[DeTomaso] Speedometer/odometer errors

[MikeLDrew at aol.com]

Hi guys,

With reference to our earlier discussion about speedometer vs. odometer 
errors, and in light of the fact that Jim Seiferling stopped by yesterday and 
hooked up my cigarette lighter outlet for me (among other things--thanks again 
Jim!), I decided to install my GPS in my Pantera and take it for a quick drive 
(also to verify that my mongo fuel filter has temporarily fixed my fuel 
starvation problem, which it has).

My car is fitted with the standard 4.22 differential, and BFG 305/50-15 rear 
tires, which are among the tallest tires out there, measuring 27.1 inches tall 
according to BFG's website (data extracted in 2005, no longer published 
there, thankfully I downloaded the PDF file).

Here are the results:

Indicated on speedometer: 80 mph                                         
GPS reading: 75 mph

Distance travelled on odometer:   22.55 miles 
Distance travelled by GPS: 21.98 miles

Engine rpm at indicated 80 mph/actual 75 mph:   3400
Max speed attained: 96.2 mph :>)

So, in both cases, the instrument reads higher than actuality, although the 
difference isn't that bad.   If the car was fitted with smaller rear tires 
(such as 335/35-17s that many of you run, which are only 26 inches and some 
change), the delta would be correspondingly greater; I would estimate 10 mph 
difference at 80 mph for most cars and a correspondingly greater odometer reading as 
well.

BTW, it's a beautiful day here in northern California, and even though I only 
had a half-hour or so to spare, it sure was good for the soul to take it for 
a spin...:>)

Mike


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