A little larger tire=what mpg loss?

MiddleAgeSubie

Forum Member
Joined
Dec 15, 2013
Messages
990
Location
AZ
Car Year
2018 / 2008
Car Model
4Runner / Tribeca
Transmission
5EAT
Hi all,

What mpg loss should I expect if I switch from 225-60-17 Geolander AT-S (stock size) to 225-65-17 Geolander AT-S? Tire diameter increase would be 0.9".

At present I usually average 20-21 mpg (US) in town and 25-27 out of town according to the computer (I think that the real consumption is about 2 mpg worse).

Thanks!

Edit: I know Machine1 reported a 3 mpg loss on his Forester XT when going double my hoped-for-increase. His car is rated comparably to mine (19/24 vs. 18/25 on mine). Should I expect a difference of about 1-2 mpg?
 
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You will see a loss in mileage if you go by the odometer for sure. After all, it won't recalibrate for the larger tires. Whether or not that loss is real is another question.

I know my 99 Outback is running at pretty high RPM on the freeway, so as long as I drive the same real speed - lower speed on the speedometer - I could see mileage staying the same due to the taller effective gearing.
 
My next set of tyres will probably be 215/65 16" instead of the stock tyres, currently 215/60 16".

I expect this to fix my speedo error almost perfectly. It currently reads about 100 kmh when I am actually doing 95-96 kmh per the tachometer, GPS unit and overhead speed check devices ... SWMBO's SH speedo is even less accurate than this! Of course, her car is running 225/55 17" stock rims and tyres.

Modern speedos and odometers are not linked as they were in the olden days of mechanical speedometer drives, AFAICT. With my '93 Impreza, the speedo would vary wildly, but the odometer was always accurate. We regularly did a run from Melbourne to our other home about 210 Kms away. The odometer reading never varied by more than 1-2 Kms, even though the speedo was reading up to ±25 Km/h different from the speed as per the (very accurate) tachometer!

Hope this blather is of some use to someone ... :iconwink:
 
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Ratbag,

What mpg difference do you expect, if any?
 
Gidday MSA

None ...
We measure fuel use in litres per 100 kms over this side of the big pond ... :biggrin: :lol:.

Seriously though, since the tyres are not particularly heavier, wider or bigger, and the rolling radius is slightly greater, I expect my revs to drop slightly for a given speed. Theoretically, this should give a very slight improvement in fuel economy. Probably just enough to offset the slight loss due to the slightly heavier carcase (slightly more inertia to overcome on acceleration ... ) and slightly higher gearing.

I think that fuel economy changes are only likely to be noticeable if one increases the tyre size, width and weight by more than just a little bit ... For most of us, slight alterations in driving behaviour, traffic flow and the road conditions are a more likely cause of fluctuations in fuel economy.

With me, it's the weight of my right foot that causes most fuel use increases ... :lol: :lildevil:.
 
taza said:
Probably. Sounds about right.

Junkie, what kind of rpm are you doing at 70mph?
Click to expand...
I think it's around 3000rpm, but let's do a little math:
205/70R15 have diameter around 26.3", circumference around 82.6". That means to go one mile, they need to go (5280ft)/(82.6/12) = 766.9 revolutions. In order to go 70mph, we need the tires to do 53681 revolutions in 60 minutes, or 895 revolutions per minute.

The internet tells me 4th gear on the 4EAT is .694. The differentials are geared at 4.444, so every time the engine turns the wheels turn 1/(.694*4.444) = .3242 times.

For the wheels to turn said 895 turns in a minute, the engine has to turn 2758 times in a minute.

Then again, both speedometers and tachometers are often a little optimistic - I know the speedometer is. I imagine the tach is off by a similar amount. That would explain why I thought it was closer to 3000rpm (or one of my numbers could be off).
 
Gidday Junkie

Just for precision's sake, one should use the rolling radius of the tyre, not its nominal diameter ... :ebiggrin: :poke: :biggrin:.

Have fun ... :iconwink: :lol:.

But seriously, I think that the fuel economy is only going to suffer noticeably is when the tyre size, width or weight (or that of the rim ... ) is changed dramatically. I know you guys in the US can do pretty much whatever you want to, but here in Oz, these things are tightly regulated under the Australian Design Rules (ADRs).
 
I went from the stock 205/55/r15 on my forester to 215/65/r15 and I did not notice any significant drop in MPGs. The only thing that really has affected mileage is my roof rack and then when I put my spare up on the roof.
 
If you stick to the same type of tyre (ie, same construction) they shouldn't be much heavier. According to rimsntires.com, about 1kg heavier. This will affect economy slightly.

Circumference is 2273mm instead of 2204mm.

I know when I put my muddies on I get a big loss in economy, but they are LT construction so much heavier :(
 
Well, I wish I had a larger set just for vacation purposes. But there is no way my spouse falls for that:). The plus is that 235-65-17 tires are much cheaper than those I use (225-60-17) and plan to use (225-65-17). They also would provide 0.7" of much needed clearance. BUT, tires that large will take more than splash guard trimming AND definitely be useless as DD tires. They may also affect the gearing in ways that I may not like, I just do not know.

By contrast, the 225-65-17s would give me 0.45 of clearance while still being fine as DD tires.

I am just trying to determine how fine:)

I will need new tires next summer anyway so I am contemplating getting the larger tires now, use them this summer, and then revert to the old set until it wears off at which point the larger ones will become the permanent set.

But I definitely cannot afford to purchase a set prematurely unless I am sure I will like it in the long term.
 
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