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nicholasandrews
08-18-2010, 01:31 PM
Hi Everyone -

Random question for you all. I recently lifted my 80 and put on 33 inch tires and was wondering what affect this will have on my odometer reading (i know it's off, but wasn't sure by how much)? I've heard and read that putting on 33's will have a 5% error rate on the speedometer if you are taveling at 100 mph so you are actually doing 105 mph (like my 80 could go that fast...) but i wasn't sure what the affect would be on my odometer - if its something similar to the speedometer - or something else... Does anyone know?

Thanks,
Nick

corsair23
08-18-2010, 05:32 PM
Nick,

It will be a % that both are off. In my LX with 286/75/16 Toyo MTs IIRC the error was around 4% under. i.e. both the speedometer and the odometer would read about 4% lower than actual.

You can get new speedo gears to correct that (or get close) from Slee etc. Or you can get fancy and get a Mark's speedo correction box or similar. Or you can do what I did if you have a Scangauge II and have it display your correct speed. Or you can just know in your head about how far off you are :D

Since not all tires are the same the best way to know for sure how far you are off is to use a GPS and then check at various speeds and do the calculations so you know. If you track you MPG then you need that to "up" the number of miles driven to get an accurate number.

IMO knowing what I know now having done most of the above I'd just go straight to buying a speedometer correction box. Buy one that allows you to change the settings so if you go up to 35s or regear down the road you can change the settings to match. Not such a big deal with 33s but sending the correct speed to the ECU does help with transmission shifting etc :thumb:

nicholasandrews
08-18-2010, 10:09 PM
Thanks for the advice!! i was using my high school geometry to try and figure this out and got an error rate of roughly 6%... see you on the trail or at the meetings! :beer:

Nick

Rzeppa
08-19-2010, 09:35 AM
Due to numerous factors, it is rather difficult to accurately calculate the difference between actual and indicated speed and miles when using tires that are different than stock. What I do is calibrate my offset every year when I go to Moab. I write down my indicated odometer reading when I hit milemarker 100, and again when I cross the Utah line. The reason for the long distance is to give good resolution in the measurement. From these observations, it is then easy to calculate the proper correction factor. This varies every year according to tire wear.

Even with proper odometer correction, that still doesn't mean the exact same correction applies to the speedometer, even though it is close. Whenever driving past those radar trailers the cops put out, try to maintain a steady speed to get a good idea of your offset.

A GPS can also be used for both mileage and speed correction.

nakman
08-19-2010, 06:03 PM
One you go 4.88 gears and 35's the error will swing the other way, so you'll get a chance to make up all those miles.. :)

Corbet
08-19-2010, 07:13 PM
6% is what mine is off with 295's (33.4")

I pretty much make sure I'm driving the posted limit on my speedometer and figure I'll get away with the little extra. If you are in the habit of driving 5 over you'll get caught eventually with the extra your not seeing.

And yes the odometer will be off too, but in your favor.

Corbet
08-19-2010, 07:15 PM
In my LX with 286/75/16 Toyo MTs IIRC the

Jeff, where did you find 286's :D

corsair23
08-19-2010, 10:24 PM
Jeff, where did you find 286's :D

Walmart...They 'wuz on special :hill:

Rzeppa
08-20-2010, 11:38 AM
And yes the odometer will be off too, but in your favor.

Only if you plan to sell your rig. It will be off to your detriment if you base your maintenance intervals and fuel economy calculations on the uncorrected odometer readings ;)

In fact, the main reason I do my annual calibration for my correction factor is so I can get accurate fuel economy readings.

nicholasandrews
08-20-2010, 12:26 PM
I definitely appreciate all the advice and stories from your own experiances. I started to look at the correction so I could figure out my mpg.... i don't plan to sell this truck, I already have too much regret for getting rid of my other 2 toyotas!

simps80
08-20-2010, 03:41 PM
i went ultra low tech/intelligence :D

drove around on 44th between ward and kipling couple times where they have the speed limit signs with the radar on them, compared what radar sign said to my speedo at a couple speeds