My spedometer is reading faster than it should be...but it's not consistently faster...it gets worse and worse the faster I go. I can be doing 45 on my speedo, but I'm really only doing 40...but on the other hand, my speedo is saying 110 when I'm at like 97.

How do I fix this?

Thank you very much in advance,
Collin

 

Simple solution: Learn what RPM correlates to what speed.

 

Stock tire size?

 

I should have stated before...the reason it's doing this is because I'm running 205/40-16s as opposed to the stock 185/70-15s. BIG difference.

Now I guess my real question is...how do I fix this without changing my tire size.

Thanks,
Collin

P.S.: wotnartd...that's not fixing it...

 

You should be running 205/55/16 (837 Revolutions per mile)
Stock size was 205/60/15...? (834 Revolutions per mile)

The tires you're running now go 927 revolutions per mile

 

uh, ya, tire size doesnt matter. im running 225/45, a little change doesnt matter, i would say tire size if you were running 35" mudders, THAT would mess things up. it sounds like your speedo cable is off my a tooth. try this, get under your car, and make sure the speedo cable is attached correctly to the tranny, grab the plastic collar and give it a soft twist, if nothing happened, thats not the problem. after that, get behind your guage panel and check the cable into the cluster. if its not off, then i have no idea!

peace

 

^^

 

sorry man but ~90 revolutions per mile is no small change. it's about 10% in this case. Hence being about 10mph off at about 100mph...

 

Unless you know someone who manufactures custom ratio speedometer gears for the FC, you don't.

 

LOL... I was thinking "do you have an engineering degree?"

 

This Mazda guy I know told me I can get the speed sensor out of a miata and use that...does anyone know anything about that?

 

uh, if you think about it, it would only be .01mph off, being 1% change, so no, it doesnt matter.
do the math, if you are running 205/55/15, the sidewall would be 3.72 inch, for a grand total of ~19 inches. ok, now, if i am running 225/45/17 then my total tire size would be ~20-21 inches. if you think about it logically, at ~90 rpm, that would only be "off" by less than 2%. like i said, if you are running 35" mudders, THAT would throw it off by a little more than 11%. a toyota 4x4, running 16" wheels with 35" tires, it results by the speedo being 5 mph off, im telling you from first hand experience.

peace

 

He's not much of a Mazda guy because thats entirely wrong in every way.

 

actually, if you would go to google and look up the tire calculator, it would show that his tires size right now is : 22.5". well the stock tire is 24.9". so your telling me that if he takes 1.5" out of his tires its not going to change his speedo...

i know it will make a difference, thats pretty much like running a stock honda civic tire on your 7



oh yea BTW i have a 86 toyota truck with stock gearing sitting on 36" super swamper tsl radials, and its off my about 15MPH... so 45 = 60.... just a lil FYI do your math!

 

k... for arguments sake... I've used the same tire in all five sizes below.

BFGoodrich G-Force Sport

205/60/15----24.7" dia----843 rpmi (stock size)
205/55/16----24.9" dia----836 rpmi (stock size)
*note less than 1% difference in dia and rpmi


225/45/17----24.3" dia----856 rpmi (your tire) ~1.5-2.5% discrepancy in dia and rpmi meh.... I could probably live with that for the extra width. but the correct size for you is ---> 215/45/17----24.6" dia----844 rpmi

205/40/16----22.4" dia----927 rpmi (OP's tire) ~10% discrepancy in dia and rpmi

 

There's the problem...

FYI Rex, I'm running 205/40ZR16 Khumo SPTs

Thanks for the insight. Can toss that math up there. I got different numbers when I did it...probably something I left out?

 

Where?

Just make a mental note 80 = X etc.

Damn well every car I've ever owned has had innaccurate speedo's due to changing wheel and tire combinations. Deal with it

 

Even if it was only 1% off...1% of 100 is 1...no .01....nice math...

 

My speedo is almost as far off as yours and I have the stock tire size. You are just going to have to deal with it as the only way to fix it other than changing the tire size is to get a custom made speedo gear.

 

Is there a way to get your speedo 'recalibrated'? I'm running light vert wheels with 50 series tires so i'm sure the speedo is off a little.

 

The FC speed "sensor" is built into the transmission and is mechanical and isn't the same as a miata.

I guess if it's off you have to "deal with it." Here's a tip though: if you can find a friend or anyone with a new Civic, just look at them while they're driving next to you and figure out what RPM corresponds to their GIGANTIC speedometer built into their dash. That thing is visible from miles away. I mean it's HUGE.

 

You can calculate a rim/tire's rolling circumference from the tire size: (2*(section width*(profile/100))+(rim diameter*25.4))*pi - will give the circumference in mm (hence the rim size *25.4 to convent the only imperial measure into metric)

For example, my stock tires: 205/60-15 - (2*(205*.60)+(15*25.4))*pi=1970mm

my current street tires: 225/50-16 (2*(225*.60)+(16*25.4)*pi=1984mm, not even 1% larger than stock, so the speedometer would read 1% slower - insignficant. Tire pressures and tread wear likely contribute more to any speedometer error in this case.

with the OP's tire sizes - original 185/70/15 (I thought the 15's always came with 205/60s, but anyway) (2*(185*.70)+(15*25.4)*pi=2011mm

and the current tires: (2(205*.40)+(16*25.4))*pi=1792mm, which is 12% smaller than stock circumference, and hence speedo error will be about 12% fast - almost exactly the percentage difference reported at speeds.
You can get this fixed by having your speedometer re-geared, but that's probably not cost effective, or you can learn to mentally adjust the indicated speed down by a bit more than 10%. Or you could put properly sized tires on it, which is probably the best idea, since this is a relatively huge difference - your engine, gearbox, diff, and wheel bearings are all turning 12% more for every mile you cover, which will translate to increased wear and fuel consumption (remember if you calculate you mileage to reduce the odometer reading by 12% to get actual miles driven).

 

I'm confused how that translates to increased fuel consumption...being that it's less rotational mass...

 

Yes, there is.
Look around for a speedo repair/ calibration service...here in Denver, the same shop that calibrates the units for the State Patrol will do it for you too.
They build a little gearbox that goes between the speedo and cable to correct for wheel/tire or transmission swaps.

As an added bonus you get a certificate of calibration so the next time a cop asks "Do you know how fast you were traveling, sir?" you can say "As a matter of fact I do, dickweed".
You and all your new cellmate friends will laugh and laugh.