Math geeks needed

[PATR8]

I was talking to a friend who happens to be a state trooper here in PA. He was asking me my 0-60 time and I told him I did not even know how accurate my odometer is. I explained my car came with 13 inch 195 tires and a 3.90 rear axel and now has 15 inch 225 tires and a 3.45 axel. I told I could ask so math smart guys and they could figure it out but he says there are too many varibles.

I suggested next time he brings his cruiser home he let me do a drive by and zap me with his gun but he seems to think it may be not an appropiate use of his position.

So help me, in the simpliest manner possible, can some one give me a formula to figure this out?

I am pretty sure I have seen this done before.

Thanks

 

[DougF]

You can get on the interstate and time yourself with the mile markers at 60 mph and figure from there.
Note: I'm not a math geek

 

[TR3driver]

What is/was the profile on the tires?

 

[PATR8]

The original tires stock on and 8 I beleive was 195/70/13. I am now running 225/50/15

 

[philman]

won't a gps tell you your ground speed?

 

[TR3driver]

The original tires stock on and 8 I beleive was 195/70/13. I am now running 225/50/15

Well, according to https://www.riverviewtire.com/Tireheightchart.html those are too close to even worry about. The difference is less than 0.5%, which is likely less than the original error in your speedometer/odometer.

That leaves the final drive ratio to worry about. 3.45/3.90 = .885, meaning your odometer should be reading about 11% low. When it says you've gone 100 miles, you've actually gone 113 miles. If your speedo says 70 mph, you're actually doing about 79 mph.

 

[69tr]

I got this formula from the forum some time back.
(rolling distance*RPM)/(gear ratio*1*constant of 1056)=speed


To find wheel travel: Pi*tire diameter

Hope this helps, Pete

 

[69tr]

Here is another site that might help

https://www.miata.net/garage/tirecalcold.html

 

[3798j]

Here are two more:
https://www.corral.net/tech/gearcalc.html (when you page down under trans gear, you'll see 1.00 or .71 as choices).
https://www.mintylamb.co.uk/?page=tyre.htm

 

[Silverghost]

I just used the navigator function in my cell phone to tell me how fast I was going when my speedo indicated 55 mph.... They matched.

 

[Darwin]

www.britishv8.org/Articles/chart.xls

Fill in the fields with the info from your car and it will give you very accurate speed and rpm data. I used my tach for several years with this info and never got a ticket. Checked it against police radar and it was very close when all the ratios, sizes, etc were right.
It is and excel spreedsheet but it will convert to open office if you are running that. Open office is free by the way.

 

[tr8todd]

Stock TR8s used 185/70/13 and All had 3.08 rear gears. TR7 verts used 3.45 and TR7 coupes used 3.90. Automatic TR7s used some weird ones. 3.27 on the early cars and 3.08 on the 81s.

 

[PATR8]

I forgot about the BritishV8 site. I used to spend alot of time there just drooling at some of the cars

 

[tr8todd]

My GPS unit seems to work very well compared to the speedo in my wife's car and in my truck. In fact, it works so well, I'm considering using a gps based speedo in the TR8 that has the Ford rear end in it rather than trying to get it close by swapping around speedo drive gears. That car will probably need a custom cluster anyway in order to be compatible with the new fuel cell, and stand alone FI system.

 

[aeronca65t]

I'm another fan of the GPS idea.

I just use the GPS to show true speeds and then I put little pieces of tape on the speedo face at those points (usually 25, 50 and 65 mph). The odometer is still wrong , but that's no big issue to me.

By the way, most GPSs will also record max speed....I often put my basic Garmin Nuvi ($100) in my race car, running it on its internal battery, just to see what I'm doing. Here's one from last weekend.......the max speed number is in the center right of the GPS screen (the other numbers are meaningless).