How Important is Tire Pressure for MPG?

Perhaps this might help:

Bob Wilson

 

What size tires does the prius have? I have a 1998 diesel Jetta that gets between 44 and 52 mpg depending on how I drive, and a 1982 vw camper bus that gets between 17 & 21 mpg (but it does have a sink, stove, refrigerator & 2 beds). I recently noticed that both vehicles have the same size tires. 185R 75 14 is the tire size on both vehicles, and both vehicles are shod with Hanooks. But the bus tires say maximum inflation 65psi while the jetta's tires say maximum inflation 44 psi. The bus tires have 8-ply side walls to reduce tire flexing. I was thinking of putting bus tires on the jetta next time I need new rubber.
These are the bus tires:
BusDepot.com - VW Bus / Camper / Vanagon / EuroVan Specialists
Would they fit a prius? You can really PUMP THEM UP!
Busdepot also has 8-ply 65psi tires that fit 15 inch wheel rims.
BusDepot.com - VW Bus / Camper / Vanagon / EuroVan Specialists

 

Bob:

(I'm sorry to reproduce the graphic, but it is important to my question.)

I have assumed this is data gathered on your Prius (or a combo with others). Do you have any data on PSIs above 40? The curve is flattening, it appears, but I'm wondering what you see.

My experience is that MPGs continued to increase even above the Max pressure. I don't run in that league now, but I did for a while, and I noticed higher mileage.

 

Rolling resistance continues to decrease. One of my other references from the California tire rolling resistance project reports it is an inverse power function.

Bob Wilson

 

Would you say, then, that at 44 PSI (OEM tire max pressure) there would be (roughly) a 23% reduction in rolling resistance?

 

Be careful when inflating the tires to higher pressures than the stock ratings. I did this on my first set of tires (40/38) and the tires wore unevenly causing them to wear out prematurely (20,000 miles). My new tires are inflated to the stock rating (35/33) and the ride is better and they should wear better. The difference in mileage is negligible.
I have had my Prius for 4 years now and it has been the greatest car I have ever owned. Finally my friends are starting to buy their own!
Changing the Planet one Prius at a time!!!!!!!!!!!!!

 

Hey John. Did your tires wear out in the middle of the tread?

I'm at 40/38 right now (only about 1,500 miles), and I'd like to get the entire life out of the tires. It doesn't make sense to gain 2 mpg, only to lose that savings and more by buying tires early.

... Brad

 

very important question............................do the tire manufacturers measure maximum tire pressure HOT OR COLD??

i have to admit that i've followed the advice here and ran 42/40, even 42/42, just for chits and giggles....................however, i am i bit nervous with this high tire pressure.

here's the bottom line, unless i'm running nitrogen, my 42psi COLD will measure UP TO 48 HOT. so, when our goodyear integrity's say max 44 psi, is it cold or hot????

THANKS.

 

I would guess that it has to be cold - for consistency sake if nothing else.

... Brad

 

If the tires needed to be replaced at 25,000, because the tread had worn unevenly due to higher tire pressure, how much less would you have paid for gasoline just to get the extra 2 mpg?

Let’s assume you get 50 mpg at the higher tire pressure and 48 mpg at the lower pressure.

Had you stayed at the lower pressure, you would have used (25,000/48) 521 gallons of gasoline. Let’s look out into the future and say that your average cost of gasoline would have been $4.00. This means at the lower pressure you would have paid $2,084 for gasoline. At the higher pressure you would have used (25,000/50) 500 gallons and paid $2,000 for the privilege. Has the $84 been worth it?

All of this analysis, though, assumes that a tire pressure of 40/38 psi will prematurely age the stock tires (due to uneven treadwear). Though I cannot disagree with the one poster who experienced this, I have not heard of this happening before. I have heard of 3-4 mpg increases due to higher tire pressure (still at or under the manufacturer’s max pressure).

Another calculus is to see what savings have occurred if the tires last 40,000 miles. Assuming the higher tire pressure would result in 51 mpg (vs. 48 mpg), then across 40,000 miles the owner will have saved a bit more money -- $3,137 vs. $3,333, or a savings of $196. At $71 per replacement for the Integrities, the 3 mpg increase has paid for nearly three of the four new tires. That’s not a bad deal in my book.

Truly, if the tires wear out at 20,000, instead of 40,000, then saving $84 in gas is not going to cover the entire cost of replacement. In that scenario, you will have saved $168 dollars in fuel across the first 40,000, but you will have paid $284 more for the extra set of tires (4 at $71 each). Thus the cost would be ($284-$168) $116. The break-even point (roughly) would require an mpg increase of 6, as opposed to 2. Again, this is just assuming tire replacement at 20,000 due to uneven tread wear related to higher tire pressure, and assuming the original tires should otherwise last 40,000. Once the assumptions change, so that replacement would be at 25K or 30K due to higher tire pressure, or so that the original tires would last beyond 40K, then the economics change, and things start to favor the higher tire pressure.

Another caveat: I am not saying higher tire pressure causes uneven treadwear. This has been reported by one poster. I run at higher tire pressure, and I’ve noticed a significant increase in MPGs. The tires look to have good wear and no uneven markings, but I am only at 11K.

Comments welcomed!

 

Why not try 44/42? You haven't exceeded max, and you will notice a difference. If the ride is too rough for you, then creep down until you are comfortable. If you are sitting on your wallet, you may decide a bit more bounce isn't that uncomfortable after all!

 

NO one really asserted my answer about the PSI - how it's rated and the maximum pressure the standard tires can hold.

i can TOTALLY decipher and agree that running the maximum tire pressure will get the best mpg..............however, if the tires maximum psi rating is 44 , that means that we should run about 38-39 COLD - in order to be safe. it's OBVIOUS that the air expands on heat, causing the PSI's to rise.

PLEASE, someone comment on this. IF the max pressure of the integritys is 44, then running 44 cold will turn into 48 hot. that's pushing the tire......................

 

I think you had a pretty quick answer from Brad (bac), and no one seems to have disagreed with the view that PSI is always measured cold.

Yes, it is obvious that air expands when heated, but there is nothing obvious about saying that a 44 psi max on the tire should somehow be read "38-39 COLD." The 44 rating is a 44.

There should be no "pushing the tire" if you run it at the manufacturer's maximum rating -- unless you are placing an exceptionally unusual weight load on the car.
I also don't understand your fear or impatience. You had one direct answer, and there are plenty of PCers who have posted their opinions. You don't have to follow anyone's suggestion about tire inflation, though the manufacturer's recommendation is not a bad start.

If a private party's interpretation still doesn't do it for you, you may want to try the government's opinion. I'm not so certain I agree with everything published by the National Highway Traffic Safety Administration, but here is their web page stating that PSI is always read cold and is required to be posted as the cold reading.

 

Cold, which ideally means, "driven no more than one mile during the previous hour".

And just because I like repeating it: the max rated pressure is molded into the side of every tire. No need to guess what it is.

If a modern radial-ply tire wears unevenly it's because the car needs an alignment, or the tire was run under-inflated, or the tire is faulty. A correctly-made radial tire changes shape negligibly over a wide range of pressures. That's what the steel belts are for, and that's why it is difficult to tell by eye whether a radial tire is inflated correctly.

 

I just swapped my stock wheels (with stock Integrity tires) back on my car after a few days of driving with aftermarket wheels.

I checked the tread wear on each tire using a quarter and my results are as follows:

Over 30,000 miles driven with pressures exceeding 44psi and 4,000 of those above 56psi.

Tire #1-3:Outside tread = top of George Washington's head
Middle tread = slightly deeper into George Washington's head
Inside tread = top of George Washington's head

Tire #4 Same as above but had noticably more center tread than the other 3 but the inside/outside tread was closer to the other 3.

This defies all "common" tire wear lore that states an overinflated tire will wear out the center tread faster than the outside tread. In fact I would almost argue that the opposite is happening but since my tires are rotated fairly often this accelerated outside/inside tread wear could be due to hard cornering or alignment issues and is probably not related to tire inflation.

 

Hmmmm.
Purchased a digital tire gauge and checked my tires today, after reading this thread. 33 in the front, 31.5 in the rear. Part of the dealer set-up, I assume, but it does seem low.

 

Good question. I'll give you the non-tech answer just in case Bob Wilson or Hobbit or some other purveyor of the true knowledge doesn't join in this thread's discussion.

First, the manufacturer has suggested having the front tires at a higher pressure than the rear tires. This should follow through at all times.

Many cars, especially those with front wheel drive, have this same requirement.

Second, I believe it has most to do with control, not only application of the drive train to the pavement through the tires, but also cornering and braking. Remember, the Prius has discs on the front only, it isn't a four-wheel disc brake car.

OK. That about does it for me!