AmDWs6

so I've been searching for a calculator online to see the actual speed i'm going at, all the calculators out there calculate as if all your tires where the same,
mine are not and most of you guys aren't either.

i have 255/50/R17-Rear 275/40/R17-Front
^^ Don't ask just run with it.

now those are two different sizes so what if i was at a speed of 20mph(meter reading), how much i'm actually going?

how do we usually fix such speed-o-meter issue?

what is the formula you used when you answered the first question? if you did.

Help

99zee

iTrader: (4)

The speedometer reads off of the rear tires. The computer can be "tuned" to the tire size you are running. I use this site: 1010tires.com. You can put in your stock size and any other sizes to see the difference. It gives you the MPH difference at 60mph along with a percentage difference. The easiest way to do it is use a GPS that shows mph and go drive. When I lowered my crew cab, I had it tuned and the tire size adjusted. It matches my GPS now.

RAMPANT

iTrader: (17)

Here is the one I use all the time.

http://www.f-body.org/gears/

I use it for selecting my gears, and estimate shift points, but can be easily be used for your needs.
As mentioned above, you will just program the cluster to match your changes. I use my 10 year old Predator and a Garmin GPS to fine tune it.

AmDWs6

from 275/??/R17 or 255/??/R17 to 295/40/R18
is it possible to go from 60 km/h to 90km/h and speedo reads 60.
why im asking this is because when i was driving my brothers car to the garage
my friend was following me to take me back
when i was driving 100km/h he said i was going 150km/h.
and when i was 60 he was 90.
his car is a brand new mazda 6 i really doubt that it would be wrong.
although my brother's car, there is no doubt in that lol

any ideas, i haven't tested further more but i'm planning to get a police patrol to follow me and radar my speed, if they would agree to help out with that.

RAMPANT

iTrader: (17)

Use a GPS at a a steady speed.

If you change the gears or tire diameter, you will require a change to the program to get the correct speedo reading.

BTW, you need to tell us what you are driving. Tire size means nothing with out the year, transmission, gears and model.

If you are talking Kms, where in Canada are you?

AmDWs6

i'm not a us citizen nor am i a canadian,
i live in the middle east a country near by saudi arabia called Bahrain
^^ Answer for the question of what the f is Bahrain or where the f.
---
since its my brothers car, it's a 99 Z28 M6. rear tire size is 295/40/R18 Nitto's.
the car has a 2G vin therefore its originating from canada. it's not a import.
gears i can't tell, my brother used to run 3.73's but he broke the differential and made the shaft spin on it self at the drag strip, so both of them got replaced and i totally don't know what he's running now & i believe neither does he.
--
The car has been tuned when the tires where installed but not after the new differential.

99zee

iTrader: (4)

You need to find out what gear is in the car. There are a few methods on how to find that out. You can use the speedometer and tach, but I would assume that is only for stock tire size. I would search online. You can track the tire making a complete revolution versus the driveshaft making a complete revolution. Not sure how that one works, but I'm sure it's online. I have a 99 M6 also. The stock gear was 3.24 and the stock tire size was 245/50/16. Once you find out the gear ratio you can have it tuned again and your set.

AmDWs6

i believe that's the only current unsure solution.
but, i have to make sure that what happened that day is true.
i can't see how 3.42 ratio can go up to 90km/h in only first gear while speedo reading 60km/h
i can't seem to find that to be possible.
i'm wasn't sure about my car (02 Ws6 M6), it had a center-force in it, i thought it was stock so i really can't tell what's in that differential could also be 3.73 but i can be sure once i open up the tune. although my point is that (in first gear) i go up to a max with engine over rev 60-65 km/h in it.

i hope more info would pop up here, my brother drives the car daily to work (except Friday-Saturday) and set's the cruise control at 100-120km/h.
which means he's going 150-180km/h according to the mazda .. if that's true he'll be getting a hell lot of speeding tickets from the camera's, could also be jailed !

RAMPANT

iTrader: (17)

Well if he had a 3.73 gear in it before and put a stock diff back in with out changing the tune it will cause an error of 3.73/3.42=1.091 error ratio. so with a stock sized tire it would be going 109.01 at 100km on the speedo. If the diff he put back in was a 2.73 out of an auto car, 3.73/2.73=1.366, so now it would read 100km on the speedo and be going 136.6 kms. Add to this your over sized tires. 27.2/25.6" diameters. So now if you factor in your larger tire diameter. The 3.42 would result in 116kms and 2.73s would result in 145kms.

If I had to guess your speedo is programmed for stock tires and a 3.73, and what you really have is a 2.73 diff, with a 27.2" tall tire. Resulting in 145km speed when your speedo says 100. Close to what you are seeing. BTW, most factory speedos are set to read low by 1-2% to ensure that the car manufacturer will never be found liable for tickets or accidents.

You must know the diff ratio. If he has messed with the transmission, there are a few with different ratios as well.

One way you might be able to get your diff ratio, is go on the highway, put it in 4th gear (which is 1:1 ratio) go 100kms by the speedo. Pace the car and see what the real speed is. Document the rpm. With the known tire size, we can likely sort it all out. It is just math and you need enough data to be sure.

99zee

iTrader: (4)

If I had to guess your speedo is programmed for stock tires and a 3.73

I would "guess" the same. Thats the only thing that makes since from what you are seeing.

AmDWs6

99zee,
its programmed on 295/40/R18 3.73
the car still runs the same tires but 3.42 now
---
Rampant,
4:45 am and i just got back driving the car and getting you some numbers,
using my blacberry gps
4th gear
Speed-o at 60km/h, RPM 1600-1700, GPS 79-82km/h
Speed-o at 100km/h, RPM 2500-2600, GPS 130km/h
---
rpm for 100km/h im not 100 percent sure might be more closer to this 2600-2700, couldn't focus on cause there isnt any power steering in the car and well turns never end lol
--
All speeds done by cruise control.

RAMPANT

iTrader: (17)

Well it looks like you do not have 3.42 gears. You have 2.73s, get it programmed accordingly. Mystery solved.

Look at the chart below. The 5th column is 4th gear. 50mph=80km 80mph=128km

AmDWs6

according to that chart you seem to be right, but somehow i still have some doubt, what was your choice on the list to get such gear ratio's?

i'll make sure & get that diff opened up before tuning, but we can still tune it twice with no prob's matter of checking i guess.

what would be my alternate if 2.73's turns out to be wrong?

RAMPANT

iTrader: (17)

I selected 98 Camaro M6, then changed to your tire size and changed the diff ratio to 2.73.

I am 99.999999% sure. I can not imagine a different set of circumstances. I am not using your speedo reading for this, just your RPMs and GPS reading. You said it was a stock diff, there were only 3 different factory gears. 2.73, 3.23 and 3.42.

There is no need to open the diff. Make the changes suggested to the program and I bet you are within 1-2%.