/// changed tire size ... how to recalculate mileage ///

Andyinchville1

Veteran Member
Joined
Apr 7, 2016
Location
Virginia
TDI
2003 Jetta TDI wagon, 5 sp, 226K miles
Hi all,

I went from the factory size tires at 195 65 r 15 to
205 70 r 15.

Based on my GPS my speedometer is now more accurate Speed wise but how does that affect my mileage calculations?

I have read that the factory speedometer reads a little on the fast side relative to real speed and upsizing the tire makes it the speedo correct but I think the odometer was set to read correctly with the factory tire size ..... so now the odoneter is not correct?

so now that I updated my tire size I think I need to use a fudge Factor when calculating my mileage...

I'm not sure how my friend calculated it, but I think my fudge factor is 5%?

Meaning I take my miles and / gallons to get miles per gallon and then multiply that figure by 1.05 to get my corrected miles per gallon does this sound right?

Thanks in advance for any and all help

Andrew
 
Last edited:

jerrymander

Veteran Member
Joined
Feb 11, 2012
Location
Cowford, FL
TDI
2011 Golf Reborn Edition, 2015 Golf, 2011 Golf w/ Factory Crushed Option
I use this website: https://www.rimsntires.com/specspro.jsp and put in my values. RIM SIZE and OFFSET aren't important for this calculation.

If I put 195 65 15 in the old and 205 70 15 in the new, it gives me this information:

OLD: 635 mm (25'')
NEW: 669 mm (26.3'') - 34 mm (5.4%) taller.

OLD: 807.1 revs per mile
NEW: 76631 revs per mile.

This means that your car rotates 5.3% less than it did before to cover the same distance. If your car says you drove 1000 miles, you really drove 1053 miles. Thus, any mileage your car tells you will be inaccurate: you need to multiply what your car says by 1.053. This is exactly what you have, so you should be good!

My VWs have all had a 0-6% lower indication on the speedometer than what I was actually doing, according to my GPS measurements, but the odometer has always been 100% accurate.
 

jmodge

Veteran Member
Joined
Jun 18, 2015
Location
Greenville, MI
TDI
2001 alh Jetta, RC2 5speed daily commuter and 2000 alh Jetta 5spd swap, 2" lift, hitch, stage 3 TDtuning w/502's backroad cruiser
My 205’s put the speedo spot on and the odometer 7 miles light per 100. So when calculating I multiply the odometer miles by 1.07, if yours is short 5%, multiply by 1.05
 

Andyinchville1

Veteran Member
Joined
Apr 7, 2016
Location
Virginia
TDI
2003 Jetta TDI wagon, 5 sp, 226K miles
HI,

Thanks for the input guys, A friend of mine somehow calculated the 5% error factor to increase my hand calculations by ... I wasn't 100% sure on the math involved but I am happy to see at least 2 people came up with a similar fudge factor.

Thanks for the input .... I feel better about my fuel mileage now! ;-)

Andrew
 
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