Gas Milage And All Terrain Tires

[HerbVick]

Just thought I'd pass this along, since I have just discovered it.

I've been feeling pretty miserable at the pump here in Michigan lately with these high fuel prices, especially since I'm still clinging to the >91 octane recommendation. When I first bought the 4R, I was getting 19 on the highway (just barely) and 15 point something in town. Couple of months ago, I bought a set of General Grabber AT2's , M&S 265/70R17. My in town gas milage dropped to 12.7 and highway was between 15 and 16 mpg. That sucked, and I was starting to look over the fence at Corollas.

Today I got out there and checked all the tire pressures. They were 32 psi, just as I had set them. The ride seemed a little soft, but the tires were showing no bulging. They appeared fine. I decided to bump the cold fill pressure up to 36 psi to see if they would feel firmer without going too far.

Long story short, my in town gas milage went from 12.7 to [get this!] 19.9 mpg avg.

That's right! I reset the computer and drove around town stopping at red lights and stop signs, but traveling mostly on main access to business roads. No Interstates. Speeds were 35 to 45 mph stop and go driving. There is no doubt that the milage increased by 7+ mpg. The only factor that changed was the tire inflation pressure. The AC was off, but it was off when getting 12.7 mpg as well.

That is better than I was getting with my stock tires. Has anyone else found a correlation between tire pressure and mpg? Was it that much with that little difference in pressure?

I no longer look at Corollas now!

 

[RonMAIDEN]

Also do a real MPG check using the divide miles by gallons you fillup with.

My D load rated Nittos didn't hurt my mileage much (maybe 1mpg) over the set of H/ts that were on my Runner when I bought it. I keep my pressure at 35psi and I get a solid 16-17 with mostly city driving. All hwy has netted me about 21mpg.

 

[HerbVick]

Thanks. I know it seems a little anal, but I do that with every tank full and I keep it in a log book in the console. After I've run through a couple of tank fulls, I'll post the calculated results here.

 

[techno]

Yeah, its a pretty well known fact that tire pressures can effect gas milage. It makes sense if you think about it, since the lower the pressure goes, the more the tire appears to bulge out, and the more tire surface will contact the road increasing rolling resistance. So conversly, as the pressure increases, less tire contacts the road and resistance (and gas consumption)is reduced.
Also, the more aggressive the tire tread, the more the rolling resistance. I noticed a drop of 1-2 mpg when I went from all terrains to mud terrains.

Did you notice any reduction in ride quality with the higher pressures? It usually makes things feel stiffer and rougher.

 

[HerbVick]

The ride was firmer. Not terribly so, but you could feel the difference on bumps and pot holes. However, it also felt more positive in steering and acceleration. Very noticeable. Being used to the lower pressure effect, I found myself over accelerating initially upon leaving the driveway. I like it!

 

[Thai]

For me, changing tires have not made a big impact on the fuel economy. When i went from stock tires to BFG AT KO (same size), there was maybe 1/2-1 change in mpg. When i went from BFG AT KO to bigger Firestone M/T, i noticed a 1 mpg drop. All this time, i was using 32 psi.

 

[HerbVick]

The thing that amazes me is the magnitude of the difference. It was slightly more than 7 mpg in this case. Now part of that could be explained away as due to the initial period of average mpg being longer than the second sample period, and thereby including a greater diversity of driving conditions such as cold engine start-n-go, shorter trips, more idling, etc... However, in this case I had been trying to learn why the mpg was so low and had been keeping a close eye on the average mpg and had reset the computer several times hoping current driving was producing better gas mileage than previous driving. All I ever got for in town driving was between 12 and 13 mpg.

After raising tire pressure to 36 from 32, my first trip produced 19.9 mpg average by the end of about a 40 minute drive [in town] that included 2 cold starts. (I took a 1 hour break) I've never gotten better than 16 mpg in town with the stock tires.

I think there is a generalization one can draw from this that applies to all tires, but may be more applicable to tires with aggressive tread patterns. I think there is a sweet spot in the tire pressure that will yield the best gas mileage on pavement. Higher or lower pressure from that sweet spot will decrease gas mileage. I say this because when the tires were mounted, the service provider gave them to me set at 41 psi, and I had poor mileage performance.

I can imagine that, with such dramatic results, the 4 psi of additional pressure was just enough to expand the center of the tire and pick up the knobbies from the pavement. The knobbies probably flex a little as the tire rolls over them, creating friction if they are in contact with the pavement. Picking them up would then have the effect to reduce roll friction considerably.

I wonder if this means that, as the tire wears, my gas mileage is going to go to a craft store in Montana? ... You know ... Helena Handbasket?

 

[TeCKis300]

I honestly do not think it did that much for you. While I do see a 1-maybe 2 mpg improvement, 7 is impossible and you are just being extra careful with the gas since you are conscious of it.

Keep driving around for a few more tanks of gas with normal driving practices, then report back.

I did do this experiment as well with my V8 T4R. Tried it at 32psi and 38psi. It did increase my miles to the tank a little bit, but no way was it an additional 7mpg.

I run 265/70/17 NTGs...I'm getting consistently 15.6 mpg on the trip meter in heavy to aggressive local commuting.

When my wife gets the car for a week, I reset the trip computer and she will typically avg 17.2mpg on the trip computer with more leisure highway commuting.

I don't remember exactly the mpg's on stock tires, but it wasn't more than a 1-2mpg difference.

 

[Falls]

I usually always run my pressures high. Thats why I get the milage I do. Its not uncommon for me to get 17-18 city/rural and 19/21 hiway with my 4x4 V8.

I have my 265/70 Revos at 40psi . Rides fine , handles fine , the wear has been good . The tire footprint is still where its supposed to be . I even did a contact patch test.

 

[HerbVick]

OK. I'll just keep tracking average mpg in my daily commute of 2 miles one way, together with sporatic supermarket and Wal Mart trips, and report back with longer term findings. It's already giving back a little. After one morning cold start, one afternoon cold start, and about 8 miles cumulative of relatively heavy stop-n-go, it is down to a nice round 18 mpg.

Stay tuned.

 

[jwyshrek]

wow...

but this why they emphasize that people should maintain tire pressures to ensure best gas mileage...

 

[Koz]

I run 40 psi.

koz

 

[Falls]

I never noticed a difference in milage with switching from the stock tires to the Revos........since I drove from the dealer on 265/65/17's directly to the tire store and left with 265/70/17 Revos.

It really depends on the tire I guess. One guy on here said he bought a set of the new Goodyear wrangler Silent Armor's and his milage dropped like a rock..........not calling him a liar , but strange to say the least. I could see if you switched to a mud terrain. MAybe a real aggressive AT like BFG AT KO's.

 

[Thai]

It really depends on the tire I guess. One guy on here said he bought a set of the new Goodyear wrangler Silent Armor's and his milage dropped like a rock..........not calling him a liar , but strange to say the least.

That is not surprising because there are more than a few owners with Goodyear SilentArmor who complained about the mileage drop. Some people say that the recent Goodyears have more rolling resistance than other tires.

 

[Falls]

That is not surprising because there are more than a few owners with Goodyear SilentArmor who complained about the mileage drop. Some people say that the recent Goodyears have more rolling resistance than other tires.

Interesting

 

[HerbVick]

There's a very interesting discussion on the accuracy of speedometers and odometers over on CR4 (T4R, CR4. Don't get confused now.)

The engineers get into the effect of tire pressure on the accuracy of speedometers. Since the mileage computer draws its data from the same place, it seemed appropriate here as well.

 

[HerbVick]

Update:

After over a week of routine driving, I have found the average mpg computer to have settled out around 14.4 mpg. Not as good as I had hoped. But, not as bad as it was at 12.7 mpg.

Good call TeCKis300.

I'm still wondering how I got over 19 mpg during my first little venture in town. Could it have been downhill both ways? :