Ertified4Runner

everything I've read defies this, but...
my speedometer is now only 2-3 mph off after I got 32" BGF MTs two months ago; before it was 5-6 mph off w/my old 31" BFG ATs. I thought with the old tires that whoever owned the rig before me put on 31"s when the speedometer was factory calibrated to whatever stock smaller size it is, giving rise to the 5-6 mph off-ness. then I got the 32"s and it's considerably and constantly less off. by my logic and everything I've read, this is backwards--and I don't think the factory (or whoever prior to me) calibrated it to 33"s. furthermore, I'm pretty sure it has 4.10's, so I don't think gearing is it.
and my data gathering isn't flawed either, cuz I checked it on a straight, flat 1 mile stretch at 45 to 70 mph at 5 mph increments with the digital readout of your speed on the side of the road (it's a "safety corridor" w/a 50 mph limit--don't ream me for speeding, either: this was a scientific study), and cross-checked with my GPS (using D=RT). still both old and new tires' speeds jive, so I'm kinda perplexed.
anyone know what gives?
cheers

curly_c

i noticed that mine was off about 3 MPH at about 30-40. but i only compared it to one of those little speed tester things that the cops put on the side of the road so you can see how fast you are going. i have 4.56 gears and the stock tries size is 31inches and thats what i have.

its wierd

AH64ID

Best way to tell is a GPS, and when you say 2-3mph off what speed are you refering to? Speedos are off in a %age, so the faster you go the more off you are. If you want to know how far off you are drive 50mph, and read the speed on the gps. Double the difference, thats how much you are off. If you are 2 mph off at 45, then you should be 3 off at 70, which is 4.4% off. Things like tire pressure, outside temp, and weight will affect the accuracy of you speedo.

Is the speedo cable or electric? There are inline correction kits available for both, mabye you have one.

Ertified4Runner

I have no idea what kinda speedometer's in there.

I meant that the difference in speed (45 or 70) makes no difference in how fast my GPS tells me I'm going or the side-of-the-road digital speedometers (~3 of em)--with 31"s my speedometer was consistently 5-6 mph too fast (guess I forgot to put that in the thread), and now it's consistently 2-3 mph faster, both irrespective of large ranges of speed

Glenn

I've heard the factory calibrates speedometers to read faster than actual speed.

Ertified4Runner

do they do it up to 5 mph?

Snorkeldepth

My speedometer has always been about 5 mph fast . . . . from day one.

My dream dash has a big analog tachometer right in the middle. Everything else is digital including speed from a gps. Now, how this all meshes with the ECM I have no idea.

I've heard that it's possible to get your speedometer calibrated. Check it out.

AH64ID

And increase in tire size causes an increase in speedo error, but it is a percentage. If it just reading fast at the same speed everywhere that is not a reult of your tires. Something else is wrong.

I have never heard of being able to calibrate the stock ones, it may be posible though. When I had a cable speedo I had a reduction gear put in it to bring it back to stock. With my electric one now I am going to install Dakota Digitals speedo calibration unit.

Some MFG's do make the speedos off a little... On my 99 Ranger it was fast at slow speeds, dead on from 35-55 then slow above 55.. But I dont beleive the toyota speedo runs thru the ecu, and there were only several years where they even had elec speedos, the rest are cable and not much you can do to a mechanical signal.

AH64ID

I dont really think my (22RE) ECU uses many signals that are put on your dash. Its a pretty old simple EFI. The 3.slow might be more hi-tech but I dont really think so. But if you want that talk with Dakota Digital, they do custom digital dashes...and there stuff looks sweet. Most the stuff is just an idiot light that needs a signal, the gauges just need input signals

midiwall

When I moved from stock to 265/75/R16, my odometer went 3% low and my speedometer went dead on across the board.

I think what you're seeing is normal.

RustBucket

iTrader: (1)

If you tell me your gear ratio in the transmission and in the diff I can tell you exactly how fast you're going. RPM x tranny gear x diff gear x tire diameter x pi = inches per minute. Convert to your pleasure and there you go.

AH64ID

Assuming you know the exact rolling radius of the tire he runs, which will be based on psi, weight, temp, tread wear.

RustBucket

iTrader: (1)

Well we can propagate error if you'd like...

AH64ID

Is this kinda what your talking about

lee

my spedo is pretty much dead on accurate with the 32's on. it was about 4mph off before.

Ertified4Runner

I've been meaning to check out those gear ratios more (all I'm pretty confident of now is that it's got 4.10s, more I don't know and have been sufficiently sidetracked to not check further).

that spreadsheet's pretty cool, but why's the tire size say 33.7" when they're 35"s? or am I an idiot?

and if speedos are off in percentages at varying speeds, but mine's off regardless of changes of higher speed (I think it's pretty close below 25 mph), what might that be indicative of?

I guess if it's an error of ~5% it doesn't really matter. I've gotten used to the discrepancy, but it's still curious. just one of those things, I guess...

thanks for all the info, tho, fellers

AH64ID

You have to use rolling radius when computing speeds from tire size. Tires deflect with weight, and this makes them smaller than what they are. My 35's are really 34.7 tall, then based on rev/mile that BFG publishes they have a rolling radius of 33.7.

If you speedo is 5% off thats 1.25mph at 25mph, 2.5mph at 50, and 3.75 at 75mph. Not sure what would cause it to always be 5mph off, weird. As long as you know what it means your good. There may be a way to calibrate, call toyota.

tc

I never checked my speedo stock, but with the factory gears and the 33's, it was within 1 MPH of the GPS at 70MPH. Now, with the regear (which should have taken it back to stock), it's about 3.5 MPH off at 70 (5%!). With my 'wheelin tires on (34") it is about dead on - something like 0.5MPH off the GPS.

So - yeah, the speedo was not very accurate from the factory and they typically read fast.

Ertified4Runner

Guess that's the sorta thing that gives em personality, eh?

TC--you've said you have no wheel well rub w/yer 33"s, how bout yer 34"s? I hadta trim that plastic stuff in the wheel well cuz it would still rub w/only 32"s, but nuttin yet since BJ spacers.

tc

The 34's rub like a fat girl in corduroy pants! (to steal waskillywabbit's phrase)

I had to cut the valance, trim the plastic, beat the pinchweld flat, and clearance (read: beat the crap out of) the lower rear of the front fenders. Not too many problems in the rear.

I think most of the issues are due to the extra 1" backspacing - they would probably fit better on the stock rims.