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Tire size vs. mpg


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9 replies to this topic

#1 fat tire

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Posted 08 May 2007 - 12:15 AM

I have a 92 Loyale 5sp with 175/70/13 tires. It gets around 25 city 33 highway. I also have a 87 gl10 turbo automatic that has brand new 185/80/13. The 92 loyale calls for a 165 according to the stamp on the drivers door. The 87 gl10 calls for a 185. Do you think if I rob the new 185's off the gl10 and put them on the loyale it will kill my gas milage? Can you go bigger than stock and still get optimum fuel milage, and if so how much bigger?

#2 daeron

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Posted 08 May 2007 - 02:31 PM

i was running ~25 mpg in any situation in my 87 GL10, NA, auto, with 185s on it, same mileage as ith 175s.. bear in mind that the odometer reading will be slightly different, too, with different sized tires on the car.

Its really an academic situation; such a minimal change will have minimal impact on mileage, and the only way to be 100% certain of all readings would be to take milage not from the odometer, but from a GPS device. I mean, didn't your GL-10 come with built in GPS navigation? :lol:

give it a spin and find out for yourself, every car is a little different.

#3 Esteban32696

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Posted 09 May 2007 - 05:43 AM

This link will take you to a site whetre you just type in the tire sizes to recalculate the change .
http://www.miata.net...e/tirecalc.html

#4 Crawlerdan

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Posted 09 May 2007 - 09:51 AM

i put 27 inch ats on my loyal and was getting 39 regularly. i was driving 400 miles in one sitting for work...but on the highway the extra tall tires acted as an overdrive, lowering cruise rpm by 7-900 rpm, allowing me to observe super economy. this probably wouldnt work for the around towners, as the taller tires would make it more difficult to leave a stop sign....but man did it help on the highway

only other mods to taht car wheer a cold air intake and flowmaster

#5 robm

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Posted 09 May 2007 - 11:04 AM

I have run 175/70's, 175/80's, 185/70's and the recommended 165/80's on my Loyale. I find that the 165's seem to get the best gas mileage, even when adjusting for the different outside diameters. It is not a huge difference, 1-2 MPG on the highway. If you want the best mileage, stick with the 165/80's. It is too bad they are getting so hard to find here in Canada. You might have more choice in the US.

#6 daeron

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Posted 09 May 2007 - 03:51 PM

I gladly sacrificed any mileage I may have lost for the added wind stability of the wider, 185-70 tires. I have never felt so much difference from such a minimal change of tire size. (This may just be an experience thing with me; that was simply the first time I had felt such a difference and noted it so.. BUT the fact remains that I had never felt such a difference.)

#7 subie_newbie

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Posted 09 May 2007 - 10:15 PM

There are things working for and against each tire. Here's how it works academically:

Larger diameter tires decrease your gear ratio making the engine run at a lower RPM for the same speed = better fuel economy

However, larger diameter tires also weigh more and take more throttle to accelerate the same = less fuel economy.

Also, your larger tires are a wider tire, which will create more rolling resistance = less fuel economy.

What happens in reality is people that can handle the slower acceleration and drive with the same light right foot will notice a difference from a major change in tire diameter to a bigger tire. Wider tires will always hurt your economy (think pedaling a mountain bike vs. a road bike). Most people will notice no difference when going to a smaller tire other than their car is more peppy, unless they drive a lot of cruising miles. Good luck!

#8 grossgary

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Posted 10 May 2007 - 11:34 AM

unless they drive a lot of cruising miles. Good luck!

that's my experience. i've noticed a 1-3 mpg increase when going up to a larger wheel/tire combo, i do very little city driving though. tricky figuring out your actual mileage though...as your odometer/speedometer is no longer correct.

i've always wondered...speedometer are typically inaccurate..up to 7% in the US, so does that mean the odometers are inaccurate as well?

#9 robm

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Posted 10 May 2007 - 01:34 PM

Depends on cruising speed, too. I am lucky enough to be able to run at 90-95 km/h on the highway, without getting run over from behind. This translates to about 2600 RPM, just below peak torque. The larger diameter tires would drop the engine speed to the point where it isn't as efficient, and has too little torque to take the hills without downshifting. As it is, I have to downshift on some of the steeper hills, especially when the car is full of gear. Ideally, I prefer to just leave it in fifth gear, on cruise control, and add a little throttle foot to help it up the hills, when the vacuum cylinder doesn't have enough force to keep the speed up. Works for me - I can get over 16 km/liter under ideal conditions.

#10 Tom63050

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Posted 10 May 2007 - 09:48 PM

tricky figuring out your actual mileage though...as your odometer/speedometer is no longer correct.

i've always wondered...speedometer are typically inaccurate..up to 7% in the US, so does that mean the odometers are inaccurate as well?


Here's how I figure out mileage on larger than stock tire sizes.

Find a piece of highway with mile markers on it. For convenience I start at a multiple of 10. Note the mileage on your odometer, including tenths (or set a tripmeter if you have one). Drive ten miles worth of mile markers, then note the mileage on your odo or tripmeter. The difference will tell you how far off the meter is, like 9.2 miles instead of exactly 10.0, an 8 percent difference. Figure your speedo is off this much too.




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