AznSky

I was wondering if regearing affects your speedo or odo. Reason being is because from straight calculation, I would be getting about 13 mpg, but I really believe that's impossible. But factor in the difference in tire size, I will be getting 16 mpg. But that's using the old gear ratio, does the new gear ratio affect it at all?

In my case:

Stock: 28" tires on 15" wheels, 3.90 gears
Now: 33" tires on 16" wheels, 4.88s

Say I get about 200 mpg and fill up with 14 gallons. Would I just take the tire change into affect? Or also the wheels and gears?

Gracias

Roadtripr

most likely it will have an effect. The easy why to test is to bring a GPS with you when you drive, that should tell you how much your speedo and odo is off.

lee

andy, is there anything you CANT do in excel?

bamachem

lol, not really... that's what us engi-nerds are good for... running numbers.

Flamedx4

Did a 96 really come with 3.90s? First I ever heard of that.

If you don't have a GPS, here's how we checked the speedo in the old days. Hold a a steady speed and time a few mileposts. Just one is not enough - they are not precisely located (gasp!) Do at least three. If for example you held an indicated 60 mph, then you should have traveled 60 seconds for each mile. 65 mph = 55 seconds per mile, 70 mph = 50 seconds per mile, etc. Simple as can be.

firemedic

On the spreadsheet, if you plug in "100" in "odo miles", you can use the "actual miles" as a percentage figure, which makes it easier to correct mpg for various mpg calculations.

bamachem

good catch. table updated and reposted...

bamachem

waskillywabbit's 00 sr5 came with 3.90's. lot's of the "base" sr5's came with them. the sport and highlander sr5's came with 4.10's or 4.30's IIRC

firemedic

Nice work Andy!

AznSky

yea that's pretty sweet. I dont' know why I didn't think of the basic old vs new relationship. Especially since I'm an engineer in training (ie college)

Flamedx4

cross multiplication is the most valuable tool I learned in school. (We had quills on papyrus scrolls in those days...)

AznSky

one thing I don't get though, is that I had my mom drive right next to me in her rental vehicle (parents are visiting) and when my speedo shows 70, she was doing 78.

So if my speedo is lower than the actual speed, shouldn't my odo be lower than my actual speed? I think there must be more that goes into account

Flamedx4

Heard stories like that before, and there may be some electronic stuff going on with an automatic transmission (if you have an auto) on some later models. Don't know for sure, and not sure I heard that about Yotas or some other cars, but there are some cars where the odo is not mechanically driven with the speedo and instead is a computed function or something.

You did not say how you checked the odometer - are you sure it's off?

One more reason to add to the list of why the 85 4Runner is the best Yota ever made!

Flamedx4

BAMACHEM - your math is wrong. [[if you have 285/75-16 revos, then they're supposed to be exactly 33.0" in diameter. if you have 4.88's and assuming that your speedo and odo were correct when you got the runner and it had 3.90's with 28's (225/75-15's), then when your speedo shows 70 MPH, you're actually doing 62.6 MPH.]]

If you had 3.90s and 28s and went to 4.88s and 33s, when your speedo reads 70 you will be doing an actual 74.32 mph.
Aperfect match would be a 4.596 ratio, but 4.88s are better (perfect actually in the real world) to help overcome the additional losses due to the big tires and increased frontal area, etc.

ANDY - your second chart should start with 4.10s with 28s (225s) 4.56 with 31s was the option.

firemedic

I think Andy is close, when going from 28" and 3.90 to 33" and 4.88 you will be going 94.19% of the indicated mph, ie. actual 94.19mph with an indicated 100mph, or 65.8mph at an indicated 70mphThe odometer should also indicate 100 miles for 94 actual miles traveled.
I double checked Andy Here

green91runner

this was posted in another thread today.. (can't remember by who) just a lil assistance for those of us who don't feel like pulling out the calculator

http://www.offroaders.com/info/tech-...alculators.htm

another useful one...

http://www.1010tires.com/TireSizeCal...?action=submit

oh, and...
i'd almost kill to get that kind of mileage

photoleif

i posted it here, fwiw. is that the one you saw?

while we're kind of on the subject, could someone point me to how i can determine my crawl ratio? i see from that calculator page above that i need the transmission ratio, transfer case ratio and the axle ratio. i admit my ignorance... can i find this by a combination of visually examining things under the vehicle and reading the door sticker? i did a bit of searching, found this YT post, and i knew about the gear page on brian894x4's site, but don't quite get how to determine the transmission ratio and transfer case ratio. i do know i've got 4.56s (that's axle ratio, right?), from decoding the door panel sticker against his list. thanks for a ref.

DOT3

I thought his question was:

Will changing his gearing affect his speedometer?

We know changing tire size will.

firemedic

EDIT: I guess not on a 3rd gen, according to Bamachem

bamachem

Background info:
Well, on third gen runners, it was either 3.90 w/ 225/75-15's, 4.10 w/ 265/70-16's or 4.30 w/ 265/75-16. Those were the only combinations of gears and tires from 1996 thru 2002. The 1996-1998 3rd gens have a mechanical odometer. The 1999-2002 models have an electronic odometer.

The 4.88's overcompensated for the larger tires. That means that his axles are now turning slower than stock at the same driveshaft/engine RPM, but when you factor in the diameter of the tires, they're turning faster than stock when you make a "feet traveled per engine RPM" comparison.

Look at it this way, Let's say that the engine is at 2000 RPM. The trans is a 1:1 ratio and the XF case is a 1:1 ratio to make it simple. Therefore, the driveshaft is turning at 2000 RPM. Now, lets assume that the two gear ratios are 4:1 and 5:1 and the diameters of the tires are such that one has a 9' circumfrence and the other has a 10' circumfrence.

So:

Stock Gears and Stock Tires:
2000 RPM Driveshaft (4:1 Gears) = 500 RPM Axle (9' Tires) = 4500' per minute is the travel velocity or approx 51MPH.

Stock Gears but with Larger Tires:
2000 RPM Driveshaft (4:1 Gears) = 500 RPM Axle (10' Tires) = 5000' per minute is the travel velocity or approx 57 MPH, but the speedo still reads 51 MPH as before.

The speedometer will read the same thing on both of these cases since the gears and driveshaft RPM are the same no matter if it's electronic or mechanical or measured at the XF case or the axle via the ABS system.

now, regear to 5:1 with the 10' tires and...

Regeared and with Larger Tires:
2000 RPM Driveshaft (5:1 Gears) = 400 RPM Axle (10' Tires) = 4000' per minute is the travel velocity or approx 45 MPH actual speed.

Now, if the speed is measured at the XF case output flange, then the speedo will read the same as the original case (51 MPH) but the actual speed is only 45 MPH. This isn't the case with anyone w/ a 3rd gen who's regeared, so therefore, this must not be true.

If the speed is measured at the Axle via the ABS System, then now the axle shaft is turning slower even though the driveshaft is at the same speed. Now the axle is turning at only 400 RPM instead of 500 RPM. Go back and calculate what the speed should show w/ the original gears and the original tires. The distance traveled is 9' * 400 RPM or 3600' in one minute. That's approximately 41 MPH according to the speedometer, but the actual speed is 45 MPH.

In other words, on a 3rd gen runner, the speed is measured at the axle via the ABS system. With that said, Regearing will have an effect on the ODO accuracy ONLY. Regearing has ZERO effect on the speedometer accuracy since the senser is at the axle and does not care what gearing you have. On an older rig where the speedometer sensor is in the XF case or is mechanically driven from the XF case, then regearing will have a overall positive effect on the speedo accuracy as long as the gearing isn't overcorrected.

In this case, we don't know if the ODO is on a different setup as the speedo. I just proved that the speedo is electonically driven via input from the ABS.