Djlarroc

I've been reading and learned that with going from a stock 265/70/16 to a 265/75/16, it corrects your speedo, but it makes your odo about 3% slow. When I punch in a 285/75/16 (which is what I have now), it says my speedo is about 7% too slow. Would that mean, that it's actually 10% slow? (according the the Toyota speedo that's already 3% slow from factory).

How does that affect gas mileage calculations? I know these trucks are not mileage champs, but I'm trying to figure out how to accurately calculate my mileage. Does gear ratio also affect speedometer/odometer? Or just power and gas use? I have the 3.4L, auto, 285/75/16, and 4:56 gears.

I'm confused.

TEPUI

Do you have a GPS by any chance? Some say that GPS speed reading is pretty accurate. Maybe with a GPS you can see how off your speedo is.

1986ToyTruckOMM

With my 86' w/ stock gears a 225 tire is stock but im running 31's. I took my gps and found out my speedo was off 2mph.

scuba

My speedo is off by 5ph, Found that out when i had a gps in my truck, im running 31 10.5's

thefatkid

To figure out your mileage you need to figure out your stock tire rolling diameter. Then the diameter of the new tire. Figure the percentage of difference between the stock and new tire size and that is your calculation.

The issue you have is that the speed is calculated at the wheels by the abs. There is only one way to correct this issue, some sort of signal converter that goes in-line between the ABS computer and the Engine computer. On older non abs vehicles it was as simple as a gear change.

Djlarroc

Thanks Brian. Well I know my truck came stock with 265/70/16 (roughly 31"), I have 285/75/16 (roughly 33"). This calculator says I'm 7.3% too slow, so that's why I'm wondering if its more like 10% too slow, since I read the trucks with the 16" wheel option never had the speedo adjusted at the factory and were already 3% slow.

I don't have a GPS, and instead of buying one or the signal converter, I'd just like to know what % off is my speedo so I can take that into account when I calculate mileage. Then again, I also read, the odo is off, when you switch tires and have the speedo accurately. I duno...

Roadtripr

No more like 4% too slow.
(However the odometer will now be showing 7% less for same distance traveled)

Before you spedometer read to fast (3%), with the larger tire it will now read slower (7% comp to stock):
with 31" when your speedo said 50mph, you were going 48.5
with new 33" when your speedo says 50mph, you will be going 52
While you are going 7% faster compared to stock. The spedometer will only be showing ~4% too slow.
with 285
speedo/actual
50/52
60/62
70/72.5
80/83

dairyjato

i have 265/75's and with the gps i was 2 mph faster than speedo says. now that 4% is nice to know i had 245/65's when i bought it and thought i was on crack b/c i thought the odometer was slower when i put the bigger tire on but waas afraid to say anything for yalled think i was crazy.

MadCityRich

It sounds like the ABS sensor is controlling the speedometer. What controls the odometer? Is it the same sensor?

FWIT - I was reading about 3 mph fast at 60mph according to my GPS. When I installed 265/75 tires, it became dead-on accurate with the GPS.

MadCityRich

Roadtripr

how is your odometer now? It is wierd that the odometer works but the spedometer is off.

idodgimus

can someone provide a little more info on this? maybe some place to read up on it? I've searched and cant find anything... I've just had some 265/75/16s installed and thought i would be reading slow, but after reading this i think i might be reading correct???

does anyone know of a way to reprogram the odo to read correct?

cackalak han

I've actually found my odo was reading 2-3% higher. Going 70mph (indicated) but 68mph according to the GPS. Going to 32's, I am right on. If you don't have a GPS, you can look at those speed traps on the highway or on the streets that display your speed. Those are pretty accurate, too.