speedometer / odometer accounts for decrease in MPG's?

[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?

 

[ARK_SWAMP_RIDER]

I agree. I installed a speedohealer after adding my 285/70/17 duratracs and unless I'm doing a lot of stop and go driving, my gas mileage has changed very little. I keep records every time I fill up. A recent trip netted an even 20 mpg. My odometer was off by 8.9%, which makes a substantial difference in the mpg.

 

[DC runner]

Your car, brand new, is allowed a variance of up to 10% in speedometer accuracy.
This means that when you adjust your wheel size beyond what was factory standard, your maximum variance could potentially be greater than 10%.

 

[nevada]

Your car, brand new, is allowed a variance of up to 10% in speedometer accuracy.
This means that when you adjust your wheel size beyond what was factory standard, your maximum variance could potentially be greater than 10%.

yup.
altho i thought it was 7%, but yes.
also, it can read fast, but never slow.
meaning, showing 60, but going 58. not the other way.

 

[03_4x4Runner]

The best way to see how far your speedometer is to use a gps to see what the speed really is. It will be off more the faster you go.

 

[JB.]

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?

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%.

 

[4eigner]

This was going through my head the whole time while sifting through the sea of posts of those claiming mpg loss after upsizing tires. Why isn't this info in a sticky (not just 4th gen specific)?

 

[BlackWorksInc]

I thought it was obvious knowledge?