Smaller Tires and MPG? (1 Viewer)

[NMC_EXP]

Greetings

Being a contrarian by nature I'm thinking of switching to smaller tires.

Bought a used '98 4-Runner. It is the wife's daily driver. Has the 3.4L V6, A340F auto trans, 4.10 gears per the door jamb data plate confirmed by spin test: 20 tire revolutions = 41 driveshaft revs.

The data plate shows 225 75 R15 tires but the vehicle is on 265 70 R16 tires. Based on GPS, the speedometer appears correct.

Her 70 mile roundtrip commute is mostly highway and she gets from 19 to 20.5 MPG.

We will relocate to Colorado in a couple of years. Here in the low altitude flatlands I am not impressed with the performance of this vehicle. In the mountains, I'm concerned it will not be able to get out of it's own way.

So I am considering changing to the 225 75 R15 tires.

According to this gear ratio calculator:

Gear Ratio Calculator

...at 60 MPH changing to the smaller tire will increase engine RPM from 1,917 to 2,074, an 8.2% increase. Tire weight would be reduced from 44 LBS to 30 LBS.

QUESTION: Is there a way to estimate the effect of the smaller tire on MPG? Does an 8.2% increase in engine RPM equate to an 8.2% increase in fuel consumption?

Thanks.

Jim

 

[IanB]

Fuel economy is tied more closely to load than rpm, the tire weight will make a bigger positive difference to fuel economy than the gearing change due to the tire size. As far as ecomomy goes, to do what you've proposed will require buying new wheels (15's) AND new tires, make sure you'll actually see overall savings in the life of those tires from the fuel you'll save before committing.

 

[MoCoNative]

I did a similar thing that you are proposing. I have a T100 with the 3.4 and 5speed manual. I get a good solid 20+ MPG prety much all the time. A little better on dedicated highway trips. I'm 4.10 ratio as well.

My truck has similar markings on the door concerning a 235 75R15. It was originally fitted with 31x10.5 tires from Toyota, like alot of trucks were. When I got it, the PO had put 16 inch Alloy wheels on it from a Sequoia and it had 265 75R16 tires on it, Load range D. A 265 70R16 is the same as a 31x10.5 tire. I had slightly oversized tires. I felt they were a tad big and heavy.

When I got new tires, I got 245 75R16 tires. They are roughly a 31 inch tire, but narrower. I thought that would improve economy and reduce rolling resistance and get the gearing closer to the original design.

I have found very little difference in economy between the two sizes. Perhaps a very small gain. However, the vehicle does not ride quite as well on the smaller size tire. I'm not sure why, and I'm no tire specialist, but I guess wider tires give a better highway ride, lower profile helps that also.

I know from years of running 235 75R15 tires on Land Cruisers they are a bit more darty and such, but a Cruiser is anyway. You would need new rims for this size. The 245's do look narrower, they change the look of my truck, they are pretty flush with the 7 inch alloys. The stance is still good though, however a T100 is wider than a 4runner.

I also put manual hubs on my T100, so I could fully disengage the front axles in the summer months. Like others on the internet, I actually saw very little difference in economy. It felt a bit peppier, and the steering was lighter, but no amazing increase in economy. I think low to mid 20's is just as good as the 3.4 will do. The automatic probably does more to lose economy than the tire size. And of course, only get all terrains, no mud tires, etc... I'm running Firestone Destinations right now. Very quite and smooth on the highway. A little disapointing in snow and mud. A more standard all terrain seems to do better than these in bad conditions.

 

[NMC_EXP]

Fuel economy is tied more closely to load than rpm, the tire weight will make a bigger positive difference to fuel economy than the gearing change due to the tire size. As far as ecomomy goes, to do what you've proposed will require buying new wheels (15's) AND new tires, make sure you'll actually see overall savings in the life of those tires from the fuel you'll save before committing.

Ian

This is driven by my concern that the current configuration will be geared to high to perform well in the Rockies. It seems underpowered to me here on small hills at 750 ft elevation. The fuel economy is a secondary issue - I just do not want to cause a significant reduction. Like everything else there is no perfect solution, only compromises.

The cost of 15" wheels would be offset somewhat by the $46 per tire difference in cost between the two tire sizes.

I assume the 32% reduction (14 LB each) in tire weight would be a plus for MPG, ride and braking but hard to quantify.

Thanks

Jim

 

[NMC_EXP]

MoCoNative

Seems the V6 with auto trans is pretty anemic. I have a '89 Toyota truck with the 3.0L and AT in Colorado. It had 31x10.5x15 on when I got it. Did OK here in the flatlands but in the mountains I'd get a leg cramp from standing on the throttle. I put small tires on it and made a big difference in the hills.

I had a '98 Tacoma with the 2.7L and a 5 spd and it would run circles around either of the V6 auto.

Regards

Jim

 

[IanB]

Keep the big tires, add a SC, problem solved, lol.

 

[Haggis]

My DD 01 4Runner came from the factory with 265/70/r16. I have stayed with that size and I get around 21mpg Hwy.

 

[40Habits]

The 15" rim does not fit on the front of the 4runner. It will rub on the calipers. You might try one be for you by them.

 

[Duane]

I would keep the 16 inch rims.
You can go narrower and smaller.
Perhaps a P235/75R16 which should be at a pretty good price point as it is a common size.
Choosing a lower rolling resistance tread and driving the speed limit will improve your mileage.