I am running a 205/40r17 on my tc. It shows that im going about 5mph faster than i really am. Is there solution to correct this? I dont want to run the stock size.

 

You can try a taller tire, it is off because the rolling diameter doesn't match up to the factory settings. The tire you have is making more revolutions than what a taller tire would.

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I know that's what's wrong but I want to keep the size tore I'm running and get an accurate speedometer reading. I guess what I'm asking is there a way to program the computer to my current size tire?

 

the inaccuracy will increase with speed, while it might only read 5 mph over at say 30 mph, at say 70 it will read even even higher than +5 mph. Your ODO and TRIP meters will also become inaccurate as you drive more.

if you are really opposed to running a wheel setup that maintains the factory overall diameter, the only solution I can think of is to buy an engine management system that can compensate for the difference. I know the APR X1 (discontinued for tC) can do this and I imagine AEM EMS could do the same. That is a VERY expensive solution though.

Alternatively, you could get a ScanGuage II. You can set it to display MPH and change the settings so that it reads the correct speed. However it will only be correct on the SG readout, not the actual factory speedometer.

 

Ok thank you I may do the sg and use that. I don't want to spend a crazy amount of money. I just wish it was easy like with trucks and the hypertech programmer.

 

Or a GPS unit with speed. If you find the stock tire circumference and your tire circumference it's quick math to find the % of error

 

I still have my stock size tires I just got tired of hearing rubbing. But I am ready to move onto coilovers and go lower. This is why I plan to keep the current tire size. I am just gathering my options to solve the speedometer from being off but it doesn't seem like there is a reasonable one. I may go back to stock size and just roll the front fenders until the rubbing stops.

 

The stock tires size is 215/45/17

running a 205/40/17 as you are results in 4.946% difference on the speedometer
when going 60 mph, your speedo will read 62.9 mph

if you were to run a 205/45/17 the difference on the speedometer would only 1.443%
when going 60 mph, your speedo will read 60.8 mph


sell the 40 series tires and get som 45 series. The difference will be negligible at that point, and you should still be stretching enough to reduce rubbing.

there isnt much to roll on the front fenders anyway...

 

Alright thank you for all your help and calculations they are about halfway wore I'll just wait until the next set to get the 45 series. Ive got 17x8 wheels too i don't remember offse off the top of my head.

 

no problem. just used a tire size calculator: http://www.1010tires.com/tiresizecalculator.asp

^internet gold right there

 

Thanks a lot man you've been extremely helpful with my question.

 

Bookmarked that on my iPhone and I'll book mark that on my laptop when I get up in the morning.