Speedometer Gear Change?

[Groundcoat]

Greetings all. I have a 1969 Pontiac Firebird 400 with a TH400 transmission and 3.08 gears. It's currently rolling on 14" rims and 215-60-14 tires. I have ordered new 15" rims and 215-65-15 tires.

Will I need to make a speedometer gear change in the transmission? Any ideas regarding the tooth count of the existing driven gear and what the tooth count might be for the new driven gear, if one is needed? I don't know the tooth count of the current drive gear or driven gear other than to say "stock".

There are many options on the market regarding speedometer gears with varying tooth counts.

Any input is greatly appreciated.

 

[melsg5]

Assuming you have a 15 tooth drive gear and a 38 tooth driven gear, going from a 24.2" tire to a 26" tire you need a 36 tooth driven gear.

 

[Groundcoat]

Thank you. I’ll give that a try. Much appreciated.

 

[cammerjeff]

The other option is to compare what the difference in the MPH readings are with what MPH you are actually traveling (with a GPS system) figure out the % of difference and add a reducer gearbox inline with the Speedo cable. An example is below.

15618397 Speedo Adaptor

ADAPTOR, SPEEDOMETER 1.2020 RATIO CODE BG (WITH O-RING) (O-RING NOT SOLD SEPATATELY)

www.wheelerfleet.com

They just tread onto the output gear of the transmission, and then you attach the speedo cable to it. Just another option I have used when the Speedo drive gear is not easily replaced in some transmissions, like a GM 2004R that does not have a removable tail shaft assembly.

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[Groundcoat]

Thanks and good to know. When I change over to the larger diameter wheels (15"), will the speedometer read faster or slower?

 

[melsg5]

Thanks and good to know. When I change over to the larger diameter wheels (15"), will the speedometer read faster or slower?

Slower