I was checking yesterday and my speedo reads higher then it's going . At low speeds its almost right. From 30-50- it reads 5 more and from 60-100 reads 10. I know tire size has to do with this but I do have a g37 diff with 3.7 gears I believe so that could also be the problem. Anyone know of a way around this. Or if these spedo corrector would work. btw tires are the same but they are 40 instead of 45

http://shop.12oclocklabs.com/index.p...&product_id=59

http://www.indysuperbike.com/custome...oductid=112818

 

you have the right idea.

 

Just trying to see if anyone has ever used one of these or if there is a better one

 

Speedo gets the signal off the rear wheel speed sensor. The gear ratio does not matter, just the tire height. With a 40 series sidewall you should have a 275 rear tire to keep it accurate.

Abs and TCS use the same sensors

 

I thought it came from all 4. And with a 3.7 gear it revs higher (obviously) I thought it also changed speed. I'm saying this because I have never seen a car have such a difference in speed from 5 inches. I had a cobra before with 305/40 in the rear and speedo was off by like 2 mph

 

I run a 285/35-18 and have a 3.7 rear gear set. It's just the tire height that does it.


What rear tires do you run.

 

What matters is the tire circumference (or height, if you want to look at it from that perspective) as compared to the original tire circumference, which is what the speedo is calibrated to.

A tire with a larger circumference (i.e. taller) will have to turn fewer times for any given distance (let's say a mile), so the speedometer will read slower than the actual speed of the car.

Now, the interesting part - you said your soeedo is reading fast, so your tire circumference is smaller than the original tire. Your tires have to turn more times to move the car any given distance, as compared to stock.

Finally, as stated already, rear ratio will change your tachometer reading for any given (actual) speed, but the speedo reads off the rear axle shaft sensors, so its accuracy is only affected by tire changes.

 

After owning many vehicles and doing tire&wheel mods,you are experiencing a common problem.without knowing the entire size of your 40 series tires...should be 275's to make the speedo right,its hard to determine the cause of your problem.I'm running 19's.My rear tire is of the right size.However,my speedo reads lower than the actual speed.
If the speed calibration is a PIA to you,I'd get it fixed.Most guys would just live with it.
I would...

 

The tires I'm running are 225/40-18 front and 245/40-18 rear
The only issue I have with the speed is that it also messes with the odometer reading and I'm not a big fan of that.

 

Well, there's your answer. Stock rear tire size is 245/45-18, which is a larger circumference (taller) tire.

Plug your numbers in here to see the differences in tire sizes:

http://tire-size-conversion.com/tire-size-calculator/

 

I did but since both my tires are
Off the percentage that it's off is bigger . So what size tire should I get to fix this. Obviously stock size but if I get a 275/40 wouldn't it be the same as 245/45?

 

275/40 is what you need

 

OP this is your answer, i just didnt think they were gonna spoon feed you lol

 

Yeah I know I just wanted to double check lol. But last thing, front diameter then does not matter since the reading are from the rear right?

 

I think I may have mentioned that

 

Ok cool in that case I guess when It's time for new tires ill just get the right one thanks guys. I might just buy the speedo corrector just in case I need it in the future. Just gotta find out how to wire it

 

yes you did.

 

you would probably need 4, one for each wheel. If you alter just the one rear that the speedo runs off, ABS/TCS/VDC will not like you. All would have to be altered.

I would love it if UpRev could crack the code and allow for change.

 

Yeah same here. Yeah I'm going to change all 4 but ill make sure I get all the right size. Prob 235/245 up front. And I only have abs and I'm not too worried about losing it.

 

When you lose it, the rear gets a heavy bias and you need to remove the ABS control unit and install a proportioning valve