Kumho Power Star 758 Tires- Observation after 6000 miles

[F8L]

It could have been just one of the MFD flip flops where it read high. After a few tanks you'll see if it has truely changed or not.

[HighBreed]

Another 758 user here. Has been for 20,000 miles. Originally set at 37F/35R, I was getting around 50MPG. Then pushed it up to 42F/40R, and MPG decreased very so slightly-- maybe by 1MPG. Did not make sense to me. Same commute everyday, every week. Brought it down to 38F/36R and now getting AT LEAST 50MPG, averaging 52MPG, and if I try really hard and has Shell in my tank, can see 54-55MPG. I leave it at 38F/36R nowadays. My alignment is perfect (Johnson's Alignment in Torrance if you want the ultimate-- I drive 50 miles to have alignment on all my cars done there-- no trial and error- just perfect every time I leave the shop), no uneven wear, no pulling to one side.

[Blegate]

Originally Posted by HighBreed

Another 758 user here. Has been for 20,000 miles. Originally set at 37F/35R, I was getting around 50MPG. Then pushed it up to 42F/40R, and MPG decreased very so slightly-- maybe by 1MPG. Did not make sense to me. Same commute everyday, every week. Brought it down to 38F/36R and now getting AT LEAST 50MPG, averaging 52MPG, and if I try really hard and has Shell in my tank, can see 54-55MPG. I leave it at 38F/36R nowadays. My alignment is perfect (Johnson's Alignment in Torrance if you want the ultimate-- I drive 50 miles to have alignment on all my cars done there-- no trial and error- just perfect every time I leave the shop), no uneven wear, no pulling to one side.

Okay I admit I thought of this theory as well but couldn't believe it. Now I see you commenting that it might be plausible! When I checked the tire pressure after getting home from the tire shop back on 7/8 it read 40F/38R; however, I took this measurement after driving 18 miles highway on a hot day. I believe the tire pressure increases temporarily due to the heat etc., but I could be wrong. Tomorrow morning before I go anywhere I'll bump it down to 38F/36R and give it a whirl.

Thanks for posting this. I'll post my findings later this weekend.
-brian

[Blegate]

Quick UPDATE

I replaced the 12v battery two days ago myself and my car seems to be getting better MPG. I did 325 miles in the last two days- mostly highway but some city- and got 51mpg. I did this same trip last month with about the same exterior conditions and got 46. Could the old faulty 12v interfered with the Prius Brains and cause more drain from the main battery therefore causing the ICE to come on more frequently = less mpg?


Thoughts?

[jdenenberg]

Originally Posted by Blegate

Okay I admit I thought of this theory as well but couldn't believe it. Now I see you commenting that it might be plausible! When I checked the tire pressure after getting home from the tire shop back on 7/8 it read 40F/38R; however, I took this measurement after driving 18 miles highway on a hot day. I believe the tire pressure increases temporarily due to the heat etc., but I could be wrong. Tomorrow morning before I go anywhere I'll bump it down to 38F/36R and give it a whirl.

Thanks for posting this. I'll post my findings later this weekend.
-brian

Brian,

I looked at the powerstar ratings on TireRack and two things jumped out:

The MAX PSI rating is 36 - Not a good candidate for 42/40

The revs/mile were not listed, but the 24.4 inch diameter (185/65R15) indicates to me that it is higher than the original Goodyear Integrity tires (24.4 inch, 855 revs/mile) and will cause slightly higher readings (0.5%) for MPG due to mis-calibration of the odometer.

This is a low cost tire and I buy better ones (eg. Michelin, Nokian) even though the tend to cost $100 per tire.

JeffD

[Blegate]

Originally Posted by jdenenberg

Brian,

I looked at the powerstar ratings on TireRack and two things jumped out:

The MAX PSI rating is 36 - Not a good candidate for 42/40

The revs/mile were not listed, but the 24.4 inch diameter (185/65R15) indicates to me that it is higher than the original Goodyear Integrity tires (24.4 inch, 855 revs/mile) and will cause slightly higher readings (0.5%) for MPG due to mis-calibration of the odometer.

This is a low cost tire and I buy better ones (eg. Michelin, Nokian) even though the tend to cost $100 per tire.

JeffD

Jeff- I understand your point but there are many posts from satisfied Prius owners who purchased this particular tire. If you read within this thread there's some debate on whether the psi should be 42/40 or more like 40/38 for this tire. BTW, I did research here as well talked with tire shops and got this recommendation for it's price range. I feel it's a good bargain. I didn't do the goodyear Integrity since we are all too familiar with the real 'integrity' of the tires wear characteristics when running on a Prius. The .5% potential difference I can live with.

I'm well into the 430 now (one tank of gas with two pips left) and I'm still 50.4 mpg @ 430 miles with the last 35 odd miles being 85% city (P&G user). I confident to say that replacing that 12v battery- be it the battery or the resetting of the Prius computer when it was disconnected- did something positive for me. I haven't experienced 430 on a tank since 11/07 (I track mileage). The car is acting better. I dunno..... if the mpg drops again my car is possessed.

-Brian

[SDJack]

"MAX PSI rating is 36".

The sidewalls of the 175R6514 HS Powerstar 758's on my 2003 say 44 MAX.

On mileage: In San Diego over the last 2 1/2 yr. and 18,000 miles on the original Bridgestone RE92 XL tires (which I ran until cord showed at 35,200 miles) I have averaged 43.7 mpg calculated, 46.6 mpg indicated, driving about 50/50 local/interstate and filling up when it beeped at me. With mostly short (5-10 mi) trips, mileage drops at least 5 mpg, but it has a warm and toasty catalytic converter. At a steady 65 mph interstate it gets about 50 mpg. In January, I increased tire pressure from 38/36 to 45/43 producing at least a 2-3 mpg improvement. AC knocks off 2-3 mpg. I have bought gas only at the same Costco station, but not the same pump.

Four weeks ago, mainly because of all the noises the R92's make even worn to the bone, I bought 758's at Discount tire ($266 including tax). Wonderful! Quiet, steers, brakes and corners like a real car, even with air at 44/42, none of which were passable on the RE92's. Oh yes, and after the first tank of gas (beeped at me after 350 instead of 450+ miles) the indicated mileage was 41.4 mpg, the calculated 35.9 mpg, a 13, 18, or 22% drop, depending on how you figure it. OOPS! Drove the same old ruts, too. This is not the expected 1-2% new tire hit.

But the reliable 3 mpg differential between indicated and calculated mpg was 6 this time. Something doesn't figure. As miles accumulated, the mpg seemed to improve. The first day, over an 8-mile route I always exceed 50 mpg on, it got 26. After 85 miles on the second tank it indicates 46.3 mpg.

The day after spiffy new shoes, the car went to True Line where EVERY measurement was made essentially spot on with no rear toe and minimal front toe in even though it was within limits to begin with. Then it went to Mossy for rough ICE start/stop and the door open light staying on. Had its "plenum" cleaned of oil gunk ($65) (smoother) and all door switches/connections removed and replaced and the aux ecu removed, given a few infamous whacks, put back; all OK (4.25 hr., $45 labor, no parts, 3 band aids). Then I topped the gas off, for a clean start.

Does the Gen I “learn”? Is there something a mechanic could do to make the engine control system use gas less frugally? Anyone have any ideas? My 30-day trial is up. So I have to decide whether to keep these, go with horrid but L.R.R. R92’s ($192 more) or Michelin Destiny’s ($232 more).

[SDJack]

Update:
Toyota service manager says the ECU "adapts" over long periods of time- tank over tank, so that optimum mileage may not return for weeks or months.

On the 2nd tank after 119 miles, indicated mileage is 43.2, down about 3mpg, or 6%.

[MR.K]

Belgate...I know an older battery will /can effect mileage because the car is trying to charge an unwilling batt. These are partly sulfated ..its like charging a batt with an auto shut off charger that wont shut off....the battery and the charger gets warm but the voltage wont go high enough to turn off...

[MR.K]

Ps...With a partly sulfated batt you would probably not notice the mileage drop like a Chevy V8....but a Prius or the first Honda Insight(I have both ) is a different story.Insights go into a Lean Burn mode on stick models 20-26 :1 (instead of 14.7:1)when every thing is warm and light load ..Tho the batt will start the car ,a sulfated battery adds enough load to prevent L/B... it can knock the mileage by 7 mpg ...when you think that Prius air pressure in the tires can make a 5 mpg differance any enegry loss can effect mileage.. There is battery desulfators that work well ,pulse tech ,vector ,and others .I charged my sulfated prius batt it would only go to 12.3 volts ...after I desulfated it would go to 13.5 ........