2nd Gen Non-Engine/Transmission Speedo/Odometer Correction for Tire & Gear Change

[Noseridorr]

I just switched out gears and tires on my truck. I am just about through my first full tank and am wanting to check mileage. I have been to all of the tire/gear calcs on the net, and I can determine what is is if I just switched either the tire/gears. But I switched both at the same time, can anybody help me out here? Originally the truck had 245/75/16 tires with 3. 54 gears, I now have 315/75/16 with 4. 10 gears. Any help would be appreciated.

Thanks,

Jason Orr

 

[Bart]

I believe Geno's garage has tables that help in this area. Email if you can't find it and I'll dig it out. I used them just a few days/week ago. Geno's is associated with this web site.

 

[Noseridorr]

Am I making to much of this? Genos is not the place for this info. My speedometer/odometer was off after going to 4. 10's than I added 315's my little pea brain cannot process this this calculation.

Thanks,

Jason

 

[nps]

Your tires result in a 13. 5% change.



The swap of the rear end results in a 15. 8% change in the opposite direction.



Therefore your net change is 2. 3%. Your spedo will show you going 2. 3% FASTER than your actual speed - no worrys about tickets.



Thus 60mph on the spedo is really 58. 6 mph. Likewise, if your odometer shows you traveled 100 miles, you really only went 97. 7 miles.

 

[bkroupa]

I just swapped from 4. 10's to 3. 54's & the speedometer read exactly the same compared to the GPS as it did before. I started to freak out thinking I just swap in the same gear, but I realized the rpm was a lot lower. The sender for the speedo is on the rear axle (on my '98 anyway) so it's just looking at the speed of the axle & gear swaps don't change the calibration. If yours is like mine, you only need to compensate for the tire size & your speedo would read 13. 5% slow.

 

[nps]

I just swapped from 4. 10's to 3. 54's & the speedometer read exactly the same compared to the GPS as it did before. I started to freak out thinking I just swap in the same gear, but I realized the rpm was a lot lower. The sender for the speedo is on the rear axle (on my '98 anyway) so it's just looking at the speed of the axle & gear swaps don't change the calibration. If yours is like mine, you only need to compensate for the tire size & your speedo would read 13. 5% slow.

His signature says he has a '97 12V so his will be different.

 

[bkroupa]

Mine's a 12V also, what year did they change?

My speedo is off 10% because of the tires, does it have to be flashed to be corrected?

 

[nps]

Mine's a 12V also, what year did they change?

My speedo is off 10% because of the tires, does it have to be flashed to be corrected?

Believe '97 was the last year for gear-driven spedos. Guess they had the electronics ready for the 98's but started putting in the 24Vs mid-year.



You don't need a reflash in the engine control programming sense, rather the setting of the tire diameter parameter. The dealer can set it, or you can get a terminal to set it also - the Superlift Truespeed is one such device.

 

[Noseridorr]

Thanks for the help.

 

[Bart]

Sorry about Geno's. I got the papes out the the site is

http;//dodgeram.org Sounds like some pretty intelligent folks got you lined out anyway.

 

[jaybigboy34]

I am dumb when it comes to these calculations. I have 3. 54 gears in my truck. I changed to a 325/70/18 tire. Could someone tell me how fast I am actually going if my speedo says 60mph? Also, is it true that my odometer will be off?



Thanks

 

[nps]

I am dumb when it comes to these calculations. I have 3. 54 gears in my truck. I changed to a 325/70/18 tire. Could someone tell me how fast I am actually going if my speedo says 60mph? Also, is it true that my odometer will be off?



Thanks

What was the original tire size? Yes your odometer is off.

 

[jaybigboy34]

What was the original tire size? Yes your odometer is off.

My original tire size was 245/75/16. Also how much will the odometer be off?



thanks

 

[bkroupa]

It's a straight percentage. If your old tires are 30. 5" tall & the new ones are 36 (that's a guess) 30. 5 divided by 36 equals . 847222 or ~85% so your speedo would be reading 85% of your actual speed. example 60/. 85 = 70. 58

 

[nps]

Got everything right except the top and bottom numbers - the new tires are 36, the old 30. 5, therefore they are 1. 18 times as big, or 36/30. 5=18%.



At 60mph on your spedo, you are really going 71mph.



Your odo says you went 100 miles, you really went 118.

 

[bkroupa]

Dang, I was half a mph off. But I did spell speedo right

 

[nps]

Dang, I was half a mph off. But I did spell speedo right

Yeah, anything over 5mph and the cop isn't going to believe the bigger tire story anyway.



What really stacks up is the 3% difference on the odometer. Those extra 3 miles every 100 will add up.

 

[jaybigboy34]

Ok, I can understand how to figure with bkroupa's formula, but I am having trouble with nps's. I found out that my stock tires where 30. 5 inches tall and my new tires are 33. 4 inches tall. Could you tell me how fast I am going at 60? And how far I have really traveled after 100 miles?



Thanks

 

[piampietro]

let the website do the work

rather than measure yer tires & and calculate, use a site like this one to figger it out:

http://www.1010tires.com/TireSizeCalculator.asp

it lets you choose your stock tire & wheel config and the up to 3 other setups, and then gives you all the stats on the diff between your stock & modified setups.

most importantly, they give you the revs per mile, which is what you will need if you are going to flash the computer's ABS module with a new value to correct your speedo, odo, & ABS functionality.

if you have a ******* do it for you, be sure to explain to them that they have to use the ABS setting, not the tire size options in the TCM to do this. the TCM has only the 2 stock tire sizes to choose from; your (presumably bigger) tires & wheels will not be listed. under ABS they can enter the correct rpm for your new tire/wheel combo.

there's lotsa sites like the one above for doing these calcs, some even let you work gearing into the mix.

incidentally, for 35x12. 5x17's (my Toyo's), the rpm value to use is 598 or so. i'm w/in an mph or so of my GPS using that value, and was off by nearly 10% before switching from the stock setting (657, from my "offroad" michelin 267/75R16s i got with the truck)



LOL i just previewed my post and noticed that "s t e a l e r" was censored! how PC!