Quote:
Originally Posted by zachb_NW
So there are a lot of threads about larger tires decreasing MPG’s but I haven’t noticed people account the MPG’s decrease to the odometer/speedometer change due to larger tires.
Say your speedometer changes 4mph when going 60mph which seems typical. Meaning when your speedometer reads 60 you’re really going 64, this is a difference of say 7%. Meaning when your calculate you get 20 mpg you really have 21.4 mpg. When I have done this in the past I have never seen as big of a MPG drop as people have claimed when upsizing their tires.
Any thoughts?
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Yeah, you gotta make the adjustment if you want reasonably accurate mpg info.
Rounding to a whole number, adding 6% to vehicle odo is about right for 265/70/17. Tire calculators suggest it should be more like 3 or 4% but that doesn't account for vehicle odo "error".
This is based on extensive GPS testing in my '05 which was then compared to Google Earth and map programs for confirmation. With my current tires with 40,000+ miles on them, I'm adding 5%.
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