Need help finding some speedo gears.

I am trying to fond some speedo gears for my car but can't find the right ones. the closest ones i can find are a 10 drive and an 34 driven when i really need a 29 but they don't make it. i have a 4.10 gear and 28.8 inch tires. i bought 7 drive and 20 driven gears but quickly found out the 20 driven gear woundn't fit. with a chevy speedo housing work in my short tail buick trans? if so that would be great, if not can some tell what gears to use and if they are available from the dealer. currently my speedo is 40 mph off. so i really can't just take off like 10 mph. any help would be appreciated.
Thanks Jeramie :beer

 

Here's a great website that you can play around with the numbers/gears to see what will work best. I guessing by the gear numbers you posted that you have a th350 trans. There are transmission shops that make gear boxes that go on the outside of the trans right at the speedo gear housing. You just tell them how much the speedo is off and they can build it to any gear ratio to correct. Used to be very common, but there are less shops out there that do it now. Good luck.

http://www.tciauto.com/tech_info/speedo_gears.htm

Here is a transmission company that sells the gear ratio boxes.

http://www.redrider.us/400_performance_transmission_par.htm

Here's a pic of one:

 

And here's their gear boxes. Call them. I've used them and they'll be very helpful and figure it all out for you and get you the right stuff so your speedo reads true.

GM Ratio Adapter

#54RA. This GM ratio adapter will slow your speedometer cable down 26.7%. Made for low rear end ratios with short tires. Offset for clearance reasons. If your speedometer reads too fast with 15 and 45 teeth gears then you need this part. Cost $78.00

#54RA18. This GM ratio adapter will slow your speedometer cable down 17.6%. Made for low rear end ratios with short tires. Offset for clearance reasons. If your speedometer reads too fast with 15 and 45 teeth gears then you need this part. Cost $74.00

#54RA3. This GM ratio adapter will slow your speedometer cable down 2.56%. Cost $59.00

 

Thanks for the advice. Thanks larry for the part numbers. i've been using teh tci site for alot of the trans info, great site. Thanks Jeramie