So after I went from 28-29s to 31s I've noticed that my trip meter is telling me that I am only getting 180 miles to my 3/4 tank opposed to what I was getting which was 280 miles to 3/4 tank. I was expecting a difference in gas mileage but not only does that seem excessive but I didn't go through gas that much faster honestly. Still my normal 2 1/2 weeks between fill ups.

So I know the speedo is off after tires changes (I also went from 3.73s to 4.10s so my speedo is actually pretty much correct now since I went from 28-29s to 31s, it evened itself out) but does the odometer also not read accurately? I figure since it takes more distance per rotation of the tires so it takes longer to measure a mile. Correct right?

 

I believe you are correct. your miles will actually accrue faster now because of larger tires.

 

Hummm so if I am reading more miles than I am actually traveling, say 1.2 miles for every real 1 mile, than I am actually doing worse on gas than I thought.

So with that basic estimate every 100 miles my truck says I travel I am actually traveling 80 and if I use 1/2 tanks of gas in that "100 miles" I actually only got 80 miles from that 1/2 tank.

That blows lol. It seems to be that the miles would rack up slower since it takes longer to make one full rotation of the tire. But I suck as math so....

 

I think thats right, you go further than 1 mile for each mile that the odometer reads.

 

That defiantly makes more sense


Anyone else with some math skill or upsized tires that has dealt with this

 

Changing to larger diameter tires would cause the odometer to register fewer miles than before and the speedometer to read a lower value than before.

The percentage that the odometer changes is always exactly equal to the percentage that the speedometer changes. They are geared together inside the speedometer head.

For 98~2000, changing the axle ratio has absolutely no effect on speedometer/odometer error because the speed is measured at the ring gear. For that reason, changing the axle ratio cannot cancel the speedo/odo error caused by changing tire diameter.

 

Yeah our 2.5l engines aren't to fond of bigger tires. I went from 25-28 mpg using stock tires to 20mpg driving on 30 inch tires. I'm pretty sure i lost 2-3 mpg having to use my defroster all the time and adding extra weight in the bed but my truck is a mule going up the hills in seattle.

Also I haven't calculated for the difference between the actual amount of miles i travel vs what my odometer reads, so i might actually be getting better mpg's that i think. But like you my average of filling up every week has not changed so i'm not complaining.

 

Thanks! I was hoping an expert would chime in. Didn't know about the speedo at the axle stuff. That was opposite of what I believed.

And that also means that I was correct about the odometer registering less miles. That's good, that means I am getting better gas mileage than my truck was showing. It also means that my truck will gather miles slower :)

 

I have 4.10s, aluminum rims, and 31s and I don't get to leave it in 5th as much as I did before and launch sucks but it's not too bad. I have sand bags in as well though.

 

when i went from 235/75/15 to 31/10.5/15 which is about what you did if you take your miles on your trip meter and multiply that by 1.07 that should be very close to the acutal miles you drove. so if your trip meter says you drove 100 miles take (100)(1.07)= 107 miles. There are calculators out there (maybe on TRS) that will tell you that information as to how much to multiply by.

 

You could get a tuner to correct your problem. Moar powa! And it will calibrate yo sheet! Nah mean?

 

I couldn't remember if it was that way or not. it made sense in my head when i gave it a quick thought. but as im staring at my truck across the lot as im typing this, i can see what you mean. I knew they were geared together, so they're both gonna be off.

 

PCM flash tuners such as those from SCT cannot correct the speedometer in 98~2000 Rangers because the correction is not done in the PCM. A dealer level tool is needed to access and change the revs/mile value in either the GEM/CTM (RABS trucks) or in the 4WABS Module (4WABS trucks).

 

i have an 2.3 08 with 31s and im pretty sure my odo is off by about 10% if that makes anything easier

 

http://www.miata.net/garage/tirecalc.html

 

Lol I'm not going to buy a tuner for my manual 2.5L, that's a waste. Especially since it's not that big of a difference.

7 miles is nothing.

Thanks for all the info guys!! Problem solved or at least explained

 

Ever put a GPS, in your truck? mine reads 2.5 to 3 mph fast, with stock size tires. after i put on the 33 inch tires for a 1 day fun trip, it read .5 to 1 mph slow, compared to the GPS. Our speedos read fast anyway, so you think your doing better than you are. not sure why manufactors do this, but its fairly common across the board.

 

a dealership can do the speedometer correction for you. measure the rolling diameter of your tires and tell them the revolutions per mile, and they can use a tool called an NGS (RWABS) or an IDS (4WABS) and correct this....or once i finally get ngs cards for my NGS i have at home i can do it

 

Why is it then when I put the 4.10 rear end in my speedo was corrected? I went from 28.5" tires to 31's. 3.73 ratio to 4.10. My speedo went back to where it needed to be. I've testing this not only with GPS but with my dad right behind me keeping the same distance on a back road out here.

 

For 98~00, the gearing change makes no difference in the speedometer calibration. If you look at it, the ring gear and VSS tone ring MUST spin at exactly the same speed as the average rear wheel speed. You can change the gear ratio upsteam of that and it cannot change the relationship between the tone ring RPM and the wheel RPM.

Going to a larger tire probably offset the inherent factory speedometer error and brought you closer to real speed, so closer to what the GPS reads. Take a look at post #17 in this thread.

 

I have read the whole thread before I posted. My speedo was right on before. I checked it before I put the 31's on. Then checked it again after I put the 4.10 rear in.

 

Bob is right...the tone ring is on the rear axle carrier. as bob said, the speedo usually reads fast from the factory

ALSO if you went from a 7.5 to an 8.8, the 8.8 has 108 teeth on the tone ring and a 7.5 has 102 teeth. this will increase your speedometer by 5.8% . now you swapped from 28.5 to 31" tall tires, this makes your speedometer read 8.7% slower (given that your 31's are actually 31" in diameter which im sure they arent) so overall your speedometer reads 2.9% slower than it did before you did anything. at 60 MPH ground speed, thats only 1.74MPH difference

 

So what are you arguing about? All I said was when I swapped it brought it back to normal. I didn't say Bob was wrong or anything. I was asking a question, not being a smartass.

 

I was just explaining to you why your speedo was accurate now. i didnt say it to be a smart ***, i was just explaining it for people with 98-00 trucks

 

Alright. It worked perfect for my setup since I didn't need to reprogram anything. Screw going to Ford and paying for it.