First off, Nitto says my tire (275/40 R17 NT555 DR) is 25.63" inflated. Fine, but their formula they give would mean that {[275mm x 0.40] /25.4} + 17"= 21.3" overall diameter. I'm going to go with 25.6" I guess.

Second problem is that I am soon going to replace the stock speedo with a 160mph mechanical Autometer gauge. I have a 1989 700R4 with god knows what color gear in there and a 3.27 BW posi that I put on there last spring. I know my mph is off a little (says 55 when I think I am going closer to 70), but what ratio gear should I put in the tranny?

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1984 z28 w/ a 357 cu in. monster engine which is looking like the posterchild for Edelbrock with the exception of the Holley 750vac... all the suspension stuff... 9-bolt posi disk is in...

-=ICON Motorsports=-

 

You forgot to multiply the first part of the formula {[275mm x 0.40] /25.4} by 2. What that first part does is calculate the sidewall width in inches, but you have 2 sidewall widths in the overall diameter. Sorry, don't know the answer to your 2nd question...

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Greg Westphal
'87 IROC 305TPI/A4

 

Duh! (hits head against wall)

Thanks that makes sense. I'm going to see if I can find something about it again in the faq section. I think I need to call Autometer and find out what gear ratio they have in the speedo, but I assume it must be a stock ratio or something.

 

I must be doing something wrong again. I plug in my numbers into the boxes and it says with 25.6" tires and 3.27 gears, in 4th gear (0.70) I should be going 133mph at 2000rpms!

While I don't know exactly how fast I am going on the highway in 4th, I do know my tire diameter and the gears ARE 3.27s and I do about 2500rpms, which is closer to 75-80mph.

ARGH!

 

As far as I can tell, the formula does not take into consideration vehicle weight, aerodynamics, etc., so that is probably why it says 133 mph @ 2000 rpm. When I had my V6 it said I could do 175 mph in OD. HAHAHA that would be funny!!!

Vman

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1969 Camaro
383, Corvette heads, 700R4
Resto is in progress

1988 Toyota 4-Runner
aka the Parts Hauler

Previous owner of an 89 RS Camaro
2.8L V6, 700R4

 

Are you using a calculator on a web page somewhere? It sounds like it's asking for tire radius, not diameter. You should be at about 67 mph at 2000 rpm in O/D.

 

That must be it. It is a in the equations section right next to the VIN and RPO decoder sections on the main page.

Thanks.