195/65R15 to 205/65R15 at both rear only initially, and then later went all up to 205/65R15's for all four tires.

@frodoz737, thanks for the link, it kinda puts things in perspective.

Like i said, i regularly get 43mpg on the stock 195's, with the first configuration i.e. 205 at the rear, it went up to 48mpg. BTW, with the configuration, i had a minimal increase in ground clearance but not significant compared to the second configuration - all 205's. I do agree that computer calculated mpgs numbers will be off with different tire sizes in front and the rear but that is a trade off which given the circumstances one might be willing to make.
With the second configuration however, i did not like the handling, let alone the mpg hit to 38mpg. While driving, I could feel the increased motive force required for the vehicle to move which potentially can be attributed somewhat to the drag from the new tires.

Think i am going to go back to my stock 195's (pictured), suck it up and be a lot more careful around those neighborhood bumps.

 

How do you mean, there where three different changes in tires sizes made and all of them affected mpg differently?
1. stock 195 / 43mpg
2. Stock 195 front and 205 Rear / 48 mpg, though skewed from confused hybrid ECU
3. 205 Front and Rear / 38 mpg with heavy handling and cornering in line new tire expectation

Driving conditions and terrain remained the same.

 

Yes, i already concluded to return to my stock 195 tires. At this time, i have the 205's on the car but will change them soon as i return to town, don't want to risk any damage whatsoever to the car and the fact that i will not be getting the true mpg with the 195 / 205 configuration is enough for me to not go back to that configuration.

 

I agree with you, i have not done any specific empirical tests with regards to how each of the tire combinations affect mpg, beyond documenting what was recorded by the vehicle under my usual driving pattern over the specified period of time. Main reason for starting this thread was really to confirm what i had in mind which was that the 195's in front and 205's at the rear presented a false computation due to the differences in computer recorded rpm's plus the likelihood of damage to the vehicle's skid control. In all honestly, this is by no means a conclusion to deduce that one tire combination is better or yields a better mpg as indicated. Personally, staying with the 205's in the front and rear would have been a nice to have given that it gives me enough ground clearance which was what i was also looking for, but i am not willing to make the trade off given the mpg hit to 38 mpg and probably less. In addition, i am already used to the driving and handling of the car with the stock 195 tires which i consider more pleasurable than the heavy handling from the 205 tires. so to be candid, yes YMMV!

 

@The Electric Me, the thread may be misleading if one does not get the general direction of the thread. Personally, it provided an avenue for me to make some some informed decision.

Yes, of course i was pleased as anything that will give me that sort of bump in mpg is worth looking into....however, at the back of my mind, i realized that something was amiss due to the two different tire sizes - it just did not feel right, what better place to have the discussion and have others wade in on it than PC. And infact, based on the discussions, went away with some key information as indicated in the next quote.

The fact still remains that there may be other people who may find themselves in the same position, wanting to have an increased ground clearance by changing the tire sizes but cannot change all 4 of them at once, or even might want to know the effect of running two tire sizes in their car just like i did. if this thread enables them understand better about the impact of that decision then it would have provided some vital information.