YotaBum

I know that lifting a truck and putting bigger tires on there will throw off the speedometer, but what about the tachometer and the odometer? Sometimes I'll give a little stomp on the gas and it will feel like I'm reving a lot higher than it's showing. Also I've been wondering: I'm going faster than my speedometer says, so theoretically I'm covering more ground than my truck is registering, right? Somebody help me out, is my thinking right? Appreciate it.
--Alex

85TurboRunner

your odo and speedo will be off but your tach isnt affected.

YotaBum

You wrote tach twice. Which was supposed to be odometer? Thanks for helping...

AxleIke

your odometer and spedometer are measured off of your transfer case. They are geared such that when you are running the stock tire size, both are accurate. When you add a different tire size, you have changed the final ratio, and both your odo and speedo are off.

Your tach is measuring the engine, and is no way affected by larger tires or lift. Your truck will have to work harder if you didn't regear the diffs when you went to a larger tire size, but your tach is still reading accurately, regardless.

AH64ID

????

tach is not affected... like AxleIke said, your odo, trip odo, and speedo all get the signal from the same place. With lager tires you are going faster, and covering more ground, than indicated.... No its not a good enough reason to get out of a speeding ticket

YotaBum

When it says I got 220 miles to the tank, I'm actually getting a bit more?? But more so, when it says 77,000 miles on the ODO there maybe more than that?? Will one of those superlift speed recalibrators take care of both the speedo and the odo?? Thanks...
--ALex

AH64ID

yep and yep... and depends if your speedo is electric or cable... if electric that will work, so will the dakota digital one, and if cable you can get an adapter spliced in from Iss Pro is portland...

CJM

Generally it wont be off enough to make it crazy though. If your truck or runner came with 31's and you up to 33's that isnt enough to make you worry depending on your axle ratio.

But yes they will be off, enough to worry about, not really unless you go up to extreme sizes.

Am I doing this right???

I have 31's and the original tires are 235x75x15's, thats a 1.07" difference over all (31" / 29" = 1.07). I get 16-17mpg, but on avg its 16.5, 16.5 x 1.07 = 17.65mpg (if I am doing this right). This means I am getting 330mpg per 20 gallons in the tank (its really 24gallons, but the light comes on and I fill at 20). so, when in reality its 17.65 x 24 = 421 miles to my tank?

Beast

4.10 gears and 33" is a 10% error
50 is really 55

Beast

does it feel like your clutch is slipping?????????

YotaBum

Its got an auto. tranny. Yeah I think thats about exactly how off I am. I drove past one of those police radar signs, my speedo said 50, the radar said 55. So basically, my trip odo is 10% off also. So really I'm getting more like 240 something to a tank instead of the 220ish?

AH64ID

I am going to assume you mean 7% not 1.07"...

First off, your math on mph is really weird... you dont take mileage times gallons to figure out miles... its miles/gallons filled (not tank size or how much when the light comes on), then you take that number times your tire correcter... and if you want to know miles its miles times number correcter...

Tires are never what they really claim to be... so use mfgr numbers.... a 31" BFG is really a 29.8" tire (at 45mph), and the oem 235's are 28.1" (at 45mph). So the difference is 1.7"/28.1" = 6%

So if you drove 330 miles (not mpg) you really went (330*1.06) 350 miles. 350 miles/20 gallons (or whatever you acctually PUT into the truck... not tank size, like you did with 24) = 17.5mpg. Or you can take 330/20 = 16.5... 16.5 * 1.06 = 17.5mpg...

AH64ID

What size did your truck come with? And what size are you running now?

YotaBum

265/70/r16 and right now I'm running 305/70/r16.

AH64ID

Based on rolling diameter... should be 6.66%off

CJM

Thank you, I figured I had the formula wrong but didnt know for sure. Funny thing is my speedo is spot on b/c I went by several of those police signs and it said the exact mph I was at. Truck was listed as having 235's on the jamb but the window sticker says 31's and it has 31 hankooks which are actually 30.5 in dimaeter.

AH64ID

Yeah, I found that with 4.88's and 35's my speedo is w/in 1%.. which is funny because there should be more difference there... With 31's and 4.10 my speedo was WAAAY off thou...

CJM

Yea it is odd, I have 4.30's and 31's and it isnt off. According to those calculators that 4Crawler has if I upped to 33's it would be off by 3mph.

Btw, whats the formula exactly for calculating corrected mileage for future refrence?

AH64ID

I assume you mean 3% not 3 mph..

I use my GPS and at 50mph double the difference... 100 would be most accurate, but illeagl and highly unlikly on 35's ... Even down hill....