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Old 07-07-2008, 07:40 PM   #1
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Need some help on MPG calculations please

Hello.
I have recently changed from the stock 185/60/15 tires to a hankook rs-2 195/50/15. The speedometer change as calculated was 4.5%.
http://www.miata.net/cgi-bin/tirescgi

But if i go with MPG, it actually dropped on my last tank. Is there a calcuation that i can use to compensate for the shorter sidewall difference?
This is for a 07 toyota yaris sedan, if anyone was interested in that.
Thanks guys!
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Old 07-07-2008, 08:05 PM   #2
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Yeah get 195 55's put on like what should have been done, whoever said we can
drop two sizes in outer diameter while increasing the width by 10?

Once we drop from 60's to 50's we've done messed up the entire speedo calibration
and there is no particular calculation to easily compensate because it varies with speed and the faster you go the worse it gets, basically.

Even with the 55's there would be some discrepancy but it's minor enough not to worry about, and there's no shortage in that size...:
http://www.tirerack.com/tires/Compar...12&RunFlat=All

Same rim size too, I don't get this...

That or get your speedometer re-calibrated, that might be another option.
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Old 07-08-2008, 08:14 AM   #3
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Did you calculate in your odometer difference before figuring MPG?

You could do a rollout test on old and new tires, if you still have an old one. Dab some paint on the tire and roll it until you have two marks on the ground, and measure the distance between them -- that's your circumference. Do it at the same pressure and load; it would be close enough if you just let all the air out of a new one and take it off the car, I don't think you need to dismount the tire from the wheel.

About your 9% mileage difference when the calculator figured 4.5%:
- You forgot to figure in the 4.5%
- Sizing inconsistency between manufacturers
- The different size may have had a bad effect on your gear ratios
- Wider tires are less aerodynamic
- New tire model may have higher RR
- Tires with new tread have higher RR than worn tires (according to more than one studies)
- New tires may be heavier

Is the tire pressure the same as before?
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Old 07-08-2008, 10:01 AM   #4
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I wouldn't really worry about the difference in circumference as much as how accurate the odometer is with the current tires you are using. I used google earth to calculate an accurate distance between 2 fwy off ramps on my commute that are about 25 miles apart and used this to determine how accurate my odometer is.
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Old 07-08-2008, 11:03 AM   #5
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Quote:
Originally Posted by mrmad View Post
I wouldn't really worry about the difference in circumference as much as how accurate the odometer is with the current tires you are using. I used google earth to calculate an accurate distance between 2 fwy off ramps on my commute that are about 25 miles apart and used this to determine how accurate my odometer is.
Interesting idea, but Google Earth/Google Maps/Yahoo Maps/MSN Mappoint/Mapquest don't always accurately measure distances, especially between addresses (if you are actually looking at the satellite images, measuring (with a ruler or an on-screen tool) and using the scale of the amp, it should be fairly accurate.

A better bet would be to use a GPS with a track recording function, if you've got one. Basically, press start, drive around, press stop, compare the GPS measured distance to the odo, and you'll have a highly accurate measurement. Repeat the test 3 times and you can probably get down to the hundredth's place.
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Old 07-08-2008, 11:21 AM   #6
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I use 50 miles of mile markers on I95 north between Richmond and DC. My car is .5% low.

I just add .5 mile to each 100 when I fill up.

So mine is reading 99.5%.

regards
gary
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