Need some mathmatical help....

[junior3382]

Okay maybe I over think things way to much... but after a few recent posts I am a little confused on something. I am trying to figure out what would infact happen to a speedometer with changing tire sizes.

Oaky, lets work with my truck specs. Stock tire size is 235/70/15 and the tire is estimated to turn about 747 times to cover the distance of a single mile. Looking at the end run, if I were to put 35" tires of the same brand then it is estimated to turn 599 revs in the same mile. That is a 20% or so decrease in rotations and therefore tuning the tranny 20% slower? Which if I have this right, means the speedometer will read a 20% drop in speed? So when it is cruising at 65mph with the stock speedometer unchanged I would actually be cruising at 78mph? or does it go the other way? meaning I would be actually cruising at 52mph? I have always been confused on which way the speedometer reacts to the change in tires....

Does this make sense? Which one is the right calculation? I know they make adjucters to compensate, and even changing the gear ratio will alter this as well, so i can get it accurate again, but for the simple purpose of it... I am just trying to figure out which way it affects the guages.

[03chevy]

you are right jr. the bigger the tire the slower the speedo will show what you are doing. a friend of mine had a 89 toyota with 35's and he got pulled over and he said he was only doing 60 what the speedo said and the cop laughed and said no try 68 and he was in a 55 mph zone.

[z07]

Electronic Ratio Adapter (ERA)
Now You Have the Solution to Your Speedometer Problems..... Qty 12345

#69. Any change in a vehicle's tire size or rear axle ratio will cause the speedometer reading to be inaccurate. The Electronic Ratio Adapter (ERA) is designed to correct this problem. With the ERA, you simply hook up four wires, set the DIP switches according to the calibration table included with your order, and you are ready to go.
What does the Electronic Ratio Adapter do?
The automotive industry has a great variety of part sizes, types, shapes and descriptions, but regardless of make, model or year, a few things are standard. One of those standards has been the number of revolutions a speedometer cable will make for each mile traveled. Nowadays, electronic pulses have replaced the rotating cable, but the same principle applies. The Pulse Ratio (the number of pulses per mile traveled) remains the same, regardless of speed, since the same distance is traveled and the same number of pulses have occurred each mile no matter what the speed was during that mile.
However, this Pulse Ratio can be made to vary from the true when modifications are made which change the number of electronic pulses per mile on a particular vehicle. This is most commonly caused by changing the tire size (increasing the outside diameter of the tires will cause the tire to travel further before making a complete revolution), but other modifications could have the same result. When the Pulse Ratio is thrown off, the speedometer/odometer will be inaccurate and corrections must be made.
This relative difference between true speed and the speed indicated on the speedometer is called the Variance Ratio, and it is corrected using the Electronic Ratio Adapter. Cost $169.00

4L60-E.com the Ultimate 4L60E Performance Website.

[mudseeker]

your post has got me thinking. when i installed the mud tires on my truck, i went into the ABS system and re-calibrated the computer for that tire size. i did this with a Tech II and was fairly simple. now here's where my question is. i tried this with a ford (not with Tech II) and could not find the options to calibrate the ABS system for optional tire sizes. so, do only some vehicles require one of these adapters or should i still put one on my truck.

[junior3382]

Thanks z07..... I was actually planning on getting one of those anyways.... I was just curious about the math. Kudos though.... And mud, I have no idea... wish i could help...lol

[75K30]

Junior:
You also have to take the gear ratio into account. Remember, changing gears also has a affect on your speedo.
While you see a 20 percent difference in total roll out, the gear ratio changes, or minimizes the effect.
The speedo will read slower than actual mph by installing larger tires.
These formulas may help with your math.......





Common gear formulas:
Gear Ratio

Crawl Ratio

P-Metric to Inch Diameter

Tire Change (find new gear ratio)

Speedometer Adjuster (with oversize tires)
(New Tire Diameter / Old Tire Diameter) * Speedometer MPH = Actual MPH Simple Gear Ratio Formula
Ring Gear Teeth Count / Pinion Gear Teeth = Gear Ratio

[72'CaminoKid]

do you not have a gear driven speedo?

[junior3382]

so is there a chart somehwere that would give me an idea of these affects? Like for example, lets stay with my blazer, currently at a 3.09 ratio and 27.5" tires roughly... and if I upgrade to 35's and leave the 3.09 I will be about 20 percent off, but how will I know the proper adjustemnt if the gear ratio was also changed with the tire size to 4.56?

[junior3382]

Originally Posted by 72'CaminoKid

do you not have a gear driven speedo?

If you mean does the speedo read off the movement of the tranny... yes... otherwise I would not know what you mean.

[72'CaminoKid]

is it electronic to your dash or is it cable driven is what i meant.