'05 Mustang GT
Stock tires = 235/55-17 -- Modified tires = 255/45-18
Stock gears = 3.55:1 -- Modified gears = 3.73:1
5-speed manual

I just got my new clutch, flywheel, and gears installed today. Clutch is a Centerforce dual-friction, flywheel is Fidanza aluminum, and gears are Ford Racing. The flywheel is 10 pounds lighter than stock (12 lbs. vs. 22 lbs.).

Anyway, back to the relevant info. I was driving back home and my speedo was reading 80MPH, but I wasn't making any appreciable gain on traffic and other cars were still passing me. A preliminary guess is that it's off by about 15MPH!!![] That CAN'T be right, can it??[&o]

For those interested, the '05 GT's stock clutch assembly weighs 22.5 lbs. The stock gears weigh 16.5 lbs. And the stock flywheel, again, weighs 22 lbs.

 

get a friend whose speedometer works correctly to drive next to you.

 

you can calibrate your speedo with most tuners.

 

Unfortunately, that's not really an option. Anybody else have any help? Is there a calculator somewhere on the net that will tell me the difference?

 

First, substituting 255/45/18s for 235/5517s changes the rolling radius smaller by 1.5% (I got the numbers from Tirerack - diamters are 27.2 for your stock tires, 26.8 for the upgrades).

Thus, due to the tire change alone, and assuming your speedo was accurate beforehand, you car is actually doing 26.8/27.2 x 60 = 59.11 mph when the speedo shows 60 mph.

The change from 3.55 to 3.73 makes a roughly 5% change, so, when the speedometer (reading the transmission output shaft) thinks you are doing 60, due to the gear ratio change alone, you are actually doing 3.55/3.73 x 60 = 57.25.

Put these together, and its safe to say you've lost about 4 mph at 60 not much more - the speedo says you are doing 60, when you are actually doing 56.

Maybe all those other cars on the road were just driving really fast.

 

Ive been using this for a while.

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

 

Thanks, Lee W. That's a good explanation, but you got it a little backwards. I went FROM 235/55-17 TO 255/45-18. According to the calculator that I also use at the Miata site (thanks anyway, TMG), that puts my speedo at 0.5% too fast. Meaning I'm doing 59.7 when the speedo reads 60.

If your gear formula is correct, subtracting that figure to the wheel/tire combo, my speedo is off by 4.5%, if I did my math right![:'(] Guess I'm gonna have to buy a tuner so I can get the speedo properly calibrated.[X(]