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Changing from 205 to 215

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RickB View Drop Down
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    Posted: March/23/15 at 11:46am

My understanding is that with the stock size tires, 205/45-17s, the speedometer is off by 2%-3%.

By switching to 215/45s, the speedometer is supposed to be more accurate.

With nearly bald 205s, the speedo read 73mph at 3000rpm.  With the new 215s, which must be something like 3/4" taller, the speedo reads more like 71mph at 3000rpm.

Is it likely that before I was doing about 71 when the speedo said 73, and now I'm actually doing 71 when the speedo reads 71?

I did a 600mi road trip to Oregon a couple of years ago, and averaged 33mpg (odometer miles divided by pump gallons).  I made almost the same trip a couple of weeks ago, and my calculations resulted in an average of not quite 31mpg; is the change due speedo/odo differences from worn tires? 

If I made the trip again with the new tires, what effect should I expect on the calculated gas mileage?


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Post Options Post Options   Thanks (0) Thanks(0)   Quote CORNERS Quote  Post ReplyReply Direct Link To This Post Posted: March/23/15 at 3:48pm
Speedometer is more accurate but if you are at all lowered (or even if not) you will get some slight wheel rub in the rear.

I have 215s and nearest I can tell it's pretty much spot on.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote SMOG Quote  Post ReplyReply Direct Link To This Post Posted: March/23/15 at 5:51pm
I have 215's on my R56 and according to my Garmin GPS it is exactly 2mph off at any speed.  When the speedo is saying 65 I am actually going 63 according to the GPS.

It stood me well going across the USA last April to drive The Dragon.  80-90mph stretches and blew by the local constabulary at 5mph over posted limits and not even a car pulled out to follow.  Pretty reassuring experience.

215x45x17 on factory rims with my winter tires(Dunlop Winter Sport) on OEM rims.  I have Bridgestone AS 960's that are the same size on my other summer rims.


Winter tires 215


Summer 215


Edited by SMOG - March/23/15 at 5:55pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote racingbmwm3 Quote  Post ReplyReply Direct Link To This Post Posted: March/24/15 at 7:46am
download a speedometer app for your phone to see what your actual speed is, it's not worth guessing.  I've noticed the 'analog' speedometer to have a built in fudge factor.  The speed in the on board computer is a little more accurate.  

You gas mileage will be affected by air temp, rain, tire compound, tread depth, tread pattern, tire pressure, and size.  Don't forget that by using a larger tire, your Odometer may be either more accurate or less accurate and you'll also want to calibrate that using your GPS.  Ideally you'll want to start your trip mileage and GPS at the same time and measure a distance of at least 20 miles, preferably straight and flat.  The further the more accurate.  I've noticed variations of up to 3% between different tires.  To make it worse, when my speedometer is most accurate, my odometer is off by ~2%.  When my odometer is spot-on, my speedometer is off 3mph.  
....And, worn tires weigh less than new tires, so that will have more effect on improving gas mileage than the smaller diameter has on reducing gas mileage.

Short answer, yes, your gas mileage difference is likely because of the odometer difference in changing tires.  
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Sixate Quote  Post ReplyReply Direct Link To This Post Posted: March/24/15 at 7:49am
I found that 205 = 3 MPH over on speedometer vs GPS and 215 = 2 MPH over in a 07 R56.

I would consider it a safety factor to prevent speeding tickets, but the police don't take the difference between actual and speedometer indicated speed when you are doing 85 or 88 MPH!  Thankfully, I haven't had to use it as an excuse in the Mini (knock on wood!)
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Post Options Post Options   Thanks (0) Thanks(0)   Quote RickB Quote  Post ReplyReply Direct Link To This Post Posted: March/24/15 at 1:07pm

Interesting info, thanks.

The only time I've been pulled-over for speeding was 35-in-a-25, in 1978.

I'm pretty selective about doing more than 5-over, which drives my wife crazy.  A former-cop buddy says 7-over is generally safe on Washington freeways. 

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Post Options Post Options   Thanks (0) Thanks(0)   Quote racingbmwm3 Quote  Post ReplyReply Direct Link To This Post Posted: March/25/15 at 7:26am
Going the same speed as everyone else is the safest :)

Out here on the Peninsula, everyone goes the speed limit or slower, which is lame because it's 50 and 55.  I always love going back to Seattle because everyone is always doing 70-75 :)
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Post Options Post Options   Thanks (0) Thanks(0)   Quote SMOG Quote  Post ReplyReply Direct Link To This Post Posted: March/25/15 at 11:30am
Originally posted by RickB RickB wrote:

Interesting info, thanks.

The only time I've been pulled-over for speeding was 35-in-a-25, in 1978.

I'm pretty selective about doing more than 5-over, which drives my wife crazy.  A former-cop buddy says 7-over is generally safe on Washington freeways. 



Same boat here about the pushing the limit issue. 

As to tickets I have not had one since the late 60's but back in the day I got one for 94 in 65 driving a hotrod Triumph, and 40 in a 25 with a bicycle!  Hard nosed cop was pissed that I was racing a car in traffic and winning.  I could keep the speed while the car had to slow down.  I was in great shape riding/pedaling 15 miles up and over some hills each way to and from work and then body surfing on my lunch break.  I think the same cop got me for turning right on a red light without coming to a full stop on my bicycle.

The last time I was pulled over was in a VW Bus for going to slowly up a hill at 0300hrs and zero traffic on the road.  He just wanted to see why we were going so slow and the 4 of us in the van almost responded in unison "whats the hurry, are we blocking traffic?".  The cop was the only other car on the road.  1970 Carmel was pretty dead at 3 am.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote RickB Quote  Post ReplyReply Direct Link To This Post Posted: March/28/15 at 11:03am
Originally posted by racingbmwm3 racingbmwm3 wrote:

Going the same speed as everyone else is the safest :)

Out here on the Peninsula, everyone goes the speed limit or slower, which is lame because it's 50 and 55.  I always love going back to Seattle because everyone is always doing 70-75 :)

"Safe" meaning not likely to get pulled-over for speeding, not a reference to relative speed in traffic.


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